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 -- returned by
/// `POST /attachments` for an upload, minted by a driver for one a tool
/// produced, and fetched back from `/sessions/{id}/files/{ref}`. Both
/// directions use the one id so the transcript renders them identically.
pub type ImageRef = String;

/// One choice offered in answer to a [`Event::Question`].
///
/// More than a label because the reader is deciding, not confirming: what
/// an option means, and what picking it would produce, are the things that
/// decide it. Both are optional -- a permission's Allow and Deny mean
/// exactly what they say.
#[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 {
    /// An option that is only its label, which is most of them.
    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 session's transcript with a sequence number, then fanned
/// out to SSE subscribers; the phone renders purely from this stream, 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 a single 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 were
// reported" case, which is a state it is allowed to be in. A wire mismatch
// that lands on a plausible state is invisible; anything added below with a
// two-word field would have hit the same thing.
#[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 full conversation from the
    /// one stream. Recorded when the session reads the message, which is
    /// what `MessageTaken` reports.
    UserMessage {
        /// The [`Event::MessageQueued`] this resolves, when it waited.
        ///
        /// A message sent between turns is read at once and never queued,
        /// so this is `None` for most of them. It is the pair to the id on
        /// `MessageQueued` and exists for the same reason `CommandSent`
        /// carries one: 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 to it, by the ref the files route serves.
        ///
        /// On the message rather than beside it. These used to be their own
        /// `Image` events emitted just before, which drew a person's
        /// screenshot as a row of its own floating above the bubble that
        /// sent it -- and left the phone to decide, from nothing but
        /// adjacency, which message an image belonged to. Belonging is not
        /// something to infer when the sender knew.
        #[serde(default, skip_serializing_if = "Vec::is_empty")]
        images: Vec<ImageRef>,
    },
    /// 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 -- see `MessageTaken` -- but something has to say it is
    /// waiting, and it has to be the server that says it: the phone used
    /// to remember its own outgoing messages, so leaving the session
    /// screen or restarting the app showed nothing pending when something
    /// was, which reads as "nothing queued" rather than "I have forgotten".
    ///
    /// Carries no row of its own. It is resolved by the `UserMessage`
    /// bearing the same id, exactly 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, skip_serializing_if = "Vec::is_empty")]
        images: Vec<ImageRef>,
    },
    /// A message taken out of the queue before the session read it, by
    /// somebody tapping the bubble that was waiting for 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 for a message that will
    /// never arrive -- and nothing later would ever resolve it, since the
    /// `UserMessage` that normally does is exactly what is not coming.
    ///
    /// Only ever sent for a message that had not been handed over. One
    /// that has is not droppable and says so instead; 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 for that turn to finish, and
    /// until then the session has not seen it -- so recording it among
    /// things already read puts it in the transcript above output that
    /// predates it, and leaves a phone drawing it as still waiting with
    /// nothing coming to say otherwise.
    MessageTaken {
        /// The `MessageQueued` this answers, or `None` when it never
        /// waited. Carried through onto the `UserMessage`.
        id: Option<String>,
        text: String,
        /// Carried through onto the `UserMessage` with everything else.
        #[serde(default, skip_serializing_if = "Vec::is_empty")]
        images: Vec<ImageRef>,
    },
    /// 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. `None` for
        /// an image a person attached to their own message, which belongs
        /// to no call.
        #[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 -- a tag beside the question rather than part of
        /// it. `None` for a permission, which is about the call above it.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        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 the app parsing Claude
        /// Code's tool input to find out -- one dialect's 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.
        ///
        /// The CLI's `can_use_tool` request carries the `tool_use_id` of
        /// the call it is asking about, 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 that is not about a tool --
        /// AskUserQuestion, and an echo session's question.
        #[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 that starts working on something nobody
    /// on this phone 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.
        ///
        /// It exists because the live Claude Code path cannot record the
        /// message where it belongs. The CLI says nothing about a peer
        /// message until the turn's `result` -- see
        /// `claude::translate` -- 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 order on screen without a second
        /// source for either.
        ///
        /// Filled in by the pump, which is the only place that knows a
        /// seq, and only where a turn was open: `None` for a message read
        /// out of a session file by `import`, which already has it in the
        /// right place, and for one that started no turn.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        turn_start: Option<u64>,
    },
    /// The manager's record of a question being answered, so a rendered
    /// question card resolves on every device, not just the one that
    /// answered it.
    ///
    /// 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.
    /// What a dialect makes of that -- Claude Code's answers map holds a
    /// string, so several become one line -- is that dialect's business
    /// and is done where it talks to it.
    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 one 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 instead put the answer on the phone before the
    /// question had been answered, and left it there when the answer was
    /// no.
    ///
    /// Either field alone, because the two are confirmed separately and
    /// by different things -- the CLI echoes a mode change, and names the
    /// model it resolved an alias to when a session starts.
    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`] for what goes into it.
        ///
        /// Carried on the event rather than summed by whoever is reading,
        /// because it is not a sum: a conversation's 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. It is also
        /// the number a reader is asking about -- how much room is left
        /// before the next compaction -- rather than what has been spent
        /// getting here.
        ///
        /// `None` where the dialect did not say, which every reader has to
        /// be able to draw: a turn whose usage the CLI omitted leaves the
        /// context unmeasured rather than unchanged, and entries written
        /// before this existed have no answer at all.
        #[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: what a reader wants
    /// afterwards is that the session went from a million tokens to ten
    /// thousand, which is measured rather than estimated. 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 -- filling in a plausible number would make
    /// it indistinguishable from one that was counted.
    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, and defaulting the unknown
        /// case to "you asked for this" would explain it away.
        #[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
    /// to the session about itself, and a session in the middle of a turn
    /// reads a line written to it as something the model should see. So
    /// they wait for the turn to end, and this is what a phone draws
    /// while they do -- otherwise pressing Compact during a long turn
    /// does nothing visible for minutes and looks like it was missed.
    CommandQueued {
        id: String,
        /// What to show for it: the command as a person would type it.
        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, and a session that dropped its history from the
    /// screen as well as from the model would lose the only copy the
    /// phone has -- so this is a divider, not a truncation, and the
    /// events before it stay exactly where they were.
    ///
    /// It is also what makes clearing mean the same thing for every
    /// driver, which is why the marker lives here rather than in one
    /// dialect: `llama` folds its conversation out of the transcript and
    /// simply folds from the last one of these, and `claude` starts a new
    /// CLI conversation behind it.
    ///
    /// **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 the reason to write this
    /// down rather than leave it to be inferred from a second example
    /// that does not exist yet.
    Cleared,
    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, so all three count;
/// output is left out because it is what the turn produced rather than
/// what continuing from here has to carry.
///
/// One function so the definition cannot drift, because it is extracted in
/// 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, since those files reach tens of megabytes.
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. Written here beside the events it reads so a fourth reader
/// finds it.
///
/// 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 is a state each of them can reach:
/// nothing has been measured yet, a compaction finished without saying
/// how much it recovered, or a clear left a conversation nobody has
/// counted since.
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: it is 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 --
/// `/context`, `/usage` -- 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,
    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. It resolves itself -- the driver keeps asking -- so what
    /// it means to a reader is "wait", not "act".
    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 that a steer reaches
/// the model at the next tool boundary rather than at the end of the turn
/// -- can never take one back, and a phone that was told only "no" would
/// have to guess whether it had 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. The message is on its way into the conversation.
    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, images: Vec<ImageRef>);
    /// 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, because they are different
    /// things to a reader -- "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: nothing of yours is waiting.
    fn unqueue(&self, _id: &str) -> Unqueued {
        Unqueued::Unknown
    }
    /// Answers one question with everything that was chosen, in the order
    /// it was offered. One answer is a list of one; 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 actually changes the setting owes an
    // [`Event::Settings`] once it has -- that event, and not the request,
    // is what the manager and the phone read. One that cannot change it
    // owes an [`Event::Error`] saying why; saying nothing leaves a phone
    // showing a setting nobody applied.

