//! Versioned banlist settings files.

// NOTE: Needed to allow the second-to-last migration to call try_into().

use super::{BANLIST_FILENAME as FILENAME, MIGRATION_UPGRADE_GUARANTEE};
use crate::settings::editable::{EditableSetting, Version};
use authc::Uuid;
use core::convert::{TryFrom, TryInto};
use serde::{Deserialize, Serialize};

/// NOTE: Always replace this with the latest banlist version. Then update the
/// BanlistRaw, the TryFrom<BanlistRaw> for Banlist, the previously most recent
/// module, and add a new module for the latest version!  Please respect the
/// migration upgrade guarantee found in the parent module with any upgrade.
pub use self::v2::*;

/// Versioned settings files, one per version (v0 is only here as an example; we
/// do not expect to see any actual v0 settings files).
#[derive(Deserialize, Serialize)]
pub enum BanlistRaw {
    V0(v0::Banlist),
    V1(v1::Banlist),
    V2(v2::Banlist),
}

impl From<Banlist> for BanlistRaw {
    fn from(value: Banlist) -> Self {
        // Replace variant with that of current latest version.
        Self::V2(value)
    }
}

impl TryFrom<BanlistRaw> for (Version, Banlist) {
    type Error = <Banlist as EditableSetting>::Error;

    fn try_from(value: BanlistRaw) -> Result<Self, <Banlist as EditableSetting>::Error> {
        use BanlistRaw::*;
        Ok(match value {
            // Old versions
            V0(value) => (Version::Old, value.try_into()?),
            V1(value) => (Version::Old, value.try_into()?),
            // Latest version (move to old section using the pattern of other old version when it
            // is no longer latest).
            V2(mut value) => (value.validate()?, value),
        })
    }
}

type Final = Banlist;

impl EditableSetting for Banlist {
    type Error = BanError;
    type Legacy = legacy::Banlist;
    type Setting = BanlistRaw;

    const FILENAME: &'static str = FILENAME;
}

#[derive(Clone, Copy, Debug)]
pub enum BanKind {
    Ban,
    Unban,
}

#[derive(Clone, Copy, Debug)]
pub enum BanErrorKind {
    /// An end date went past a start date.
    InvalidDateRange {
        start_date: chrono::DateTime<chrono::Utc>,
        end_date: chrono::DateTime<chrono::Utc>,
    },
    /// Cannot unban an already-unbanned user.
    AlreadyUnbanned,
    /// Permission denied to perform requested action.
    PermissionDenied(BanKind),
    /// Cannot have a UUID linked IP ban if there is not a corresponding UUID
    /// ban. In this case the corresponding entry in the UUID ban map is missing
    /// completely.
    CorrespondingUuidBanMissing,
    /// Cannot have a UUID linked IP ban if there is not a corresponding UUID
    /// ban. In this case there is a corresponding entry, but it may be an unban
    /// instead of a ban, the expiration date may not match, or the ban info
    /// doesn't match.
    CorrespondingUuidBanMismatch,
    /// Ban info is an optional field to support legacy data, of which IP bans
    /// and their linked UUID bans are not included.
    NonLegacyBanMissingBanInfo,
    /// Multiple active IP bans should not link to the same UUID since that UUID
    /// should also be banned (and thus no IPs can be banned via that user).
    ActiveIpBansShareUuid,
}

#[derive(Debug)]
pub enum BanError {
    Uuid {
        kind: BanErrorKind,
        /// Uuid of affected user
        uuid: Uuid,
        /// Username of affected user (as of ban/unban time).
        username: String,
    },
    // Note, we specifically don't expose the IP address here since this is
    // shown to users of the ban commands.
    Ip {
        kind: BanErrorKind,
        /// Uuid of affected user
        uuid: Option<Uuid>,
        /// `username_when_performed` from the associated uuid ban entry, if the
        /// associated entry is missing (which would cause a validation
        /// error) or there is no associated entry (uuid is None) then
        /// this will be None.
        username_from_uuid_entry: Option<String>,
    },
}

/// NOTE: This isn't serialized so we can place it outside the versioned
/// modules.
///
/// `BanAction` name already taken.
///
/// Note, `IpBan` will also apply a regular ban, while `UnbanIp` will only
/// remove the IP ban.
pub enum BanOperation {
    // We don't use `Ban` struct because the info field is optional for
    // legacy reasons.
    Ban {
        reason: String,
        info: BanInfo,
        /// NOTE: Should always be higher than the `now` date provided to
        /// [`BanList::ban_operation`] , if this is present!
        end_date: Option<chrono::DateTime<chrono::Utc>>,
    },
    BanIp {
        reason: String,
        info: BanInfo,
        /// NOTE: Should always be higher than the `now` date provided to
        /// [`BanList::ban_operation`] , if this is present!
        end_date: Option<chrono::DateTime<chrono::Utc>>,
        ip: NormalizedIpAddr,
    },
    Unban {
        info: BanInfo,
    },
    UnbanIp {
        info: BanInfo,
        /// The Uuid linked to the IP ban (currently no functionality to created
        /// or remove non-uuid linked IP bans even though the model can
        /// support them)
        uuid: Uuid,
    },
}

#[derive(Debug)]
pub enum BanOperationError {
    /// Operation cancelled without performing any changes for some reason.
    NoEffect,
    /// Validation or IO error.
    EditFailed(crate::settings::editable::Error<Final>),
}

mod legacy {
    use super::{v0 as next, Final, MIGRATION_UPGRADE_GUARANTEE};
    use authc::Uuid;
    use core::convert::TryInto;
    use hashbrown::HashMap;
    use serde::{Deserialize, Serialize};

    #[derive(Deserialize, Serialize)]
    pub struct BanRecord {
        pub username_when_banned: String,
        pub reason: String,
    }

    #[derive(Deserialize, Serialize, Default)]
    #[serde(transparent)]
    pub struct Banlist(pub(super) HashMap<Uuid, BanRecord>);

    impl From<Banlist> for Final {
        /// Legacy migrations can be migrated to the latest version through the
        /// process of "chaining" migrations, starting from
        /// `next::Banlist`.
        ///
        /// Note that legacy files are always valid, which is why we implement
        /// From rather than TryFrom.
        fn from(value: Banlist) -> Self {
            next::Banlist::migrate(value)
                .try_into()
                .expect(MIGRATION_UPGRADE_GUARANTEE)
        }
    }
}

/// This module represents a banlist version that isn't actually used.  It is
/// here and part of the migration process to provide an example for how to
/// perform a migration for an old version; please use this as a reference when
/// constructing new migrations.
mod v0 {
    use super::{legacy as prev, v1 as next, Final, MIGRATION_UPGRADE_GUARANTEE};
    use crate::settings::editable::{EditableSetting, Version};
    use authc::Uuid;
    use core::convert::{TryFrom, TryInto};
    use hashbrown::HashMap;
    use serde::{Deserialize, Serialize};

