synaps-core 0.9.0

//! Task 35 — opt-in session journal + periodic atomic snapshots (spec §9.8;
//! decision record: docs/decisions/T35-session-journal-opt-in.md).
//!
//! The DEFAULT persistence (`SessionPersistence::Json`) is byte-for-byte the
//! legacy path: every save atomically rewrites `sessions/<id>.json`. The
//! opt-in `Journal` mode is purely ADDITIVE: the snapshot file keeps the
//! unchanged legacy `Session` schema, and an append-only `sessions/<id>.journal`
//! (JSONL, schema v1) carries the deltas since the snapshot, so a steady-state
//! save costs O(delta) instead of O(total history).
//!
//! Replay is IDEMPOTENT by construction — `msg` records carry absolute
//! history indices (`i == len` appends, `i < len` skips, `i > len` stops at
//! the last consistent prefix) and `meta` records only apply when their
//! `updated_at` is not older than the loaded state. Torn tails (kill during
//! append), stale journals (kill between snapshot and journal reset), and
//! manual journal deletion therefore all recover to a consistent session.
//!
//! Everything here writes through the T4 `private_fs` helpers: 0700 dirs,
//! 0600 files, symlink-refusing, atomic snapshot replacement, synced appends.

use crate::core::session::Session;
use chrono::{DateTime, Utc};
use serde::{Deserialize, Serialize};
use std::io::Write;
use std::path::{Path, PathBuf};

/// Journal record schema version (line-level `"v"` field).
pub const JOURNAL_SCHEMA_VERSION: u8 = 1;

/// Journal size floor before a periodic snapshot is forced.
pub const JOURNAL_SNAPSHOT_MIN_BYTES: u64 = 256 * 1024;

/// A snapshot is due when the journal outgrows `snapshot / RATIO` (bounded
/// write amplification: large sessions stretch the threshold proportionally).
pub const JOURNAL_SNAPSHOT_RATIO: u64 = 4;

/// Bounded tail window scanned for the freshest `meta` record.
const META_TAIL_WINDOW: u64 = 64 * 1024;

/// Hard cap on any single persisted-session artifact read (snapshot or
/// journal). Reads are bounded on the OPENED handle; an artifact past the
/// cap is refused rather than slurped.
pub const MAX_PERSISTED_READ_BYTES: u64 = 1024 * 1024 * 1024; // 1 GiB

// ─── confined resolution (fix1 I1 + fix2) ───────────────────────────────────
//
// STRICT TRUSTED-ROOT SEMANTICS (fix2): the sessions directory path is
// resolved with EVERY component — ancestors AND the final one — opened
// handle-relatively from `/` with symlinks refused
// (`ConfinedDir::{open,create}_absolute_no_symlinks`; Linux uses one atomic
// `openat2 RESOLVE_BENEATH|RESOLVE_NO_SYMLINKS` from the root handle).
// Nothing on the path is trusted; there is no check-then-open race — a
// component swapped to a symlink at any moment fails the open itself.
// Artifacts inside the directory are then opened/written/removed relative
// to that ONE handle. Operators whose base dir legitimately sits behind
// ancestor symlinks (e.g. `/home` → `var/home`) must point
// `SYNAPS_BASE_DIR` at the canonical path.
//
// Non-unix keeps the crate's documented best-effort pathname fallback
// (final-component symlink refusal only).

#[cfg(unix)]
type SessionsDirHandle = crate::core::private_fs::ConfinedDir;

/// Open the sessions dir strictly. `Ok(None)` when a path component does
/// not exist; symlinks anywhere are errors.
#[cfg(unix)]
fn open_sessions_dir(dir: &Path) -> std::io::Result<Option<SessionsDirHandle>> {
    match crate::core::private_fs::ConfinedDir::open_absolute_no_symlinks(dir) {
        Ok(handle) => Ok(Some(handle)),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(None),
        Err(e) => Err(e),
    }
}

