seshat-cli 0.5.0

//! Implementation of the `seshat serve` command.
//!
//! Discovers the project database via smart resolution (explicit repo argument,
//! current working directory, git root walk-up, or single-DB fallback), displays
//! startup information, and starts the MCP server on stdio transport with
//! graceful Ctrl+C shutdown.

use std::collections::HashMap;
use std::path::{Path, PathBuf};
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, Ordering};
use std::time::Instant;

use seshat_core::{BranchId, Language, ScanConfig};
use seshat_mcp::{ProjectConnection, ScanState};
use seshat_scanner::{read_and_parse_file, record_branch_scan_complete, scan_project};
use seshat_storage::{
    BranchRepository, Database, FileIRRepository, SqliteBranchRepository, SqliteFileIRRepository,
    SqliteSubmoduleRepository, SubmoduleRepository, SubmoduleRow,
};
use seshat_watcher::{WatcherError, WatcherParams, start_watcher};
use tokio::sync::oneshot;

use crate::config::AppConfig;
use crate::db::{ServeTarget, detect_branch, gc_branch_snapshots};
use crate::error::CliError;

/// Handle for the GC background task.
///
/// Call [`GcHandle::shutdown`] (or simply drop) to stop the periodic GC task.
pub struct GcHandle {
    shutdown_tx: oneshot::Sender<()>,
    task: tokio::task::JoinHandle<()>,
}

impl GcHandle {
    /// Signal the GC task to stop and await its completion.
    pub async fn shutdown(self) {
        let _ = self.shutdown_tx.send(());
        let _ = tokio::time::timeout(std::time::Duration::from_secs(5), self.task).await;
    }
}

/// Metadata about a discovered scanned project database.
struct RepoInfo {
    /// Human-readable project name (derived from DB filename).
    name: String,
    /// Path to the `.db` file.
    db_path: PathBuf,
    /// Current branch stored in the database.
    branch: BranchId,
    /// Number of indexed files.
    file_count: usize,
    /// Number of convention nodes.
    convention_count: usize,
}

/// Resolve the call log path from CLI flag and config.
///
/// Priority: CLI flag > config value > disabled.
/// - `Some("")` (bare `--call-log`) → default path `$XDG_DATA_HOME/seshat/call-log.jsonl`
/// - `Some("/path")` → explicit path
/// - `None` + `Some(config)` → config value
/// - `None` + `None` config → disabled
fn resolve_call_log_path(cli_flag: Option<PathBuf>, config_value: Option<&str>) -> Option<PathBuf> {
    match cli_flag {
        Some(p) if p.as_os_str().is_empty() => {
            // Bare --call-log with no value → use default path
            let data_dir = dirs::data_dir().unwrap_or_else(|| PathBuf::from("."));
            Some(data_dir.join("seshat").join("call-log.jsonl"))
        }
        Some(p) => Some(p),
        None => config_value.map(PathBuf::from),
    }
}

/// Decide whether the file watcher should start for this `serve` invocation.
///
/// Watcher is gated on **both**:
/// - The user has not disabled it via `[watcher] enabled = false` (`enabled`
///   parameter), AND
/// - The auto-scan (if any) did not fail. A failed scan means the project
///   is in an indeterminate state (e.g. too many files, scan timeout); we
///   refuse to walk the filesystem with `notify-debouncer-full` because
///   that is exactly what blew up to 91.8 GB in the original bug report.
///
/// `state.error_message()` returns `None` when the scan was not needed,
/// is in progress, or completed successfully — so this gate proceeds in
/// all the normal paths and only blocks the explicit failure case.
fn watcher_should_start(enabled: bool, state: &ScanState) -> bool {
    enabled && state.error_message().is_none()
}