    #[derive(Clone, Deserialize, Serialize)]
    pub struct BanRecord {
        pub username_when_banned: String,
        pub reason: String,
    }

    #[derive(Clone, Deserialize, Serialize, Default)]
    #[serde(transparent)]
    pub struct Banlist(pub(super) HashMap<Uuid, BanRecord>);

    impl Banlist {
        /// One-off migration from the previous version.  This must be
        /// guaranteed to produce a valid settings file as long as it is
        /// called with a valid settings file from the previous version.
        pub(super) fn migrate(prev: prev::Banlist) -> Self {
            Banlist(
                prev.0
                    .into_iter()
                    .map(
                        |(
                            uid,
                            prev::BanRecord {
                                username_when_banned,
                                reason,
                            },
                        )| {
                            (uid, BanRecord {
                                username_when_banned,
                                reason,
                            })
                        },
                    )
                    .collect(),
            )
        }

        /// Perform any needed validation on this banlist that can't be done
        /// using parsing.
        ///
        /// The returned version being "Old" indicates the loaded setting has
        /// been modified during validation (this is why validate takes
        /// `&mut self`).
        pub(super) fn validate(&mut self) -> Result<Version, <Final as EditableSetting>::Error> {
            Ok(Version::Latest)
        }
    }

    /// Pretty much every TryFrom implementation except that of the very last
    /// version should look exactly like this.
    impl TryFrom<Banlist> for Final {
        type Error = <Final as EditableSetting>::Error;

        fn try_from(mut value: Banlist) -> Result<Final, Self::Error> {
            value.validate()?;
            Ok(next::Banlist::migrate(value)
                .try_into()
                .expect(MIGRATION_UPGRADE_GUARANTEE))
        }
    }
}

mod v1 {
    use super::{
        v0 as prev, v2 as next, BanError, BanErrorKind, BanKind, Final, MIGRATION_UPGRADE_GUARANTEE,
    };
    use crate::settings::editable::{EditableSetting, Version};
    use authc::Uuid;
    use chrono::{prelude::*, Utc};
    use common::comp::AdminRole;
    use core::ops::Deref;
    use hashbrown::HashMap;
    use serde::{Deserialize, Serialize};
    use tracing::warn;

    /// Important: even if the role we are storing here appears to be identical
    /// to one used in another versioned store (like admin::Role), we *must*
    /// have our own versioned copy!  This ensures that if there's an update
    /// to the role somewhere else, the conversion function between them
    /// will break, letting people make an intelligent decision.
    ///
    /// In particular, *never remove variants from this enum* (or any other enum
    /// in a versioned settings file) without bumping the version and
    /// writing a migration that understands how to properly deal with
    /// existing instances of the old variant (you can delete From instances
    /// for the old variants at this point).  Otherwise, we will lose
    /// compatibility with old settings files, since we won't be able to
    /// deserialize them!
    #[derive(Clone, Copy, Debug, Deserialize, Eq, Ord, PartialEq, PartialOrd, Serialize)]
    pub enum Role {
        Moderator = 0,
        Admin = 1,
    }

    impl From<AdminRole> for Role {
        fn from(value: AdminRole) -> Self {
            match value {
                AdminRole::Moderator => Self::Moderator,
                AdminRole::Admin => Self::Admin,
            }
        }
    }

    impl From<Role> for AdminRole {
        fn from(value: Role) -> Self {
            match value {
                Role::Moderator => Self::Moderator,
                Role::Admin => Self::Admin,
            }
        }
    }

    #[derive(Clone, Deserialize, Serialize)]
    /// NOTE: May not be present if performed from the command line or from a
    /// legacy file.
    pub struct BanInfo {
        pub performed_by: Uuid,
        /// NOTE: May not be up to date, if we allow username changes.
        pub performed_by_username: String,
        /// NOTE: Role of the banning user at the time of the ban.
        pub performed_by_role: Role,
    }

    #[derive(Clone, Deserialize, Serialize)]
    pub struct Ban {
        pub reason: String,
        /// NOTE: Should only be None for migrations from legacy data.
        pub info: Option<BanInfo>,
        /// NOTE: Should always be higher than start_date, if both are
        /// present!
        pub end_date: Option<DateTime<Utc>>,
    }

    impl Ban {
        /// Returns true if the ban is expired, false otherwise.
        pub fn is_expired(&self, now: DateTime<Utc>) -> bool {
            self.end_date.map_or(false, |end_date| end_date <= now)
        }

        pub fn performed_by_role(&self) -> Role {
            self.info.as_ref().map(|info| info.performed_by_role)
                // We know all legacy bans were performed by an admin, since we had no other roles
                // at the time.
                .unwrap_or(Role::Admin)
        }
    }

    type Unban = BanInfo;

    #[derive(Clone, Deserialize, Serialize)]
    pub enum BanAction {
        Unban(Unban),
        Ban(Ban),
    }

    #[derive(Clone, Deserialize, Serialize)]
    pub struct BanRecord {
        /// Username of the user upon whom the action was performed, when it was
        /// performed.
        pub username_when_performed: String,
        pub action: BanAction,
        /// NOTE: When migrating from legacy versions, this will just be the
        /// time of the first migration (only applies to BanRecord).
        pub date: DateTime<Utc>,
    }

    impl BanRecord {
        /// Returns true if this record represents an expired ban, false
        /// otherwise.
        fn is_expired(&self, now: DateTime<Utc>) -> bool {
            match &self.action {
                BanAction::Ban(ban) => ban.is_expired(now),
                BanAction::Unban(_) => true,
            }
        }