/// Create-or-open the sessions dir strictly (0700 leaf).
#[cfg(unix)]
fn create_sessions_dir(dir: &Path) -> std::io::Result<SessionsDirHandle> {
    crate::core::private_fs::ConfinedDir::create_absolute_no_symlinks(dir)
}

/// Open `<name>` inside an already-strictly-opened sessions dir.
/// `Ok(None)` when the file does not exist; a symlinked artifact errors.
#[cfg(unix)]
fn open_artifact(handle: &SessionsDirHandle, name: &str) -> std::io::Result<Option<std::fs::File>> {
    match handle.open_file(&[name.to_string()]) {
        Ok(file) => Ok(Some(file)),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(None),
        Err(e) => Err(e),
    }
}

/// One-shot strict open of `<name>` inside `dir`.
#[cfg(unix)]
pub(crate) fn confined_open(dir: &Path, name: &str) -> std::io::Result<Option<std::fs::File>> {
    let Some(handle) = open_sessions_dir(dir)? else {
        return Ok(None);
    };
    open_artifact(&handle, name)
}

// ── Non-unix best-effort fallbacks (documented, matching `private_fs`) ──

#[cfg(not(unix))]
struct SessionsDirHandle {
    dir: std::path::PathBuf,
}

#[cfg(not(unix))]
fn open_sessions_dir(dir: &Path) -> std::io::Result<Option<SessionsDirHandle>> {
    if !dir.exists() {
        return Ok(None);
    }
    Ok(Some(SessionsDirHandle {
        dir: dir.to_path_buf(),
    }))
}

#[cfg(not(unix))]
fn create_sessions_dir(dir: &Path) -> std::io::Result<SessionsDirHandle> {
    crate::core::private_fs::ensure_private_dir(dir)?;
    Ok(SessionsDirHandle {
        dir: dir.to_path_buf(),
    })
}

#[cfg(not(unix))]
impl SessionsDirHandle {
    fn write_atomic(&self, name: &str, data: &[u8]) -> std::io::Result<()> {
        crate::core::private_fs::write_atomic_private(&self.dir.join(name), data)
            .map_err(std::io::Error::other)
    }
    fn append_file(&self, name: &str) -> std::io::Result<std::fs::File> {
        crate::core::private_fs::open_private_append(&self.dir.join(name))
            .map_err(std::io::Error::other)
    }
    fn remove_file(&self, name: &str) -> std::io::Result<()> {
        std::fs::remove_file(self.dir.join(name))
    }
}

#[cfg(not(unix))]
fn open_artifact(handle: &SessionsDirHandle, name: &str) -> std::io::Result<Option<std::fs::File>> {
    let path = handle.dir.join(name);
    match std::fs::symlink_metadata(&path) {
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => return Ok(None),
        Ok(meta) if meta.file_type().is_symlink() => {
            return Err(std::io::Error::other(format!(
                "refusing symlinked session artifact {name:?}"
            )))
        }
        _ => {}
    }
    match std::fs::File::open(&path) {
        Ok(f) => Ok(Some(f)),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(None),
        Err(e) => Err(e),
    }
}

#[cfg(not(unix))]
pub(crate) fn confined_open(dir: &Path, name: &str) -> std::io::Result<Option<std::fs::File>> {
    let Some(handle) = open_sessions_dir(dir)? else {
        return Ok(None);
    };
    open_artifact(&handle, name)
}

/// Bounded read of a whole artifact from an already-confined handle.
fn read_artifact_bytes(handle: &SessionsDirHandle, name: &str) -> std::io::Result<Option<Vec<u8>>> {
    use std::io::Read;
    let Some(file) = open_artifact(handle, name)? else {
        return Ok(None);
    };
    let mut buf = Vec::new();
    file.take(MAX_PERSISTED_READ_BYTES + 1)
        .read_to_end(&mut buf)?;
    if buf.len() as u64 > MAX_PERSISTED_READ_BYTES {
        return Err(std::io::Error::other(format!(
            "session artifact {name:?} exceeds the {MAX_PERSISTED_READ_BYTES}-byte read bound"
        )));
    }
    Ok(Some(buf))
}