/// Handle branch switching and snapshot logic for the serve flow.
///
/// For ExistingDb: if detected branch differs from DB's current branch,
/// switch to it (creating a snapshot from source if target has no data).
/// For AutoScan: if detected branch differs from "main" and "main" has data,
/// create a snapshot from "main" to the detected branch.
///
/// Returns the final branch ID after any switch.
fn handle_branch_switch(
    db: &Database,
    detected_branch: &str,
    current_branch: &BranchId,
    _is_auto_scan: bool,
) -> Result<BranchId, CliError> {
    let branch_repo = SqliteBranchRepository::new(db.connection().clone());

    // Check if we need to switch branches.
    if detected_branch == current_branch.0 {
        return Ok(current_branch.clone());
    }

    let detected_id = BranchId::from(detected_branch);

    // Check if target branch already has data.
    let branches = branch_repo
        .list_branches()
        .map_err(|e| CliError::CommandFailed {
            command: "serve".to_owned(),
            reason: format!("failed to list branches: {e}"),
        })?;

    let target_has_data = branches.iter().any(|b| b.0 == detected_branch);

    if !target_has_data {
        // Target branch has no data — check if source has data to snapshot.
        let source_branch = current_branch.clone();

        // Check source has actual data (not just registered).
        let source_branches = branch_repo
            .list_branches()
            .map_err(|e| CliError::CommandFailed {
                command: "serve".to_owned(),
                reason: format!("failed to list branches: {e}"),
            })?;
        let source_has_data = source_branches.iter().any(|b| b.0 == source_branch.0);

        if !source_has_data {
            tracing::info!(
                source_branch = %source_branch.0,
                target_branch = %detected_branch,
                "Source branch has no data — switching without snapshot"
            );
        } else {
            tracing::info!(
                source_branch = %source_branch.0,
                target_branch = %detected_branch,
                "Target branch has no data — creating snapshot from source"
            );
            branch_repo
                .create_snapshot(&source_branch, &detected_id)
                .map_err(|e| CliError::CommandFailed {
                    command: "serve".to_owned(),
                    reason: format!("failed to create snapshot: {e}"),
                })?;
        }
    }

    // Switch to the detected branch.
    tracing::info!(
        from = %current_branch.0,
        to = %detected_branch,
        "Switching branch"
    );
    branch_repo
        .switch_branch(&detected_id)
        .map_err(|e| CliError::CommandFailed {
            command: "serve".to_owned(),
            reason: format!("failed to switch branch: {e}"),
        })?;

    Ok(detected_id)
}

/// Handle branch snapshot for AutoScan path.
///
/// If detected branch differs from "main" and "main" has data,
/// create a snapshot from "main" to the detected branch.
///
/// Returns the final branch ID after any switch.
fn handle_auto_scan_snapshot(db: &Database, detected_branch: &str) -> Result<BranchId, CliError> {
    let branch_repo = SqliteBranchRepository::new(db.connection().clone());

    if detected_branch == "main" {
        return Ok(BranchId::from(detected_branch));
    }

    let detected_id = BranchId::from(detected_branch);

    // Check if "main" has data.
    let branches = branch_repo
        .list_branches()
        .map_err(|e| CliError::CommandFailed {
            command: "serve".to_owned(),
            reason: format!("failed to list branches: {e}"),
        })?;

    let main_has_data = branches.iter().any(|b| b.0 == "main");

    if !main_has_data {
        return Ok(detected_id);
    }

    // Create snapshot from "main" to detected branch.
    let main_branch = BranchId::from("main");
    tracing::info!(
        source_branch = "main",
        target_branch = %detected_branch,
        "Auto-scan on non-main branch — creating snapshot from main"
    );
    branch_repo
        .create_snapshot(&main_branch, &detected_id)
        .map_err(|e| CliError::CommandFailed {
            command: "serve".to_owned(),
            reason: format!("failed to create snapshot: {e}"),
        })?;

    // Switch to the detected branch.
    branch_repo
        .switch_branch(&detected_id)
        .map_err(|e| CliError::CommandFailed {
            command: "serve".to_owned(),
            reason: format!("failed to switch branch: {e}"),
        })?;