        /// The history vector in a BanEntry is stored forwards (from oldest
        /// entry to newest), so `prev_record` is the previous entry in
        /// this vector when iterating forwards (by array index).
        ///
        /// Errors are:
        ///
        /// AlreadyUnbanned if an unban comes after anything but a ban.
        ///
        /// Permission(Unban) if an unban attempt is by a user with a lower role
        /// level than the original banning party.
        ///
        /// PermissionDenied(Ban) if a ban length is made shorter by a user with
        /// a role level than the original banning party.
        ///
        /// InvalidDateRange if the end date of the ban exceeds the start date.
        fn validate(&self, prev_record: Option<&BanRecord>) -> Result<(), BanErrorKind> {
            // Check to make sure the actions temporally line up--if they don't, we will
            // prevent warn an administrator (since this may indicate a system
            // clock issue and could require manual editing to resolve).
            // However, we will not actually invalidate the ban list for this, in case
            // this would otherwise prevent people from adding a new ban.
            //
            // We also deliberately leave the bad order intact, in case this reflects
            // history more accurately than the system clock does.
            if let Some(prev_record) = prev_record {
                if prev_record.date > self.date {
                    warn!(
                        "Ban list history is inconsistent, or a just-added ban was behind a \
                         historical entry in the ban
                          record; please investigate the contents of the file (might indicate a \
                         system clock change?)."
                    );
                }
            }
            let ban = match (&self.action, prev_record.map(|record| &record.action)) {
                // A ban is always valid if it follows an unban.
                (BanAction::Ban(ban), None) | (BanAction::Ban(ban), Some(BanAction::Unban(_))) => {
                    ban
                },
                // A ban record following a ban is valid if either the role of the person doing the
                // banning is at least the privilege level of the person who did the ban, or the
                // ban's new end time is at least the previous end time.
                (BanAction::Ban(new_ban), Some(BanAction::Ban(old_ban))) => {
                    match (new_ban.end_date, old_ban.end_date) {
                        // New role ≥ old role
                        _ if new_ban.performed_by_role() >= old_ban.performed_by_role() => new_ban,
                        // Permanent ban retracted to temp ban.
                        (Some(_), None) => {
                            return Err(BanErrorKind::PermissionDenied(BanKind::Ban));
                        },
                        // Temp ban retracted to shorter temp ban.
                        (Some(new_date), Some(old_date)) if new_date < old_date => {
                            return Err(BanErrorKind::PermissionDenied(BanKind::Ban));
                        },
                        // Anything else (extension to permanent ban, or temp ban extension to
                        // longer temp ban).
                        _ => new_ban,
                    }
                },
                // An unban record is invalid if it does not follow a ban.
                (BanAction::Unban(_), None) | (BanAction::Unban(_), Some(BanAction::Unban(_))) => {
                    return Err(BanErrorKind::AlreadyUnbanned);
                },
                // An unban record following a ban is valid if the role of the person doing the
                // unbanning is at least the privilege level of the person who did the ban.
                (BanAction::Unban(unban), Some(BanAction::Ban(ban))) => {
                    return if unban.performed_by_role >= ban.performed_by_role() {
                        Ok(())
                    } else {
                        Err(BanErrorKind::PermissionDenied(BanKind::Unban))
                    };
                },
            };

            // End date of a ban must be at least as big as the start date.
            if let Some(end_date) = ban.end_date {
                if self.date > end_date {
                    return Err(BanErrorKind::InvalidDateRange {
                        start_date: self.date,
                        end_date,
                    });
                }
            }
            Ok(())
        }
    }

    #[derive(Clone, Deserialize, Serialize)]
    pub struct BanEntry {
        /// The latest ban record for this user.
        pub current: BanRecord,
        /// Historical ban records for this user, stored in order from oldest to
        /// newest.
        pub history: Vec<BanRecord>,
        /// A *hint* about whether the system thinks this entry is expired,
        /// mostly to make it easier for someone manually going through
        /// a file to see whether an entry is currently in effect or
        /// not.  This is based off the contents of `current`.
        pub expired: bool,
    }

    impl Deref for BanEntry {
        type Target = BanRecord;

        fn deref(&self) -> &Self::Target { &self.current }
    }

    impl BanEntry {
        /// Both validates, and updates the hint bit if it's inconsistent with
        /// reality.
        ///
        /// If we were invalid, returns an error.  Otherwise, returns Ok(v),
        /// where v is Latest if the hint bit was modified, Old
        /// otherwise.
        fn validate(
            &mut self,
            now: DateTime<Utc>,
            uuid: Uuid,
        ) -> Result<Version, <Final as EditableSetting>::Error> {
            let make_error = |current_entry: &BanRecord| {
                let username = current_entry.username_when_performed.clone();
                move |kind| BanError::Uuid {
                    kind,
                    uuid,
                    username,
                }
            };
            // First, go forwards through history (also forwards in terms of the iterator
            // direction), validating each entry in turn.
            let mut prev_entry = None;
            for current_entry in &self.history {
                current_entry
                    .validate(prev_entry)
                    .map_err(make_error(current_entry))?;
                prev_entry = Some(current_entry);
            }

            // History has now been validated, so validate the current entry.
            self.current
                .validate(prev_entry)
                .map_err(make_error(&self.current))?;

            // Make sure the expired hint is correct, and if not indicate that we should
            // resave the file.
            let is_expired = self.current.is_expired(now);
            if self.expired != is_expired {
                self.expired = is_expired;
                Ok(Version::Old)
            } else {
                Ok(Version::Latest)
            }
        }
    }

    #[derive(Clone, Deserialize, Serialize, Default)]
    #[serde(transparent)]
    pub struct Banlist(pub(super) HashMap<Uuid, BanEntry>);

    impl Banlist {
        /// One-off migration from the previous version.  This must be
        /// guaranteed to produce a valid settings file as long as it is
        /// called with a valid settings file from the previous version.
        pub(super) fn migrate(prev: prev::Banlist) -> Self {
            // The ban start date for migrations from legacy is the current one; we could
            // record that they actually have an unknown start date, but this
            // would just complicate the format.
            let date = Utc::now();
            Banlist(
                prev.0
                    .into_iter()
                    .map(
                        |(
                            uuid,
                            prev::BanRecord {
                                username_when_banned,
                                reason,
                            },
                        )| {
                            (uuid, BanEntry {
                                current: BanRecord {
                                    username_when_performed: username_when_banned,
                                    // We only recorded unbans pre-migration.
                                    action: BanAction::Ban(Ban {
                                        reason,
                                        // We don't know who banned this user pre-migration.
                                        info: None,
                                        // All bans pre-migration are of unlimited duration.
                                        end_date: None,
                                    }),
                                    date,
                                },
                                // Old bans never expire, so set the expiration hint to false.
                                expired: false,
                                // There is no known ban history yet.
                                history: Vec::new(),
                            })
                        },
                    )
                    .collect(),
            )
        }

        /// Perform any needed validation on this banlist that can't be done
        /// using parsing.
        ///
        /// The returned version being "Old" indicates the loaded setting has
        /// been modified during validation (this is why validate takes
        /// `&mut self`).
        pub(super) fn validate(&mut self) -> Result<Version, <Final as EditableSetting>::Error> {
            let mut version = Version::Latest;
            let now = Utc::now();
            for (&uuid, value) in self.0.iter_mut() {
                if matches!(value.validate(now, uuid)?, Version::Old) {
                    // Update detected.
                    version = Version::Old;
                }
            }
            Ok(version)
        }
    }

    impl TryFrom<Banlist> for Final {
        type Error = <Final as EditableSetting>::Error;

