ai-app/server/src/session/driver.rs

//! The common event model and the `Driver` trait -- the one abstraction
//! everything hangs off (see PLAN.md).
//!
//! A driver translates its child process's JSONL dialect into [`Event`]s
//! and accepts the small inbound vocabulary below. The transcript, the SSE
//! stream, and the phone UI work purely in this model; nothing downstream
//! of a driver may branch on the session kind.

use serde::{Deserialize, Serialize};
use tokio::sync::mpsc;

/// The name a session's image is stored and served under -- minted for an
/// upload or for one a tool produced, and fetched back from
/// `/sessions/{id}/files/{ref}`. One id both directions, so the transcript
/// renders them identically.
pub type ImageRef = String;

/// The name an upload is stored and served under: an image is
/// `<hex>.<extension>` and is an [`ImageRef`] like any other; any other file
/// keeps its own name after the hex, `<hex>-<name>`, because the name is what
/// the reader attached and what the session is told. Told apart by
/// `crate::media::media_type_for`.
pub type AttachmentRef = String;

/// One choice offered in answer to a [`Event::Question`]. More than a label
/// because the reader is deciding rather than confirming: what an option
/// means, and what picking it would produce, are what decide it.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct QuestionOption {
    pub label: String,
    /// A sentence about what this option means.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub description: Option<String>,
    /// A block to show as written -- a mockup, a diff, a config file.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub preview: Option<String>,
}

impl QuestionOption {
    pub fn plain(label: impl Into<String>) -> Self {
        Self {
            label: label.into(),
            description: None,
            preview: None,
        }
    }
}