    Ok(detected_id)
}

/// Background sync after a branch switch.
///
/// Thin wrapper around [`incremental_sync_blocking`] for the serve-startup
/// path: runs in a `std::thread::spawn`, no progress callback, and lets the
/// caller's `sync_in_progress` flag track completion.
#[allow(clippy::too_many_arguments)]
fn background_sync(
    project_root: &Path,
    sync_root: &Path,
    old_branch: Option<&str>,
    new_branch: &str,
    db: &Database,
    branch_id: &BranchId,
    scan_config: &ScanConfig,
    detection_config: &seshat_core::DetectionConfig,
) {
    incremental_sync_blocking(
        project_root,
        sync_root,
        old_branch,
        new_branch,
        db,
        branch_id,
        scan_config,
        detection_config,
        None,
    );
}

/// Synchronous incremental sync of a branch's `files_ir` to match `new_branch`'s
/// HEAD commit, with an optional 1-arg progress callback `(processed, total)`.
///
/// Collects file trees from the old and new branch HEAD commits via `gix`,
/// then diffs at the path level: new/changed files are re-parsed and upserted,
/// removed files are deleted from the new branch's `files_ir`. On `gix` failures,
/// falls back to a full rescan. Runs the detection cycle on completion to rebuild
/// conventions for the new branch, and records HEAD as `last_scanned_commit`.
///
/// The progress callback (if provided) fires on every iteration of the upsert
/// loop with `(processed_so_far, new_paths_total)` and once more with
/// `(total, total)` after the loop. Callers are responsible for any throttling
/// they need (e.g. the `seshat review` blocking sync emits at 1 Hz to stderr).
///
/// Used by:
/// - [`background_sync`] (no callback) — serve startup, runs in a spawned thread.
/// - `run_review` — runs synchronously before opening the TUI so the user sees
///   fresh data (US-011).
///
/// `project_root` is the actual working directory of the project (the
/// worktree path for git worktrees; the repo root otherwise). All file reads
/// and HEAD-commit recording happen against this path.
///
/// `sync_root` is the path used for ref/tree lookups via gix. For plain
/// repos this is identical to `project_root`. For worktrees both paths
/// resolve to the same shared common-dir refs under the hood, but keeping
/// the parameters separate makes the contract explicit: never read source
/// files through `sync_root` (it can point at a sibling worktree).
#[allow(clippy::too_many_arguments)]
pub(crate) fn incremental_sync_blocking(
    project_root: &Path,
    sync_root: &Path,
    old_branch: Option<&str>,
    new_branch: &str,
    db: &Database,
    branch_id: &BranchId,
    scan_config: &ScanConfig,
    detection_config: &seshat_core::DetectionConfig,
    progress: Option<&dyn Fn(usize, usize)>,
) {
    let new_paths = match resolve_branch_tree_paths(sync_root, new_branch) {
        Some(p) => p,
        None => {
            tracing::warn!(
                "incremental_sync_blocking: could not resolve new branch tree, falling back to full rescan"
            );
            fallback_rescan(project_root, db, branch_id, scan_config, detection_config);
            return;
        }
    };

    let old_paths = old_branch.and_then(|b| resolve_branch_tree_paths(sync_root, b));

    let file_ir_repo = SqliteFileIRRepository::new(db.connection().clone());

    let exclude_set = if scan_config.exclude_paths.is_empty() {
        None
    } else {
        let mut builder = globset::GlobSetBuilder::new();
        for p in &scan_config.exclude_paths {
            match globset::Glob::new(p) {
                Ok(g) => {
                    builder.add(g);
                }
                Err(e) => {
                    tracing::warn!(pattern = %p, error = %e, "incremental_sync_blocking: invalid exclude pattern");
                }
            }
        }
        match builder.build() {
            Ok(set) => Some(set),
            Err(e) => {
                tracing::warn!(error = %e, "incremental_sync_blocking: failed to build exclude globset");
                None
            }
        }
    };