        #[allow(clippy::useless_conversion)]
        fn try_from(mut value: Banlist) -> Result<Final, Self::Error> {
            value.validate()?;
            Ok(next::Banlist::migrate(value)
                .try_into()
                .expect(MIGRATION_UPGRADE_GUARANTEE))
        }
    }
}

mod v2 {
    use super::{
        v1 as prev, BanError, BanErrorKind, BanKind, BanOperation, BanOperationError, Final,
    };
    use crate::settings::editable::{EditableSetting, Version};
    use authc::Uuid;
    use chrono::{prelude::*, Utc};
    use common::comp::AdminRole;
    use core::{mem, ops::Deref};
    use hashbrown::{hash_map, HashMap};
    use serde::{Deserialize, Serialize};
    use std::net::{IpAddr, Ipv6Addr};
    use tracing::warn;
    /* use super::v3 as next; */

    /// Important: even if the role we are storing here appears to be identical
    /// to one used in another versioned store (like admin::Role), we *must*
    /// have our own versioned copy!  This ensures that if there's an update
    /// to the role somewhere else, the conversion function between them
    /// will break, letting people make an intelligent decision.
    ///
    /// In particular, *never remove variants from this enum* (or any other enum
    /// in a versioned settings file) without bumping the version and
    /// writing a migration that understands how to properly deal with
    /// existing instances of the old variant (you can delete From instances
    /// for the old variants at this point).  Otherwise, we will lose
    /// compatibility with old settings files, since we won't be able to
    /// deserialize them!
    #[derive(Clone, Copy, Debug, Deserialize, Eq, Ord, PartialEq, PartialOrd, Serialize)]
    pub enum Role {
        Moderator = 0,
        Admin = 1,
    }

    impl From<AdminRole> for Role {
        fn from(value: AdminRole) -> Self {
            match value {
                AdminRole::Moderator => Self::Moderator,
                AdminRole::Admin => Self::Admin,
            }
        }
    }

    impl From<Role> for AdminRole {
        fn from(value: Role) -> Self {
            match value {
                Role::Moderator => Self::Moderator,
                Role::Admin => Self::Admin,
            }
        }
    }

    #[derive(Clone, PartialEq, Eq, Deserialize, Serialize)]
    /// NOTE: May not be present if performed from the command line or from a
    /// legacy file.
    pub struct BanInfo {
        pub performed_by: Uuid,
        /// NOTE: May not be up to date, if we allow username changes.
        pub performed_by_username: String,
        /// NOTE: Role of the banning user at the time of the ban.
        pub performed_by_role: Role,
    }

    impl BanInfo {
        fn migrate(
            prev::BanInfo {
                performed_by,
                performed_by_username,
                performed_by_role,
            }: prev::BanInfo,
        ) -> Self {
            Self {
                performed_by,
                performed_by_username,
                performed_by_role: match performed_by_role {
                    prev::Role::Moderator => Role::Moderator,
                    prev::Role::Admin => Role::Admin,
                },
            }
        }
    }

    #[derive(Clone, Deserialize, Serialize)]
    pub struct Ban {
        pub reason: String,
        /// NOTE: Should only be None for migrations from legacy data.
        pub info: Option<BanInfo>,
        /// NOTE: Should always be higher than the `date` in the record
        /// containing this, if this is present!
        pub end_date: Option<DateTime<Utc>>,
    }

    impl Ban {
        /// Returns true if the ban is expired, false otherwise.
        pub fn is_expired(&self, now: DateTime<Utc>) -> bool {
            self.end_date.map_or(false, |end_date| end_date <= now)
        }

        pub fn performed_by_role(&self) -> Role {
            self.info.as_ref().map(|info| info.performed_by_role)
                // We know all legacy bans were performed by an admin, since we had no other roles
                // at the time.
                .unwrap_or(Role::Admin)
        }

        pub fn info(&self) -> common_net::msg::server::BanInfo {
            common_net::msg::server::BanInfo {
                reason: self.reason.clone(),
                until: self.end_date.map(|date| date.timestamp()),
            }
        }
    }

    type Unban = BanInfo;

    #[derive(Clone, Deserialize, Serialize)]
    pub enum BanAction {
        Unban(Unban),
        Ban(Ban),
    }

    impl BanAction {
        pub fn ban(&self) -> Option<&Ban> {
            match self {
                BanAction::Unban(_) => None,
                BanAction::Ban(ban) => Some(ban),
            }
        }
    }

    #[derive(Clone, Deserialize, Serialize)]
    pub struct BanRecord {
        /// Username of the user upon whom the action was performed, when it was
        /// performed.
        pub username_when_performed: String,
        pub action: BanAction,
        /// NOTE: When migrating from legacy versions, this will just be the
        /// time of the first migration (only applies to BanRecord).
        pub date: DateTime<Utc>,
    }

    impl BanRecord {
        /// Returns true if this record represents an expired ban, false
        /// otherwise.
        fn is_expired(&self, now: DateTime<Utc>) -> bool {
            match &self.action {
                BanAction::Ban(ban) => ban.is_expired(now),
                BanAction::Unban(_) => true,
            }
        }

        fn migrate(
            prev::BanRecord {
                username_when_performed,
                action,
                date,
            }: prev::BanRecord,
        ) -> Self {
            BanRecord {
                username_when_performed,
                action: match action {
                    prev::BanAction::Ban(prev::Ban {
                        reason,
                        info,
                        end_date,
                    }) => BanAction::Ban(Ban {
                        reason,
                        info: info.map(BanInfo::migrate),
                        end_date,
                    }),
                    prev::BanAction::Unban(info) => BanAction::Unban(BanInfo::migrate(info)),
                },
                date,
            }
        }