    /// 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 own config, which is
    /// what a phone lists and the only place the name has to be. So a
    /// driver whose process has no notion of a name does nothing here and
    /// says nothing -- there is no failure to report, and an error beside
    /// a rename that plainly worked would be a puzzle rather than a
    /// warning.
    ///
    /// Claude Code has one: `--name` when a session is created 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.
    ///
    /// For the ones this app has no opinion about -- `/context`, `/usage`,
    /// anything a CLI adds next month. A driver whose process has 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 instead.
    ///
    /// Like [`Driver::compact`] and [`Driver::set_title`], this is called
    /// only when the session is between turns; the waiting is done above,
    /// once, for every driver.
    fn run_command(&self, text: &str);
    /// pi: native compaction; claude: `/compact`.
    fn compact(&self);

    /// Drops the conversation so far without ending the session.
    ///
    /// The cheap half of managing a long session, and the reason 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 a single 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 when it comes
    /// back.
    ///
    /// This is deliberately not a shutdown. A backend restart -- a
    /// rebuild, a service restart, a crash -- 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`. A driver with no
    /// process of its own has nothing to do here.
    ///
    /// 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 sees every line it wrote and every line that came back, and it
    /// updates this the instant it writes rather than when output returns.
    /// The manager's `SessionStatus` cannot answer it -- that 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 [`detach`] preserves.
    ///
    /// Two callers, and the difference between them is only 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 (see [`SessionConfig::throwaway`]).
    ///
    /// [`detach`]: Driver::detach
    /// [`SessionConfig::throwaway`]: crate::config::SessionConfig::throwaway
    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, and the
        // one thing that is certainly wrong is the figure from before it.
        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)
        );
    }
}