/// One strictly-resolved directory entry for session listings.
#[derive(Debug, Clone)]
pub struct SessionDirEntry {
    pub name: String,
    pub mtime: Option<std::time::SystemTime>,
    /// Stored bytes for metadata-only accounting. Obtained from the directory
    /// entry stat; reading it never opens session content.
    pub byte_len: u64,
}

/// Enumerate the sessions dir through the SAME strict handle resolution as
/// every other T35 operation (fix2): handle-relative `readdir` +
/// `fstatat(AT_SYMLINK_NOFOLLOW)` — a symlinked ancestor refuses, a
/// missing dir lists empty, and entry mtimes are of the entries
/// themselves, never symlink targets.
pub fn session_dir_entries(dir: &Path) -> std::io::Result<Vec<SessionDirEntry>> {
    #[cfg(unix)]
    {
        let Some(handle) = open_sessions_dir(dir)? else {
            return Ok(Vec::new());
        };
        Ok(handle
            .entries()?
            .into_iter()
            .filter(|e| e.is_file)
            .map(|e| SessionDirEntry {
                name: e.name,
                mtime: e.mtime_unix_ms.map(|ms| {
                    std::time::SystemTime::UNIX_EPOCH + std::time::Duration::from_millis(ms)
                }),
                byte_len: e.byte_len,
            })
            .collect())
    }
    #[cfg(not(unix))]
    {
        if !dir.exists() {
            return Ok(Vec::new());
        }
        let mut out = Vec::new();
        for entry in std::fs::read_dir(dir)? {
            let entry = entry?;
            if !entry.path().is_file() {
                continue;
            }
            let Ok(name) = entry.file_name().into_string() else {
                continue;
            };
            let byte_len = entry
                .metadata()
                .map(|metadata| metadata.len())
                .unwrap_or_default();
            out.push(SessionDirEntry {
                name,
                mtime: entry.metadata().and_then(|m| m.modified()).ok(),
                byte_len,
            });
        }
        Ok(out)
    }
}

/// Which on-disk persistence strategy `Session::save` uses.
///
/// `Json` (the default) is the unchanged legacy behavior. `Journal` is the
/// spec §9.8 opt-in and is only ever selected explicitly via the
/// `session_persistence = journal` config key.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum SessionPersistence {
    #[default]
    Json,
    Journal,
}

impl SessionPersistence {
    /// Parse a config value; anything unrecognized yields `None` so the
    /// caller keeps the safe default and surfaces a typed warning.
    pub fn parse(val: &str) -> Option<Self> {
        match val.trim() {
            "json" => Some(Self::Json),
            "journal" => Some(Self::Journal),
            _ => None,
        }
    }
}

/// How a save landed on disk.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SaveMode {
    /// Full legacy-schema snapshot written (and, in journal mode, the
    /// journal reset to a lone `open` record).
    FullSnapshot,
    /// Delta append: `messages` new history entries plus one meta record.
    Append { messages: usize },
}

/// Machine-readable receipt for benchmarks and delta-proportionality tests.
#[derive(Debug, Clone, Copy)]
pub struct SaveReceipt {
    pub mode: SaveMode,
    pub bytes_written: u64,
}

/// Freshest journaled metadata, read from a bounded tail window — lets
/// session listings stay accurate without parsing the full journal.
#[derive(Debug, Clone)]
pub struct JournalMetaTail {
    pub updated_at: DateTime<Utc>,
    pub session_cost: f64,
}

/// `sessions/<id>.journal` — single-extension name so the session id stays
/// the file stem (chain-head protection and retention pair on the stem).
pub fn journal_path(dir: &Path, id: &str) -> PathBuf {
    dir.join(format!("{id}.journal"))
}

/// Snapshot threshold: journal bytes ≥ max(256 KiB, snapshot / 4).
pub fn snapshot_due(journal_bytes: u64, snapshot_bytes: u64) -> bool {
    journal_bytes >= JOURNAL_SNAPSHOT_MIN_BYTES.max(snapshot_bytes / JOURNAL_SNAPSHOT_RATIO)
}