        /// The history vector in a BanEntry is stored forwards (from oldest
        /// entry to newest), so `prev_record` is the previous entry in
        /// this vector when iterating forwards (by array index).
        ///
        /// Errors are:
        ///
        /// AlreadyUnbanned if an unban comes after anything but a ban.
        ///
        /// Permission(Unban) if an unban attempt is by a user with a lower role
        /// level than the original banning party.
        ///
        /// PermissionDenied(Ban) if a ban length is made shorter by a user with
        /// a role level than the original banning party.
        ///
        /// InvalidDateRange if the end date of the ban exceeds the start date.
        fn validate(&self, prev_record: Option<&BanRecord>) -> Result<(), BanErrorKind> {
            // Check to make sure the actions temporally line up--if they don't, we will
            // prevent warn an administrator (since this may indicate a system
            // clock issue and could require manual editing to resolve).
            // However, we will not actually invalidate the ban list for this, in case
            // this would otherwise prevent people from adding a new ban.
            //
            // We also deliberately leave the bad order intact, in case this reflects
            // history more accurately than the system clock does.
            if let Some(prev_record) = prev_record {
                if prev_record.date > self.date {
                    warn!(
                        "Ban list history is inconsistent, or a just-added ban was behind a \
                         historical entry in the ban
                          record; please investigate the contents of the file (might indicate a \
                         system clock change?)."
                    );
                }
            }
            let ban = match (&self.action, prev_record.map(|record| &record.action)) {
                // A ban is always valid if it follows an unban.
                (BanAction::Ban(ban), None) | (BanAction::Ban(ban), Some(BanAction::Unban(_))) => {
                    ban
                },
                // A ban record following a ban is valid if either the role of the person doing the
                // banning is at least the privilege level of the person who did the ban, or the
                // ban's new end time is at least the previous end time.
                (BanAction::Ban(new_ban), Some(BanAction::Ban(old_ban))) => {
                    match (new_ban.end_date, old_ban.end_date) {
                        // New role ≥ old role
                        _ if new_ban.performed_by_role() >= old_ban.performed_by_role() => new_ban,
                        // Permanent ban retracted to temp ban.
                        (Some(_), None) => {
                            return Err(BanErrorKind::PermissionDenied(BanKind::Ban));
                        },
                        // Temp ban retracted to shorter temp ban.
                        (Some(new_date), Some(old_date)) if new_date < old_date => {
                            return Err(BanErrorKind::PermissionDenied(BanKind::Ban));
                        },
                        // Anything else (extension to permanent ban, or temp ban extension to
                        // longer temp ban).
                        _ => new_ban,
                    }
                },
                // An unban record is invalid if it does not follow a ban.
                (BanAction::Unban(_), None) | (BanAction::Unban(_), Some(BanAction::Unban(_))) => {
                    return Err(BanErrorKind::AlreadyUnbanned);
                },
                // An unban record following a ban is valid if the role of the person doing the
                // unbanning is at least the privilege level of the person who did the ban.
                (BanAction::Unban(unban), Some(BanAction::Ban(ban))) => {
                    return if unban.performed_by_role >= ban.performed_by_role() {
                        Ok(())
                    } else {
                        Err(BanErrorKind::PermissionDenied(BanKind::Unban))
                    };
                },
            };

            // End date of a ban must be at least as big as the start date.
            if let Some(end_date) = ban.end_date {
                if self.date > end_date {
                    return Err(BanErrorKind::InvalidDateRange {
                        start_date: self.date,
                        end_date,
                    });
                }
            }
            Ok(())
        }
    }

    #[derive(Clone, Deserialize, Serialize)]
    pub struct BanEntry {
        /// The latest ban record for this user.
        pub current: BanRecord,
        /// Historical ban records for this user, stored in order from oldest to
        /// newest.
        pub history: Vec<BanRecord>,
        /// A *hint* about whether the system thinks this entry is expired,
        /// mostly to make it easier for someone manually going through
        /// a file to see whether an entry is currently in effect or
        /// not.  This is based off the contents of `current`.
        pub expired: bool,
    }

    impl Deref for BanEntry {
        type Target = BanRecord;

        fn deref(&self) -> &Self::Target { &self.current }
    }

    impl BanEntry {
        fn migrate(
            prev::BanEntry {
                current,
                history,
                expired,
            }: prev::BanEntry,
        ) -> Self {
            Self {
                current: BanRecord::migrate(current),
                history: history.into_iter().map(BanRecord::migrate).collect(),
                expired,
            }
        }

        /// Both validates, and updates the hint bit if it's inconsistent with
        /// reality.
        ///
        /// If we were invalid, returns an error.  Otherwise, returns Ok(v),
        /// where v is Latest if the hint bit was modified, Old
        /// otherwise.
        fn validate(
            &mut self,
            now: DateTime<Utc>,
            uuid: Uuid,
        ) -> Result<Version, <Final as EditableSetting>::Error> {
            let make_error = |kind, current_entry: &BanRecord| BanError::Uuid {
                kind,
                uuid,
                username: current_entry.username_when_performed.clone(),
            };
            // First, go forwards through history (also forwards in terms of the iterator
            // direction), validating each entry in turn.
            let mut prev_entry = None;
            for current_entry in &self.history {
                current_entry
                    .validate(prev_entry)
                    .map_err(|kind| make_error(kind, current_entry))?;
                prev_entry = Some(current_entry);
            }

            // History has now been validated, so validate the current entry.
            self.current
                .validate(prev_entry)
                .map_err(|kind| make_error(kind, &self.current))?;

            // Make sure the expired hint is correct, and if not indicate that we should
            // resave the file.
            let is_expired = self.current.is_expired(now);
            if self.expired != is_expired {
                self.expired = is_expired;
                Ok(Version::Old)
            } else {
                Ok(Version::Latest)
            }
        }
    }

    #[derive(Clone, Deserialize, Serialize)]
    pub struct IpBanRecord {
        /// Uuid of the user through which this IP ban was applied.
        ///
        /// This is optional to allow for the potenital of non-user-associated
        /// IP bans.
        pub uuid_when_performed: Option<Uuid>,
        pub action: BanAction,
        /// NOTE: When migrating from legacy versions, this will just be the
        /// time of the first migration (only applies to BanRecord).
        pub date: DateTime<Utc>,
    }

    impl IpBanRecord {
        /// Returns true if this record represents an expired ban, false
        /// otherwise.
        fn is_expired(&self, now: DateTime<Utc>) -> bool {
            match &self.action {
                BanAction::Ban(ban) => ban.is_expired(now),
                BanAction::Unban(_) => true,
            }
        }