/// Everything a session can tell the outside world. Every event is appended
/// to the transcript with a sequence number, then fanned out to SSE
/// subscribers, so reconnecting is just "events after seq N" -- no separate
/// history path to drift from the live one.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
// `rename_all` renames the variants; `rename_all_fields` renames what is
// inside them. Both are needed and only the first is obvious: every field
// here was one lowercase word until `pre_tokens` arrived, so a multi-word
// field went out as snake_case, the app looked for camelCase and found
// nothing, and the event still rendered -- as the "no counts reported" case,
// which is a state it is allowed to be in.
#[serde(
    tag = "type",
    rename_all = "camelCase",
    rename_all_fields = "camelCase"
)]
pub enum Event {
    /// What the user sent, written into the transcript by the manager (not by
    /// drivers) so every device renders the conversation from one stream.
    /// Recorded when the session reads it, which is what `MessageTaken` reports.
    UserMessage {
        /// The [`Event::MessageQueued`] this resolves, when it waited. The
        /// phone has a bubble on screen for the waiting message and needs to
        /// know *which* one this is, rather than matching on the text and
        /// clearing the wrong one when the same thing was sent twice.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        id: Option<String>,
        text: String,
        /// What was attached, by the ref the files route serves. On the
        /// message rather than beside it: these used to be their own `Image`
        /// events just before, which left the phone deciding from adjacency
        /// which message an image belonged to. `images` on disk until
        /// 2026-09-03, when files joined them; the alias reads the older rows.
        #[serde(default, alias = "images", skip_serializing_if = "Vec::is_empty")]
        attachments: Vec<AttachmentRef>,
    },
    /// A message accepted from the phone that the session cannot read yet.
    ///
    /// Recorded, unlike the message itself, and that difference is the point:
    /// the message belongs in the transcript where the session read it, but
    /// something has to say it is waiting, and it has to be the server. The
    /// phone used to remember its own outgoing messages, so leaving the
    /// screen showed nothing pending when something was.
    ///
    /// Carries no row of its own; resolved by the `UserMessage` bearing the
    /// same id, as `CommandQueued` is resolved by `CommandSent`.
    MessageQueued {
        id: String,
        text: String,
        /// Carried for the same reason [`Event::UserMessage`] carries it,
        /// and it matters more here: a waiting message is on screen for as
        /// long as the turn runs, so its attachment has nowhere else to be.
        #[serde(default, alias = "images", skip_serializing_if = "Vec::is_empty")]
        attachments: Vec<AttachmentRef>,
    },
    /// A message taken out of the queue before the session read it.
    ///
    /// Recorded for the same reason `MessageQueued` is: the queue is the
    /// server's, so what is waiting has to be answerable from the transcript
    /// alone. Without it a phone that reconnects replays the `MessageQueued`
    /// and puts back a bubble nothing will ever resolve -- the `UserMessage`
    /// that normally does is exactly what is not coming.
    ///
    /// Only ever sent for a message that had not been handed over; see
    /// [`Unqueued::AlreadySent`].
    MessageDropped {
        id: String,
    },
    /// A driver has taken one of the user's messages and started reading it.
    /// The manager turns this into the `UserMessage` above, so it never
    /// reaches a phone itself.
    ///
    /// It exists because sending and being read are not the same moment. A
    /// message sent into a running turn waits, and recording it among things
    /// already read puts it in the transcript above output that predates it.
    MessageTaken {
        /// The `MessageQueued` this answers, or `None` when it never waited.
        /// Carried through onto the `UserMessage`.
        id: Option<String>,
        text: String,
        #[serde(default, alias = "images", skip_serializing_if = "Vec::is_empty")]
        attachments: Vec<AttachmentRef>,
    },
    /// Streaming assistant text; the phone renders the concatenation as
    /// markdown.
    AssistantText {
        delta: String,
    },
    ToolStart {
        id: String,
        tool: String,
        input: serde_json::Value,
    },
    ToolUpdate {
        id: String,
        output: String,
    },
    ToolEnd {
        id: String,
        output: String,
    },
    /// An image the session produced or was sent, saved under the session
    /// dir and referenced by id; the phone fetches it by URL.
    Image {
        #[serde(rename = "ref")]
        image: ImageRef,
        /// The tool call whose result carried it, when one did. A screenshot
        /// belongs under the call that took it, not floating beside it -- the
        /// reader has to pair them by position otherwise, and position is
        /// exactly what a page boundary breaks.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        about: Option<String>,
    },
    /// Anything the session needs a human for: AskUserQuestion, and
    /// permission requests, are the same shape with different options.
    Question {
        id: String,
        prompt: String,
        /// A few words naming what the question is about, when the asker
        /// offered one. `None` for a permission, which is about the call
        /// above it.
        header: Option<String>,
        options: Vec<QuestionOption>,
        /// Whether several options may be chosen at once. Here rather than
        /// left for a phone to work out from the dialect underneath: how many
        /// answers a question takes is a fact about the question, and the
        /// alternative was Claude Code's tool-input schema written out a
        /// second time in Kotlin, where no other dialect could reach it.
        #[serde(default, skip_serializing_if = "std::ops::Not::not")]
        multi_select: bool,
        /// The tool call this is permission for, when it is one, so a phone
        /// can draw the ask on the tool's own row rather than as a second
        /// card repeating its input. `None` for anything not about a tool.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        about: Option<String>,
    },
    /// A message another agent sent this session.
    ///
    /// Its own kind rather than a `UserMessage`, because it is not something
    /// the reader said and a transcript that renders it in their voice is
    /// claiming they did. It also explains what would otherwise be
    /// inexplicable: a session working on something nobody here asked for.
    PeerMessage {
        /// The sending session's own name, which is what the reader
        /// recognises it by -- the socket path it came from is not.
        from: String,
        text: String,
        /// The seq of the `Status::Running` that opened the turn this message
        /// started, so a reader can draw it above that turn.
        ///
        /// The CLI says nothing about a peer message until the turn's
        /// `result`, so the event is appended after everything it caused, and
        /// an append-only transcript cannot go back and insert it. Carrying
        /// the position instead keeps one order on the wire and one on screen.
        ///
        /// Filled in by the pump, the only place that knows a seq, and only
        /// where a turn was open: `None` for a message replayed by `import`,