// ─── journal records ─────────────────────────────────────────────────────────

#[derive(Serialize, Deserialize)]
#[serde(tag = "k")]
enum JournalRecord {
    /// First line of every journal: the snapshot held `base` messages.
    #[serde(rename = "open")]
    Open { v: u8, base: usize },
    /// Message at ABSOLUTE history index `i`.
    #[serde(rename = "msg")]
    Msg {
        v: u8,
        i: usize,
        m: serde_json::Value,
    },
    /// Full session metadata (the `Session` object minus `api_messages`).
    #[serde(rename = "meta")]
    Meta { v: u8, meta: Box<SessionMeta> },
}

/// Mirror of [`Session`] WITHOUT `api_messages`, with identical serde
/// attributes. `deny_unknown_fields` + the `session_meta_stays_in_sync_…`
/// test make silent drift between the two types impossible.
#[derive(Clone, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
struct SessionMeta {
    id: String,
    title: String,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    name: Option<String>,
    model: String,
    thinking_level: String,
    system_prompt: Option<String>,
    created_at: DateTime<Utc>,
    updated_at: DateTime<Utc>,
    total_input_tokens: u64,
    total_output_tokens: u64,
    session_cost: f64,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    abort_context: Option<String>,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    parent_session: Option<String>,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    compacted_into: Option<String>,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    prompt_provenance: Option<crate::prompt::PromptProvenance>,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    compaction: Option<crate::core::compaction::CompactionRecord>,
}

impl SessionMeta {
    fn of(s: &Session) -> Self {
        Self {
            id: s.id.clone(),
            title: s.title.clone(),
            name: s.name.clone(),
            model: s.model.clone(),
            thinking_level: s.thinking_level.clone(),
            system_prompt: s.system_prompt.clone(),
            created_at: s.created_at,
            updated_at: s.updated_at,
            total_input_tokens: s.total_input_tokens,
            total_output_tokens: s.total_output_tokens,
            session_cost: s.session_cost,
            abort_context: s.abort_context.clone(),
            parent_session: s.parent_session.clone(),
            compacted_into: s.compacted_into.clone(),
            prompt_provenance: s.prompt_provenance.clone(),
            compaction: s.compaction.clone(),
        }
    }

    /// Apply onto a loaded session. Only metadata moves — never history.
    fn apply(self, s: &mut Session) {
        s.title = self.title;
        s.name = self.name;
        s.model = self.model;
        s.thinking_level = self.thinking_level;
        s.system_prompt = self.system_prompt;
        s.created_at = self.created_at;
        s.updated_at = self.updated_at;
        s.total_input_tokens = self.total_input_tokens;
        s.total_output_tokens = self.total_output_tokens;
        s.session_cost = self.session_cost;
        s.abort_context = self.abort_context;
        s.parent_session = self.parent_session;
        s.compacted_into = self.compacted_into;
        s.prompt_provenance = self.prompt_provenance;
        s.compaction = self.compaction;
    }
}

/// Parsed view of an on-disk journal: the durable message count implied by
/// the contiguous valid prefix, the last durable message (tripwire), and
/// whether every line parsed cleanly (a torn tail forces a resnapshot so
/// later appends are never shadowed behind an unparseable line).
struct JournalState {
    durable_len: usize,
    last_msg: Option<(usize, serde_json::Value)>,
    bytes: u64,
    clean: bool,
}

fn read_journal_state(
    handle: &SessionsDirHandle,
    id: &str,
) -> std::io::Result<Option<JournalState>> {
    let Some(raw) = read_artifact_bytes(handle, &format!("{id}.journal"))? else {
        return Ok(None);
    };
    let bytes = raw.len() as u64;
    let text = String::from_utf8_lossy(&raw);
    let mut lines = text.lines();