        /// The history vector in a IpBanEntry is stored forwards (from oldest
        /// entry to newest), so `prev_record` is the previous entry in
        /// this vector when iterating forwards (by array index).
        ///
        /// Errors are:
        ///
        /// AlreadyUnbanned if an unban comes after anything but a ban.
        ///
        /// Permission(Unban) if an unban attempt is by a user with a lower role
        /// level than the original banning party.
        ///
        /// PermissionDenied(Ban) if a ban length is made shorter by a user with
        /// a role level than the original banning party.
        ///
        /// InvalidDateRange if the end date of the ban exceeds the start date.
        fn validate(&self, prev_record: Option<&IpBanRecord>) -> Result<(), BanErrorKind> {
            // Check to make sure the actions temporally line up--if they don't, we will
            // prevent warn an administrator (since this may indicate a system
            // clock issue and could require manual editing to resolve).
            // However, we will not actually invalidate the ban list for this, in case
            // this would otherwise prevent people from adding a new ban.
            //
            // We also deliberately leave the bad order intact, in case this reflects
            // history more accurately than the system clock does.
            if let Some(prev_record) = prev_record {
                if prev_record.date > self.date {
                    warn!(
                        "Ban list history is inconsistent, or a just-added ban was behind a \
                         historical entry in the ban
                          record; please investigate the contents of the file (might indicate a \
                         system clock change?)."
                    );
                }
            }
            let ban = match (&self.action, prev_record.map(|record| &record.action)) {
                // A ban is always valid if it follows an unban.
                (BanAction::Ban(ban), None) | (BanAction::Ban(ban), Some(BanAction::Unban(_))) => {
                    ban
                },
                // A ban record following a ban is valid if either the role of the person doing the
                // banning is at least the privilege level of the person who did the ban, or the
                // ban's new end time is at least the previous end time.
                (BanAction::Ban(new_ban), Some(BanAction::Ban(old_ban))) => {
                    match (new_ban.end_date, old_ban.end_date) {
                        // New role ≥ old role
                        _ if new_ban.performed_by_role() >= old_ban.performed_by_role() => new_ban,
                        // Permanent ban retracted to temp ban.
                        (Some(_), None) => {
                            return Err(BanErrorKind::PermissionDenied(BanKind::Ban));
                        },
                        // Temp ban retracted to shorter temp ban.
                        (Some(new_date), Some(old_date)) if new_date < old_date => {
                            return Err(BanErrorKind::PermissionDenied(BanKind::Ban));
                        },
                        // Anything else (extension to permanent ban, or temp ban extension to
                        // longer temp ban).
                        _ => new_ban,
                    }
                },
                // An unban record is invalid if it does not follow a ban.
                (BanAction::Unban(_), None) | (BanAction::Unban(_), Some(BanAction::Unban(_))) => {
                    return Err(BanErrorKind::AlreadyUnbanned);
                },
                // An unban record following a ban is valid if the role of the person doing the
                // unbanning is at least the privilege level of the person who did the ban.
                (BanAction::Unban(unban), Some(BanAction::Ban(ban))) => {
                    return if unban.performed_by_role >= ban.performed_by_role() {
                        Ok(())
                    } else {
                        Err(BanErrorKind::PermissionDenied(BanKind::Unban))
                    };
                },
            };

            // End date of a ban must be at least as big as the start date.
            if let Some(end_date) = ban.end_date {
                if self.date > end_date {
                    return Err(BanErrorKind::InvalidDateRange {
                        start_date: self.date,
                        end_date,
                    });
                }
            }
            Ok(())
        }
    }

    #[derive(Clone, Deserialize, Serialize)]
    pub struct IpBanEntry {
        /// The latest ban record for this IP.
        ///
        /// Note: If this IP is currently banned and the current `BanRecord`
        /// contains a Uuid, then this user must also be banned in the
        /// Uuid ban map. This is enforced by the validation.
        pub current: IpBanRecord,
        /// Historical ban records for this user, stored in order from oldest to
        /// newest.
        pub history: Vec<IpBanRecord>,
        /// A *hint* about whether the system thinks this entry is expired,
        /// mostly to make it easier for someone manually going through
        /// a file to see whether an entry is currently in effect or
        /// not.  This is based off the contents of `current`.
        pub expired: bool,
    }

    impl Deref for IpBanEntry {
        type Target = IpBanRecord;

        fn deref(&self) -> &Self::Target { &self.current }
    }

    impl IpBanEntry {
        /// Both validates, and updates the hint bit if it's inconsistent with
        /// reality.
        ///
        /// If we were invalid, returns an error.  Otherwise, returns Ok(v),
        /// where v is Latest if the hint bit was modified, Old
        /// otherwise.
        fn validate(
            &mut self,
            now: DateTime<Utc>,
            uuid_bans: &HashMap<Uuid, BanEntry>,
        ) -> Result<Version, <Final as EditableSetting>::Error> {
            let make_error = |kind, current_entry: &IpBanRecord| {
                let uuid = current_entry.uuid_when_performed;

                BanError::Ip {
                    kind,
                    uuid,
                    username_from_uuid_entry: uuid
                        .and_then(|u| uuid_bans.get(&u))
                        .map(|e| e.current.username_when_performed.clone()),
                }
            };
            // First, go forwards through history (also forwards in terms of the iterator
            // direction), validating each entry in turn.
            let mut prev_entry = None;
            for current_entry in &self.history {
                current_entry
                    .validate(prev_entry)
                    .map_err(|kind| make_error(kind, current_entry))?;
                prev_entry = Some(current_entry);
            }

            // History has now been validated, so validate the current entry.
            self.current
                .validate(prev_entry)
                .map_err(|kind| make_error(kind, &self.current))?;

            // If the current entry is an unexpired ban and is linked to a uuid,
            // then that uuid must also be banned. These bans must also have the
            // same expiration and have matching `BanInfo`.
            if let Some(uuid) = self.current.uuid_when_performed {
                let uuid_entry = uuid_bans.get(&uuid).ok_or_else(|| {
                    make_error(BanErrorKind::CorrespondingUuidBanMissing, &self.current)
                })?;

                if let BanAction::Ban(ip_ban) = &self.current.action
                    && !self.current.is_expired(now)
                {
                    if let BanAction::Ban(uuid_ban) = &uuid_entry.current.action {
                        let ip_info = ip_ban.info.as_ref().ok_or_else(|| {
                            make_error(BanErrorKind::NonLegacyBanMissingBanInfo, &self.current)
                        })?;
                        let uuid_info = uuid_ban.info.as_ref().ok_or_else(|| {
                            make_error(BanErrorKind::NonLegacyBanMissingBanInfo, &self.current)
                        })?;

                        // Expiration time must match, so that the banned user
                        // cannot join and be banned from another IP address.
                        //
                        // BanInfo must match as well since these bans should
                        // have been performed by the same user.
                        if ip_ban.end_date == uuid_ban.end_date && ip_info == uuid_info {
                            Ok(())
                        } else {
                            Err(make_error(
                                BanErrorKind::CorrespondingUuidBanMismatch,
                                &self.current,
                            ))
                        }
                    } else {
                        Err(make_error(
                            BanErrorKind::CorrespondingUuidBanMismatch,
                            &self.current,
                        ))
                    }?;
                }
            }

            // Make sure the expired hint is correct, and if not indicate that we should
            // resave the file.
            let is_expired = self.current.is_expired(now);
            if self.expired != is_expired {
                self.expired = is_expired;
                Ok(Version::Old)
            } else {
                Ok(Version::Latest)
            }
        }
    }