        /// which already has it in the right place.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        turn_start: Option<u64>,
    },
    /// **Retired on 2026-09-06, hours after it was added.** Kept only so the
    /// transcripts written while it existed still read: a session that ran a
    /// background task in that window has these lines for ever, and a
    /// transcript is append-only, so there is no pass that could remove them.
    ///
    /// Never constructed. It reported a background task finishing, and drawing
    /// a row per one turned out to be a screen of notices about work the
    /// reader was not asking after -- see PLAN.md's "Two turns must never be
    /// drawn as one". The phone folds it to no row at all, which is what makes
    /// keeping it cheap.
    ///
    /// Deleting the variant instead is what broke every live session, and
    /// [`Event::Unreadable`] is the reason that cannot happen again. This is
    /// still here rather than left to that: an unreadable line draws a
    /// placeholder, correctly, and one per background task is the same wall
    /// the row was removed for.
    TaskNote {
        about: String,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        title: Option<String>,
        status: String,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        summary: Option<String>,
    },
    /// A line in a transcript that this build cannot read: a kind a newer
    /// server wrote, a kind an older one wrote that has since been dropped, or
    /// a line whose contents do not fit the kind it names.
    ///
    /// **Only ever made when reading, never sent by a driver**, and it is the
    /// reason a transcript can outlive a change to this enum. See
    /// `Indexed::parse_at` for the incident: removing a variant after
    /// transcripts had recorded it made every read of those files fail, so
    /// every session in them lost its status, its history and its ability to
    /// be sent to.
    ///
    /// It carries the word the line called itself so a reader is told what
    /// they are missing rather than that something is missing. `kind` is not
    /// an enum for the obvious reason: the whole point of this variant is the
    /// words that are not in one.
    Unreadable {
        kind: String,
    },
    /// The manager's record of a question being answered, so a rendered
    /// question card resolves on every device rather than only the one that
    /// answered.
    ///
    /// A list because a question can take several answers, and one that took
    /// one is the list of length one rather than a different shape.
    Answered {
        id: String,
        answers: Vec<String>,
    },
    Status {
        state: SessionStatus,
    },
    /// What the session is set to, as the session itself reports it.
    ///
    /// Asking for a change and having one are different things, and only this
    /// is a measurement: a model name the dialect does not know, a mode it
    /// refuses, or a driver whose model is fixed at startup all leave a
    /// request that was sent and nothing that changed. Reporting from the
    /// request put the answer on the phone before the question was answered.
    ///
    /// Either field alone, because the two are confirmed separately.
    Settings {
        #[serde(default, skip_serializing_if = "Option::is_none")]
        model: Option<String>,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        permission_mode: Option<String>,
    },
    /// Per-turn token counts, where the dialect reports them.
    UsageDelta {
        /// What this turn cost: the tokens it was charged for.
        tokens: u64,
        /// What the model was holding when the turn ended -- see
        /// [`context_tokens`].
        ///
        /// Carried rather than summed by whoever is reading, because it is
        /// not a sum: context goes *down* at a compaction and a clear, so
        /// adding turns up would report a figure the session stopped being
        /// true of long ago.
        ///
        /// `None` where the dialect did not say, which every reader has to be
        /// able to draw.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        context: Option<u64>,
    },
    /// A compaction that finished, and how much context it recovered.
    ///
    /// The counts are the point, and a spinner is not. They are optional
    /// because the record has shipped without them, and "the compaction
    /// happened, we don't know by how much" is a state this has to be able to
    /// say -- a plausible number would be indistinguishable from a counted one.
    Compacted {
        #[serde(default, skip_serializing_if = "Option::is_none")]
        pre_tokens: Option<u64>,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        post_tokens: Option<u64>,
        /// What asked for it, in the dialect's own word -- `auto` when the
        /// session compacted on its own. Carried rather than reduced to a bool
        /// so an unrecognised trigger stays unrecognised: an automatic
        /// compaction is the one worth naming, because it explains a wait
        /// nobody asked for.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        trigger: Option<String>,
    },
    /// A command the session was asked to run on itself, held because it
    /// cannot run yet. These are not messages: `/compact` and `/rename` are
    /// instructions about the session, and a session mid-turn reads a line
    /// written to it as something the model should see. So they wait, and
    /// this is what a phone draws while they do.
    CommandQueued {
        id: String,
        text: String,
    },
    /// The same command, now handed to the session. Its [`CommandQueued`]
    /// stops being pending when this arrives, matched by `id`; a command
    /// that ran immediately has only this.
    CommandSent {
        id: String,
        text: String,
    },
    /// The conversation was cleared: everything above this is still in the
    /// record but is no longer in the session's context.
    ///
    /// Nothing is deleted. A transcript is the thing a person scrolls back
    /// through, so this is a divider, not a truncation.
    ///
    /// **Load-bearing, not decorative.** For any driver that rebuilds its
    /// conversation from the transcript, this marker decides what the model
    /// is given -- dropping it, or treating it as something only the phone
    /// draws, silently puts a cleared conversation back in front of the model
    /// at full cost. Today `llama::conversation` is the only fold that reads
    /// it, which is why this is written down rather than left to be inferred
    /// from a second example that does not exist.
    Cleared,
    /// The account behind this session has no quota left, so the turn stopped
    /// without finishing.
    ///
    /// Its own event rather than an [`Event::Error`] carrying the dialect's
    /// sentence, because two things act on it that cannot read English: the
    /// transcript draws it as a state the session is in rather than as a
    /// failure of something it did, and `crate::resume` schedules the message
    /// that picks the work back up. Recognising it belongs to the driver, which
    /// is the only layer that knows its dialect's wording -- above here nothing
    /// matches on strings.
    ///
    /// `resets_at` is epoch seconds, and `None` is a real state: the dialect
    /// said the limit was hit without saying when it lifts. Nothing here
    /// invents one -- what the wait is actually decided against is the usage
    /// endpoint, and this is the hint that starts the waiting.
    LimitReached {
        #[serde(default, skip_serializing_if = "Option::is_none")]
        resets_at: Option<f64>,
    },
    Error {
        message: String,
    },
}