    let Some(first) = lines.next() else {
        return Ok(Some(JournalState {
            durable_len: 0,
            last_msg: None,
            bytes,
            clean: false,
        }));
    };
    let base = match serde_json::from_str::<JournalRecord>(first) {
        // fix1 M1: only the CURRENT schema version is interpretable. An
        // unsupported open version invalidates the whole journal — its
        // record semantics are unknown, so nothing may replay or be
        // appended behind it.
        Ok(JournalRecord::Open { v, base }) if v == JOURNAL_SCHEMA_VERSION => base,
        _ => {
            // No/invalid/unsupported open record: the journal is unusable
            // as an append target — resnapshot.
            return Ok(Some(JournalState {
                durable_len: 0,
                last_msg: None,
                bytes,
                clean: false,
            }));
        }
    };

    let mut durable_len = base;
    let mut last_msg = None;
    let mut clean = true;
    for line in lines {
        match serde_json::from_str::<JournalRecord>(line) {
            // fix1 M1: a record from an unknown schema version ends the
            // valid prefix — the next save must resnapshot.
            Ok(JournalRecord::Msg { v, .. }) | Ok(JournalRecord::Meta { v, .. })
                if v != JOURNAL_SCHEMA_VERSION =>
            {
                clean = false;
                break;
            }
            Ok(JournalRecord::Msg { i, m, .. }) => {
                if i == durable_len {
                    durable_len += 1;
                    last_msg = Some((i, m));
                } else if i > durable_len {
                    clean = false; // gap — inconsistent suffix
                    break;
                }
                // i < durable_len: stale duplicate, ignore.
            }
            Ok(JournalRecord::Meta { .. }) => {}
            Ok(JournalRecord::Open { .. }) | Err(_) => {
                clean = false; // torn tail or nested open
                break;
            }
        }
    }
    Ok(Some(JournalState {
        durable_len,
        last_msg,
        bytes,
        clean,
    }))
}

// ─── save ────────────────────────────────────────────────────────────────────

/// Persist `session` into `dir` under the given persistence mode.
///
/// `Json` (default): the unchanged legacy path — one atomic private
/// `<id>.json` — plus rollback folding: any journal left over from a
/// previous opt-in is deleted, because the fresh snapshot supersedes it.
///
/// `Journal`: append the delta since the last durable state; write a fresh
/// snapshot instead when the journal is missing/unusable, the history
/// shrank or its durable tail was edited (append-only tripwires), or the
/// journal outgrew [`snapshot_due`].
pub fn save_session_in_dir(
    dir: &Path,
    session: &Session,
    mode: SessionPersistence,
) -> std::io::Result<SaveReceipt> {
    // ONE strict resolution per save (fix2); every artifact operation below
    // is relative to this handle.
    let handle = create_sessions_dir(dir)?;
    match mode {
        SessionPersistence::Json => {
            let json = serde_json::to_string(session).map_err(std::io::Error::other)?;
            handle.write_atomic(&format!("{}.json", session.id), json.as_bytes())?;
            // Rollback fold: the snapshot now holds everything; a stale
            // journal must not shadow future legacy-only readers.
            remove_artifact_if_exists(&handle, &format!("{}.journal", session.id))?;
            Ok(SaveReceipt {
                mode: SaveMode::FullSnapshot,
                bytes_written: json.len() as u64,
            })
        }
        SessionPersistence::Journal => save_journal_mode(&handle, session),
    }
}

fn save_journal_mode(
    handle: &SessionsDirHandle,
    session: &Session,
) -> std::io::Result<SaveReceipt> {
    let journal_name = format!("{}.journal", session.id);

    let snapshot = open_artifact(handle, &format!("{}.json", session.id))?;
    let state = match read_journal_state(handle, &session.id)? {
        Some(state) if snapshot.is_some() => state,
        // First journal-mode save of a new or legacy session (or the
        // snapshot vanished out-of-band): full snapshot + fresh journal.
        _ => return full_snapshot_reset(handle, session),
    };

    // Append-only tripwires — anything the journal cannot express safely
    // becomes a fresh atomic snapshot instead.
    let rewrite_needed = !state.clean
        || session.api_messages.len() < state.durable_len
        || state.last_msg.as_ref().is_some_and(|(i, m)| {
            session
                .api_messages
                .get(*i)
                .map_or(true, |live| live.as_ref() != m)
        });
    if rewrite_needed {
        return full_snapshot_reset(handle, session);
    }