    /// The last 64 bits of IPv6 addresess may vary a lot even when coming from
    /// the same client, and taking the full IPv6 for IP bans is thus useless.
    ///
    /// This newtype ensures that all the last 64 bits of an IPv6 address are
    /// set to zero to counter this.
    #[derive(Clone, Copy, Debug, Deserialize, Serialize, Hash, PartialEq, Eq)]
    #[serde(transparent)]
    pub struct NormalizedIpAddr(IpAddr);

    impl From<IpAddr> for NormalizedIpAddr {
        fn from(ip: IpAddr) -> Self {
            Self(match ip {
                // Take IPv4 adddresses as-is
                IpAddr::V4(ip) => IpAddr::V4(ip),
                // Ignore the last 64 bits for IPv6 addresses
                IpAddr::V6(ip) => IpAddr::V6(Ipv6Addr::from_bits(
                    ip.to_bits() & 0xffff_ffff_ffff_ffff_0000_0000_0000_0000_u128,
                )),
            })
        }
    }

    impl Deref for NormalizedIpAddr {
        type Target = IpAddr;

        fn deref(&self) -> &Self::Target { &self.0 }
    }

    #[derive(Clone, Deserialize, Serialize, Default)]
    pub struct Banlist {
        pub(super) uuid_bans: HashMap<Uuid, BanEntry>,
        pub(super) ip_bans: HashMap<NormalizedIpAddr, IpBanEntry>,
    }

    impl Banlist {
        pub fn uuid_bans(&self) -> &HashMap<Uuid, BanEntry> { &self.uuid_bans }

        pub fn ip_bans(&self) -> &HashMap<NormalizedIpAddr, IpBanEntry> { &self.ip_bans }

        /// Attempt to perform the ban operation `operation` for the user with
        /// UUID `uuid` and username `username`, starting from time `now` (the
        /// information about the banning party will be in the `operation`
        /// record), with a settings file maintained at path root `data_dir`.
        ///
        /// Banning an IP via a user will also ban that user's UUID.
        /// Additionally, a regular UUID unban will also produce an IP unban if
        /// a corresponding one is active and linked to the unbanned UUID.
        ///
        /// If trying to unban an already unbanned player, or trying to ban but
        /// the ban status would not immediately change, the "overwrite" boolean
        /// should also be set to true.
        ///
        /// We try to detect duplicates (bans that would have no effect) and
        /// return `Err(BanOperationError::NoEffect)` if such effects are
        /// encountered.
        ///
        /// If the errors outlined above are successfully avoided, we attempt
        /// the edit either succeeding and returning `Ok(())` or returning
        /// `Err(BanOperationError::EditFailed(error))`, which works as follows.
        ///
        /// If the ban was invalid for any reason, then neither the in-memory
        /// banlist nor the on-disk banlist are modified.  If the ban
        /// entry is valid but the file encounters an error that
        /// prevents it from being atomically written to disk, we return an
        /// error but retain the change in memory.  Otherwise, we
        /// complete successfully and atomically write the banlist to
        /// disk.
        ///
        /// Note that the IO operation is only *guaranteed* atomic in the weak
        /// sense that either the whole page is written or it isn't; we
        /// cannot guarantee that the data we read in order to modify
        /// the file was definitely up to date, so we could be missing
        /// information if the file was manually edited or a function
        /// edits it without going through the usual specs resources.
        /// So, please be careful with ad hoc modifications to the file while
        /// the server is running.
        pub fn ban_operation(
            &mut self,
            data_dir: &std::path::Path,
            now: DateTime<Utc>,
            uuid: Uuid,
            username_when_performed: String,
            operation: BanOperation,
            overwrite: bool,
        ) -> Result<Option<common_net::msg::server::BanInfo>, BanOperationError> {
            let make_record = |action| BanRecord {
                username_when_performed,
                action,
                date: now,
            };
            let make_ip_record = |action| IpBanRecord {
                // Note: we may support banning IPs without associated user in the future.
                uuid_when_performed: Some(uuid),
                action,
                date: now,
            };

            // Perform an atomic edit.
            let edit_result = self.edit(data_dir.as_ref(), |banlist| {
                match operation {
                    BanOperation::Ban {
                        reason,
                        info,
                        end_date,
                    } => {
                        let ban = Ban {
                            reason,
                            info: Some(info),
                            end_date,
                        };
                        let frontend_info = ban.info();
                        let action = BanAction::Ban(ban);
                        let ban_record = make_record(action);
                        // NOTE: If there is linked IP ban, `overwrite` based changes may fail. In
                        // the future, we may want to switch this to autoupdate the IP ban.
                        banlist
                            .apply_ban_record(uuid, ban_record, overwrite, now)
                            .map(|_| Some(frontend_info))
                    },
                    BanOperation::BanIp {
                        reason,
                        info,
                        end_date,
                        ip,
                    } => {
                        let ban = Ban {
                            reason,
                            info: Some(info),
                            end_date,
                        };
                        let frontend_info = ban.info();
                        let action = BanAction::Ban(ban);
                        let ban_record = make_record(action.clone());
                        let ip_ban_record = make_ip_record(action);

                        // If a user is able to connect with a banned IP (e.g. a
                        // moderator), and then `overwrite` is used with the IP
                        // ban operation on them, this will result in changing
                        // the user which is linked to this IP ban (the IP will
                        // no longer be unbanned if the previous user is
                        // unbanned). This should not cause any issues with our
                        // validated invariant of having a UUID linked to at
                        // most 1 active IP ban.
                        let ban_effect = banlist.apply_ban_record(uuid, ban_record, overwrite, now);
                        let ip_ban_effect =
                            banlist.apply_ip_ban_record(ip, ip_ban_record, overwrite, now);
                        // Only submit edit if one of these had an effect.
                        ban_effect.or(ip_ban_effect).map(|_| Some(frontend_info))
                    },
                    BanOperation::Unban { info } => {
                        let action = BanAction::Unban(info);
                        let ban_record = make_record(action.clone());
                        let ban_effect = banlist.apply_ban_record(uuid, ban_record, overwrite, now);
                        // If there is a matching IP ban we should remove that as well.
                        //
                        // Validation checks that there is only one active IP ban for a particular
                        // uuid, since if we ensure IP bans also ban the uuid and regular unbans
                        // remove an existing IP ban if it exists then a user won't be able to
                        // connect from another IP while there is an active IP ban linked to that
                        // user.
                        let ip = banlist
                            .ip_bans
                            .iter()
                            .find(|(_ip, entry)| {
                                entry.current.uuid_when_performed == Some(uuid)
                                    && !entry.current.is_expired(now)
                            })
                            .map(|(ip, _)| *ip);

                        ip.and_then(|ip| {
                            let ip_ban_record = make_ip_record(action);
                            banlist.apply_ip_ban_record(ip, ip_ban_record, overwrite, now)
                        })
                        // Only submit edit if one of these had an effect.
                        .or(ban_effect).map(|_| None)
                    },
                    BanOperation::UnbanIp { info, uuid } => {
                        let ip = banlist
                            .ip_bans
                            .iter()
                            .find(|(_ip, entry)| {
                                entry.current.uuid_when_performed == Some(uuid)
                                    && !entry.current.is_expired(now)
                            })
                            .map(|(ip, _)| *ip);