/// How much the model was holding, from the three figures a turn reports:
/// the input side only, prompt plus both cache figures. A cached token is
/// cheaper but it is still one the model was given; output is what the turn
/// produced rather than what continuing has to carry.
///
/// One function so the definition cannot drift, because it is extracted two
/// quite different ways -- the live translators have the usage object parsed,
/// and `import::context_tokens` scans it out of a raw line without parsing.
pub fn context_tokens(input: u64, cache_creation: u64, cache_read: u64) -> u64 {
    input + cache_creation + cache_read
}

/// The context after `event`, given what it was before.
///
/// The whole rule in one place, because three readers need the same answer:
/// the pump keeping a live session's figure, the transcript seeding it at
/// startup, and the phone folding the same events into what it draws.
///
/// The two that *lower* it are the point. A clear takes the conversation away
/// and a compaction replaces it with a summary, so a figure measured before
/// either stopped being true at that moment -- and carrying it forward is how
/// a session that had just been cleared went on reporting the context it no
/// longer had.
///
/// `None` is "we don't know", which each of them can reach.
pub fn context_after(current: Option<u64>, event: &Event) -> Option<u64> {
    match event {
        // `or`, so a turn the dialect reported no usage for leaves the last
        // measurement standing: stale by a turn, which every context figure
        // is, rather than wrong.
        Event::UsageDelta { context, .. } => context.or(current),
        Event::Compacted { post_tokens, .. } => *post_tokens,
        Event::Cleared => None,
        _ => current,
    }
}

/// Something a session can be asked to do to itself.
///
/// A closed set rather than a string, because the two that are not
/// dialect-specific have to reach every provider: compaction is a capability
/// an llama session may one day have, and a name is this server's own. `Raw`
/// is the escape for a dialect's own commands, which only the thing running
/// the session can interpret.
#[derive(Debug, Clone, PartialEq)]
pub enum SessionCommand {
    Compact,
    Clear,
    SetTitle(String),
    Raw(String),
}

impl SessionCommand {
    /// What a person would have typed to ask for this, which is what a phone
    /// shows while it waits.
    pub fn label(&self) -> String {
        match self {
            Self::Compact => "/compact".to_string(),
            Self::Clear => "/clear".to_string(),
            Self::SetTitle(title) => format!("/rename {title}"),
            Self::Raw(text) => text.clone(),
        }
    }

    /// Runs it. Called only at a boundary -- see [`Event::CommandQueued`].
    pub fn apply(&self, driver: &dyn Driver) {
        match self {
            Self::Compact => driver.compact(),
            Self::Clear => driver.clear(),
            Self::SetTitle(title) => driver.set_title(title),
            Self::Raw(text) => driver.run_command(text),
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub enum SessionStatus {
    Idle,
    Running,
    AwaitingInput,
    Compacting,
    /// The session's own turn is over, but work it started is still going:
    /// a backgrounded subagent, or a command left running.
    ///
    /// Its own state rather than `Idle` because the two differ in kind and
    /// only one of them is an invitation. `Idle` means the session is
    /// waiting for a person; this means it is waiting for itself, and a
    /// notification saying the work had finished would have been wrong. It
    /// is also not `Running`: nothing is being written to the transcript,
    /// the reply that ended the turn is finished, and a spinner on a session
    /// that will not speak again until a task reports back is a promise
    /// nobody can keep.
    Waiting,
    Exited,
    /// There is a process recorded for this session and the machine will not
    /// say whether it is still running.
    ///
    /// Its own state rather than the nearest of the others, because both
    /// neighbours are lies with consequences: `Exited` invites starting a
    /// second process against a conversation that may already have one, and
    /// `Idle` claims a session is waiting for you when nobody has checked.
    Unknown,
}