    // Delta append: new messages (absolute indices) + one meta record.
    let mut buf = Vec::new();
    for (offset, msg) in session.api_messages[state.durable_len..].iter().enumerate() {
        let record = JournalRecord::Msg {
            v: JOURNAL_SCHEMA_VERSION,
            i: state.durable_len + offset,
            m: msg.as_ref().clone(),
        };
        serde_json::to_writer(&mut buf, &record).map_err(std::io::Error::other)?;
        buf.push(b'\n');
    }
    let meta = JournalRecord::Meta {
        v: JOURNAL_SCHEMA_VERSION,
        meta: Box::new(SessionMeta::of(session)),
    };
    serde_json::to_writer(&mut buf, &meta).map_err(std::io::Error::other)?;
    buf.push(b'\n');

    let appended = session.api_messages.len() - state.durable_len;
    let mut file = handle.append_file(&journal_name)?;
    file.write_all(&buf)?;
    file.sync_data()?;
    drop(file);

    // Periodic snapshot: fold an oversized journal back into the atomic
    // snapshot. Crash between the two steps leaves a stale journal whose
    // records replay idempotently (see module docs).
    let journal_bytes = state.bytes + buf.len() as u64;
    let snapshot_bytes = snapshot
        .and_then(|f| f.metadata().ok())
        .map(|m| m.len())
        .unwrap_or(0);
    if snapshot_due(journal_bytes, snapshot_bytes) {
        let reset = full_snapshot_reset(handle, session)?;
        return Ok(SaveReceipt {
            mode: SaveMode::FullSnapshot,
            bytes_written: buf.len() as u64 + reset.bytes_written,
        });
    }

    Ok(SaveReceipt {
        mode: SaveMode::Append { messages: appended },
        bytes_written: buf.len() as u64,
    })
}

/// Atomic full snapshot (unchanged legacy schema) followed by an atomic
/// journal reset to a lone `open` record. Snapshot strictly first: a crash
/// between the two leaves a stale-but-idempotent journal, never data loss.
fn full_snapshot_reset(
    handle: &SessionsDirHandle,
    session: &Session,
) -> std::io::Result<SaveReceipt> {
    let json = serde_json::to_string(session).map_err(std::io::Error::other)?;
    handle.write_atomic(&format!("{}.json", session.id), json.as_bytes())?;

    let open = JournalRecord::Open {
        v: JOURNAL_SCHEMA_VERSION,
        base: session.api_messages.len(),
    };
    let mut line = serde_json::to_vec(&open).map_err(std::io::Error::other)?;
    line.push(b'\n');
    handle.write_atomic(&format!("{}.journal", session.id), &line)?;

    Ok(SaveReceipt {
        mode: SaveMode::FullSnapshot,
        bytes_written: json.len() as u64 + line.len() as u64,
    })
}

// ─── load ────────────────────────────────────────────────────────────────────

/// Load `<id>.json` and, when a journal exists, replay it idempotently.
/// Old-format sessions (no journal) load exactly as before; a session with
/// a torn or stale journal recovers to its last consistent state. Both
/// reads are confined nofollow bounded handle reads (fix1 I1).
pub fn load_session_in_dir(dir: &Path, id: &str) -> std::io::Result<Session> {
    // ONE strict resolution per load (fix2); both artifact reads below are
    // relative to this handle.
    let handle = open_sessions_dir(dir)?.ok_or_else(|| {
        std::io::Error::new(
            std::io::ErrorKind::NotFound,
            format!("no sessions directory for '{id}'"),
        )
    })?;
    let snapshot = read_artifact_bytes(&handle, &format!("{id}.json"))?.ok_or_else(|| {
        std::io::Error::new(
            std::io::ErrorKind::NotFound,
            format!("no session snapshot for '{id}'"),
        )
    })?;
    let mut session: Session = serde_json::from_slice(&snapshot).map_err(std::io::Error::other)?;