                        ip.and_then(|ip| {
                            // Note: It is kind of redundant to include uuid here (since it's not
                            // going to change from the ban).
                            banlist.apply_ip_ban_record(
                                ip,
                                make_ip_record(BanAction::Unban(info)),
                                overwrite,
                                now,
                            )
                        })
                        .map(|_| None)
                    },
                }
            });

            match edit_result {
                Some((info, Ok(()))) => Ok(info),
                Some((_, Err(err))) => Err(BanOperationError::EditFailed(err)),
                None => Err(BanOperationError::NoEffect),
            }
        }

        /// Only meant to be called by `Self::ban_operation` within the `edit`
        /// closure.
        ///
        /// Returns `None` to cancel early and abandon the edit.
        #[must_use]
        fn apply_ban_record(
            &mut self,
            uuid: Uuid,
            record: BanRecord,
            overwrite: bool,
            now: DateTime<Utc>,
        ) -> Option<()> {
            match self.uuid_bans.entry(uuid) {
                hash_map::Entry::Vacant(v) => {
                    // If this is an unban, it will have no effect, so return early.
                    if matches!(record.action, BanAction::Unban(_)) {
                        return None;
                    }
                    // Otherwise, this will at least potentially have an effect
                    // (assuming it succeeds).
                    v.insert(BanEntry {
                        current: record,
                        history: Vec::new(),
                        // This is a hint anyway, but expired will also be set to true
                        // before saving by the call `edit`
                        // makes to `validate` (through `try_into`), which will set it
                        // to true in the event that
                        // the ban time was so short
                        // that it expired during the interval
                        // between creating the action and saving it.
                        //
                        // TODO: Decide if we even care enough about this case to worry
                        // about the gap. Probably not, even
                        // though it does involve time!
                        expired: false,
                    });
                },
                hash_map::Entry::Occupied(mut o) => {
                    let entry = o.get_mut();
                    // If overwrite is off, check that this entry (if successful) would
                    // actually change the ban status.
                    if !overwrite && entry.current.is_expired(now) == record.is_expired(now) {
                        return None;
                    }
                    // Push the current (most recent) entry to the back of the history
                    // list.
                    entry.history.push(mem::replace(&mut entry.current, record));
                },
            }
            Some(())
        }

        /// Only meant to be called by `Self::ban_operation` within the `edit`
        /// closure.
        ///
        /// Returns `None` to cancel early and abandon the edit.
        #[must_use]
        fn apply_ip_ban_record(
            &mut self,
            ip: NormalizedIpAddr,
            record: IpBanRecord,
            overwrite: bool,
            now: DateTime<Utc>,
        ) -> Option<()> {
            match self.ip_bans.entry(ip) {
                hash_map::Entry::Vacant(v) => {
                    // If this is an unban, it will have no effect, so return early.
                    if matches!(record.action, BanAction::Unban(_)) {
                        return None;
                    }
                    // Otherwise, this will at least potentially have an effect
                    // (assuming it succeeds).
                    v.insert(IpBanEntry {
                        current: record,
                        history: Vec::new(),
                        // This is a hint anyway, but expired will also be set to true
                        // before saving by the call `edit`
                        // makes to `validate` (through `try_into`), which will set it
                        // to true in the event that
                        // the ban time was so short
                        // that it expired during the interval
                        // between creating the action and saving it.
                        //
                        // TODO: Decide if we even care enough about this case to worry
                        // about the gap. Probably not, even
                        // though it does involve time!
                        expired: false,
                    });
                },
                hash_map::Entry::Occupied(mut o) => {
                    let entry = o.get_mut();
                    // If overwrite is off, check that this entry (if successful) would
                    // actually change the ban status.
                    if !overwrite && entry.current.is_expired(now) == record.is_expired(now) {
                        return None;
                    }
                    // Push the current (most recent) entry to the back of the history
                    // list.
                    entry.history.push(mem::replace(&mut entry.current, record));
                },
            }
            Some(())
        }

        /// One-off migration from the previous version.  This must be
        /// guaranteed to produce a valid settings file as long as it is
        /// called with a valid settings file from the previous version.
        pub(super) fn migrate(prev: prev::Banlist) -> Self {
            let prev::Banlist(uuid_map) = prev;
            // Mostly the same structure but we introduce a new type of ban in a separate
            // map.
            Banlist {
                uuid_bans: uuid_map
                    .into_iter()
                    .map(|(uuid, entry)| (uuid, BanEntry::migrate(entry)))
                    .collect(),
                // Previous version had no concept of IP bans
                ip_bans: HashMap::new(),
            }
        }

        /// Perform any needed validation on this banlist that can't be done
        /// using parsing.
        ///
        /// The returned version being "Old" indicates the loaded setting has
        /// been modified during validation (this is why validate takes
        /// `&mut self`).
        pub(super) fn validate(&mut self) -> Result<Version, <Final as EditableSetting>::Error> {
            let mut version = Version::Latest;
            let now = Utc::now();
            let Self { uuid_bans, ip_bans } = self;
            for (&uuid, value) in uuid_bans.iter_mut() {
                if matches!(value.validate(now, uuid)?, Version::Old) {
                    // Update detected.
                    version = Version::Old;
                }
            }

            let mut uuids = hashbrown::HashSet::new();
            for (&_ip_addr, value) in ip_bans.iter_mut() {
                // Validate that there are not multiple active IP bans
                // linked to the same UUID. (since if timing happens to match
                // the per entry validation won't catch this)
                //
                // collapsible_if: more clear not to have side effects in the if condition
                #[allow(clippy::collapsible_if)]
                if let Some(uuid) = value.current.uuid_when_performed
                    && !value.current.is_expired(now)
                {
                    if !uuids.insert(uuid) {
                        return Err(BanError::Ip {
                            kind: BanErrorKind::ActiveIpBansShareUuid,
                            uuid: Some(uuid),
                            username_from_uuid_entry: uuid_bans
                                .get(&uuid)
                                .map(|e| e.current.username_when_performed.clone()),
                        });
                    }
                }
                if matches!(value.validate(now, uuid_bans)?, Version::Old) {
                    // Update detected.
                    version = Version::Old;
                }
            }
            Ok(version)
        }
    }

    // NOTE: Whenever there is a version upgrade, copy this note as well as the
    // commented-out code below to the next version, then uncomment the code
    // for this version.
    /* impl TryFrom<Banlist> for Final {
        type Error = <Final as EditableSetting>::Error;

        #[allow(clippy::useless_conversion)]
        fn try_from(mut value: Banlist) -> Result<Final, Self::Error> {
            value.validate()?;
            Ok(next::Banlist::migrate(value).try_into().expect(MIGRATION_UPGRADE_GUARANTEE))
        }
    } */
}