/// What became of a request to take a queued message back.
///
/// Three states rather than a bool because the two failures are not the same
/// fact. A driver that writes into its session the moment a message arrives
/// -- which is what `ClaudeDriver` does, so a steer reaches the model at the
/// next tool boundary -- can never take one back, and a phone told only "no"
/// would have to guess whether it asked too late or asked about nothing.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Unqueued {
    /// Out of the queue; the session will never read it.
    Dropped,
    /// Already handed to the session, so there is nothing left to take back.
    AlreadySent,
    /// Nothing is waiting under that id.
    Unknown,
}

/// Where a driver reports events. Unbounded because producers are child
/// processes a slow phone must never be able to stall; the transcript file
/// is the backpressure-free buffer of record.
pub type EventSink = mpsc::UnboundedSender<Event>;

/// The inbound half of a session. Deliberately small; see PLAN.md for the
/// per-driver mapping of each method onto its dialect.
///
/// `send_user_message` during a run is the point of the whole app: both
/// real dialects queue it for injection at the next tool boundary rather
/// than the end of the turn.
pub trait Driver: Send + Sync {
    /// Takes a message, now or once the session is free for it.
    ///
    /// Every driver owes exactly one `MessageTaken` per message, at the moment
    /// it actually starts reading it: that event is what puts the message in
    /// the transcript, so a driver that never sends it drops the message from
    /// the conversation entirely.
    fn send_user_message(&self, text: String, attachments: Vec<AttachmentRef>);
    /// Takes back a message that is still waiting, named by the id its
    /// [`Event::MessageQueued`] carried.
    ///
    /// Answering is the whole of the contract: a driver that drops the message
    /// owes an [`Event::MessageDropped`], and one that cannot must say which
    /// of the two reasons it is -- "the session has already been told" is
    /// worth knowing, and "there is nothing under that id" means the bubble on
    /// screen is stale. The default is the honest answer for a driver with no
    /// queue at all.
    fn unqueue(&self, _id: &str) -> Unqueued {
        Unqueued::Unknown
    }
    /// Answers one question with everything that was chosen, in the order it
    /// was offered. A driver whose dialect takes a single value joins them
    /// where it writes it.
    fn answer_question(&self, id: &str, answers: &[String]);
    /// Stop mid-run; the session survives.
    fn interrupt(&self);
    fn set_model(&self, model: &str);
    /// How much the session asks about before acting. Live rather than
    /// spawn-only: the answer changes with what is being done, and a phone is
    /// the worst place to answer "may I run this?" forty times.
    fn set_permission_mode(&self, mode: &str);
    // Both of the above are requests, and neither reports the outcome by
    // returning. A driver that changes the setting owes an [`Event::Settings`]
    // once it has -- that event, not the request, is what the manager and the
    // phone read. One that cannot owes an [`Event::Error`] saying why.

    /// Tells the process what this conversation is called, when it has
    /// somewhere to put it.
    ///
    /// Unlike the two above, this is not a request that can fail: the rename
    /// has already happened in this server's config, which is what a phone
    /// lists. So a driver whose process has no notion of a name does nothing
    /// and says nothing. Claude Code has one: `--name` at creation and
    /// `/rename` afterwards, which is what puts the same name in its own
    /// session picker and in what other agents see.
    fn set_title(&self, title: &str);
    /// Runs a command this session's own dialect understands, verbatim --
    /// `/context`, `/usage`, anything a CLI adds next month. A driver with no
    /// such vocabulary says so with an [`Event::Error`] rather than sending it
    /// as a message, which would put a line meant for the session in front of
    /// the model.
    ///
    /// Called only when the session is between turns; the waiting is done
    /// above, once, for every driver.
    fn run_command(&self, text: &str);
    /// llama: not built, and refused; claude: `/compact`.
    fn compact(&self);