    let Some(raw) = read_artifact_bytes(&handle, &format!("{id}.journal"))? else {
        return Ok(session);
    };
    let text = String::from_utf8_lossy(&raw);
    let mut lines = text.lines();
    // The first line must be an open record OF THE SUPPORTED VERSION;
    // otherwise the whole journal is untrusted and the snapshot alone is
    // the consistent state (fix1 M1).
    match lines.next().map(serde_json::from_str::<JournalRecord>) {
        Some(Ok(JournalRecord::Open { v, .. })) if v == JOURNAL_SCHEMA_VERSION => {}
        _ => return Ok(session),
    }
    for line in lines {
        match serde_json::from_str::<JournalRecord>(line) {
            // fix1 M1: an unknown-version record ends the valid prefix.
            Ok(JournalRecord::Msg { v, .. }) | Ok(JournalRecord::Meta { v, .. })
                if v != JOURNAL_SCHEMA_VERSION =>
            {
                break;
            }
            Ok(JournalRecord::Msg { i, m, .. }) => {
                if i == session.api_messages.len() {
                    session.api_messages.push(std::sync::Arc::new(m));
                } else if i > session.api_messages.len() {
                    break; // gap — stop at the consistent prefix
                }
                // i < len: already in the snapshot — idempotent skip.
            }
            Ok(JournalRecord::Meta { meta, .. }) => {
                if meta.id == session.id && meta.updated_at >= session.updated_at {
                    meta.apply(&mut session);
                }
            }
            Ok(JournalRecord::Open { .. }) | Err(_) => break, // torn tail
        }
    }
    Ok(session)
}

/// Freshest `meta` record from a bounded journal tail window, for listing
/// freshness without a full journal read. `None` when no journal, no
/// complete supported-version meta record, or a refused (non-confined)
/// artifact — a symlinked journal discloses nothing.
pub fn journal_meta_tail(dir: &Path, id: &str) -> Option<JournalMetaTail> {
    use std::io::{Read, Seek, SeekFrom};
    let mut file = confined_open(dir, &format!("{id}.journal")).ok()??;
    let len = file.metadata().ok()?.len();
    let start = len.saturating_sub(META_TAIL_WINDOW);
    file.seek(SeekFrom::Start(start)).ok()?;
    let mut tail = String::new();
    file.take(META_TAIL_WINDOW).read_to_string(&mut tail).ok()?;

    let mut freshest = None;
    for line in tail.lines() {
        if let Ok(JournalRecord::Meta { v, meta }) = serde_json::from_str::<JournalRecord>(line) {
            if v == JOURNAL_SCHEMA_VERSION {
                freshest = Some(JournalMetaTail {
                    updated_at: meta.updated_at,
                    session_cost: meta.session_cost,
                });
            }
        }
    }
    freshest
}

// ─── deletion ────────────────────────────────────────────────────────────────

/// Remove a session's snapshot AND journal (compaction rollback, retention).
/// Idempotent — missing files are not errors.
pub fn delete_session_files_in_dir(dir: &Path, id: &str) -> std::io::Result<()> {
    let Some(handle) = open_sessions_dir(dir)? else {
        return Ok(()); // no directory — idempotently nothing to delete
    };
    remove_artifact_if_exists(&handle, &format!("{id}.json"))?;
    remove_artifact_if_exists(&handle, &format!("{id}.journal"))
}

fn remove_artifact_if_exists(handle: &SessionsDirHandle, name: &str) -> std::io::Result<()> {
    match handle.remove_file(name) {
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(()),
        other => other,
    }
}

/// Strict-handle snapshot write shared with the legacy
/// `session::save_json_in_dir` path (fix2): same root resolution and the
/// same handle-relative atomic private write as every journal operation.
#[cfg(test)]
pub(crate) fn write_json_snapshot(dir: &Path, id: &str, json: &[u8]) -> std::io::Result<()> {
    let handle = create_sessions_dir(dir)?;
    handle.write_atomic(&format!("{id}.json"), json)
}

// ─── tests ───────────────────────────────────────────────────────────────────

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

    /// Drift guard: `SessionMeta` must mirror every non-history `Session`
    /// field. Serializing a full `Session`, dropping `api_messages`, and
    /// parsing the rest as `SessionMeta` (deny_unknown_fields) fails the
    /// moment `Session` grows a field this module does not journal.
    #[test]
    fn session_meta_stays_in_sync_with_session_schema() {
        let mut s = Session::new("model-x", "medium", Some("prompt"));
        s.name = Some("named".into());
        s.abort_context = Some("ctx".into());
        s.parent_session = Some("parent".into());
        s.compacted_into = Some("child".into());
        let mut value = serde_json::to_value(&s).unwrap();
        value.as_object_mut().unwrap().remove("api_messages");
        let meta: SessionMeta = serde_json::from_value(value)
            .expect("Session grew a field SessionMeta does not mirror — extend SessionMeta");
        assert_eq!(meta.id, s.id);
        assert_eq!(meta.name.as_deref(), Some("named"));
    }

    #[test]
    fn meta_roundtrip_applies_every_field() {
        let mut a = Session::new("model-a", "high", Some("sys"));
        a.title = "t".into();
        a.total_input_tokens = 7;
        a.session_cost = 0.5;
        a.updated_at = chrono::Utc::now();
        let mut b = Session::new("model-b", "low", None);
        b.id = a.id.clone();
        SessionMeta::of(&a).apply(&mut b);
        assert_eq!(b.model, "model-a");
        assert_eq!(b.title, "t");
        assert_eq!(b.total_input_tokens, 7);
        assert_eq!(b.session_cost, 0.5);
        assert_eq!(b.updated_at, a.updated_at);
    }

    /// Private modes (spec §5.4): journal-mode saves keep the 0700 dir and
    /// 0600 files under a permissive umask, exactly like the legacy path.
    #[cfg(unix)]
    mod private_modes {
        use super::*;
        use crate::core::private_fs::test_support::UmaskGuard;
        use serial_test::serial;
        use std::os::unix::fs::PermissionsExt;

        fn mode_of(path: &Path) -> u32 {
            std::fs::metadata(path).unwrap().permissions().mode() & 0o777
        }

        #[test]
        #[serial(umask)]
        fn journal_files_are_0600_in_0700_dir_under_permissive_umask() {
            let _umask = UmaskGuard::set(0);
            let tmp = tempfile::TempDir::new().unwrap();
            let dir = tmp.path().join("sessions");
            let mut s = Session::new("m", "medium", None);
            save_session_in_dir(&dir, &s, SessionPersistence::Journal).unwrap();
            s.api_messages.push(std::sync::Arc::new(
                serde_json::json!({"role":"user","content":"x"}),
            ));
            save_session_in_dir(&dir, &s, SessionPersistence::Journal).unwrap();

            assert_eq!(mode_of(&dir), 0o700, "sessions dir must be 0700");
            assert_eq!(
                mode_of(&dir.join(format!("{}.json", s.id))),
                0o600,
                "snapshot must be 0600"
            );
            assert_eq!(
                mode_of(&journal_path(&dir, &s.id)),
                0o600,
                "journal must be 0600"
            );
        }

        #[test]
        fn journal_append_refuses_symlink_target() {
            let tmp = tempfile::TempDir::new().unwrap();
            let dir = tmp.path().join("sessions");
            std::fs::create_dir_all(&dir).unwrap();
            let mut s = Session::new("m", "medium", None);
            let victim = tmp.path().join("victim");
            std::fs::write(&victim, "original").unwrap();
            std::os::unix::fs::symlink(&victim, journal_path(&dir, &s.id)).unwrap();
            // Snapshot write succeeds; the journal step must refuse.
            s.api_messages.push(std::sync::Arc::new(
                serde_json::json!({"role":"user","content":"x"}),
            ));
            let res = save_session_in_dir(&dir, &s, SessionPersistence::Journal);
            assert!(res.is_err(), "journal write onto a symlink must fail");
            assert_eq!(std::fs::read_to_string(&victim).unwrap(), "original");
        }
    }
}