    /// Drops the conversation so far without ending the session.
    ///
    /// The cheap half of managing a long session, and why it is a driver
    /// operation rather than a manager one: compaction *reads* the whole
    /// conversation in order to summarise it, so on a large context it is
    /// itself one of the most expensive requests the session will make --
    /// measured at 1.7 million tokens for one automatic compaction on
    /// 2026-08-29. Clearing costs nothing, because nothing is sent.
    ///
    /// Every implementation emits [`Event::Cleared`] so the transcript carries
    /// the divider whatever the dialect did behind it.
    fn clear(&self);
    /// Stop attending to the process but leave it running, because this
    /// server is going away and means to adopt it again.
    ///
    /// Deliberately not a shutdown: a backend restart must not end a turn that
    /// is in flight, so a session's process outlives the server that started
    /// it and is found again through `session::process`.
    ///
    /// Its counterpart is [`Driver::stop`]. Every driver owes exactly one of
    /// the two on the way out, and which one is the difference between "back
    /// shortly" and "this conversation is over".
    /// Whether a line written *now* would start a turn of its own, rather than
    /// landing inside one already in flight.
    ///
    /// Asked of the driver because the driver is the only thing that knows: it
    /// updates this the instant it writes rather than when output returns. The
    /// manager's `SessionStatus` is built from what has been *recorded*, so
    /// between writing a line and the CLI's first output it still reads idle,
    /// and a second line sent in that gap lands inside the turn the first one
    /// started. For a command that is the difference between being executed
    /// and being read to the model as text, which is silent both ways.
    ///
    /// Defaults to true for a driver with no turn of its own to be inside.
    fn between_turns(&self) -> bool {
        true
    }

    fn detach(&self);
    /// End the process for good, because it must not survive this. The path
    /// out for everything [`Driver::detach`] preserves.
    ///
    /// Two callers, differing only in what is being ended: a session being
    /// deleted, whose conversation goes with it, and a throwaway session at a
    /// server's exit, whose transcript stays and whose process does not.
    fn stop(&self);
}

#[cfg(test)]
mod tests {
    use super::*;

    /// A tripwire for the wire format, not for serde. The app reads these
    /// names, and getting one wrong does not fail loudly: a field the app
    /// cannot find reads as a field the server chose not to send, which
    /// several of them are allowed to be.
    #[test]
    fn multi_word_fields_go_out_in_camel_case() {
        let json = serde_json::to_value(Event::Compacted {
            pre_tokens: Some(28719),
            post_tokens: Some(1125),
            trigger: Some("manual".to_string()),
        })
        .expect("serialize");
        assert_eq!(
            json,
            serde_json::json!({
                "type": "compacted",
                "preTokens": 28719,
                "postTokens": 1125,
                "trigger": "manual",
            })
        );
    }

    /// The two events that take the context *down* are the point of the fold:
    /// a figure measured before a compaction or a clear stopped being true at
    /// that moment, and carrying it forward is how a session that had just
    /// been cleared went on reporting the context it no longer had.
    #[test]
    fn a_compaction_and_a_clear_move_the_context_a_turn_cannot() {
        let after = |current, event| context_after(current, &event);

        assert_eq!(
            after(
                Some(500),
                Event::UsageDelta {
                    tokens: 12,
                    context: Some(30_100),
                }
            ),
            Some(30_100)
        );
        assert_eq!(
            after(
                Some(128_402),
                Event::Compacted {
                    pre_tokens: Some(128_402),
                    post_tokens: Some(9_617),
                    trigger: Some("auto".to_string()),
                }
            ),
            Some(9_617)
        );
        assert_eq!(after(Some(9_617), Event::Cleared), None);

        // A compaction that did not say how much it recovered leaves the
        // context unknown rather than stale: it definitely moved.
        assert_eq!(
            after(
                Some(128_402),
                Event::Compacted {
                    pre_tokens: None,
                    post_tokens: None,
                    trigger: None,
                }
            ),
            None
        );

        // A turn the dialect reported no context for is stale by a turn,
        // which every context figure is, rather than unknown.
        assert_eq!(
            after(
                Some(30_100),
                Event::UsageDelta {
                    tokens: 12,
                    context: None,
                }
            ),
            Some(30_100)
        );

        // Everything else leaves it alone.
        assert_eq!(
            after(
                Some(30_100),
                Event::Status {
                    state: SessionStatus::Idle,
                }
            ),
            Some(30_100)
        );
    }
}