    let total = new_paths.len();
    let mut synced = 0usize;
    let mut removed = 0usize;
    // Build a full source_map covering EVERY file in the new tree, not just
    // the diff. The detection cycle below DELETEs all auto-detected nodes
    // and re-emits them — feeding it an empty map (the previous bug) would
    // drop snippets for all unchanged files. Reading the unchanged files
    // costs a few hundred milliseconds even on large repos and matches the
    // semantics of a full scan, where `scan_project` retains source for
    // every file it touches.
    let mut source_map: HashMap<PathBuf, String> = HashMap::with_capacity(total);

    for (idx, (rel_path, oid)) in new_paths.iter().enumerate() {
        // Fire progress at the top of each iteration so `continue` paths still
        // advance the counter — otherwise large skipped runs would stall the UI.
        if let Some(cb) = progress {
            cb(idx, total);
        }

        let path_str = rel_path.as_str();
        // Absolute path is rooted at `project_root` (the worktree), NOT
        // `sync_root` — for a git worktree the latter points at a sibling
        // checkout where these files do not exist.
        let abs_path = project_root.join(rel_path);
        // Store paths relative to the worktree root, matching the
        // full-scan orchestrator. gix tree-walk already yields relative paths
        // here, so PathBuf::from(rel_path) is the canonical IR key.
        let stored_path = PathBuf::from(rel_path);

        let ext = match abs_path.extension().and_then(|e| e.to_str()) {
            Some(e) => e,
            None => continue,
        };
        let language = match Language::from_extension(ext) {
            Some(l) => l,
            None => continue,
        };

        if let Some(ref exclude_set) = exclude_set {
            if exclude_set.is_match(&abs_path) {
                continue;
            }
        }

        let max_bytes = scan_config.max_file_size_kb * 1024;
        if max_bytes > 0 {
            if let Ok(meta) = std::fs::metadata(&abs_path) {
                if meta.len() > max_bytes {
                    continue;
                }
            }
        }

        // Read every file (changed or not) so the detection cycle below has
        // source available for snippet construction. The IR upsert is still
        // skipped for unchanged files (oid match) — we only need source, not
        // a fresh parse, for detectors to attach snippets.
        let oid_unchanged = old_paths
            .as_ref()
            .is_some_and(|old| old.get(path_str) == Some(oid));

        let (project_file, source) = match read_and_parse_file(
            &abs_path,
            &stored_path,
            language,
            &scan_config.local_packages,
        ) {
            Ok(pair) => pair,
            Err(e) => {
                tracing::warn!(path = %abs_path.display(), error = %e, "incremental_sync_blocking: cannot read file");
                continue;
            }
        };

        if !oid_unchanged {
            // Write IR + symbol-index together so the new branch's HEAD
            // index includes every file whose oid changed.  Failure here
            // leaves the symbol-index inconsistent with files_ir for this
            // path — log at error so the user sees it in default log
            // configurations, not just `--verbose`.
            if let Err(e) = file_ir_repo.upsert_with_symbol_index(branch_id, &project_file, None) {
                tracing::error!(
                    path = %path_str,
                    error = %e,
                    "incremental_sync_blocking: upsert failed — symbol-index may be inconsistent for this file until next save",
                );
            }
            synced += 1;
        }
        // source_map keyed by relative path so it lines up with
        // ProjectFile.path that detectors look up against.
        source_map.insert(stored_path, source);
    }

    // Final tick so the UI snaps to "X / X" instead of stalling at "(X-1) / X".
    if let Some(cb) = progress {
        cb(total, total);
    }

    if let Some(ref old) = old_paths {
        for rel_path in old.keys() {
            if !new_paths.contains_key(rel_path.as_str()) {
                let path_str = rel_path.as_str();
                // Drop files_ir AND matching symbol-index rows in a single
                // transaction so the new branch's index can't observe one
                // half gone while the other half lingers.
                if let Err(e) = file_ir_repo.delete_with_symbol_index(branch_id, path_str) {
                    match &e {
                        seshat_storage::StorageError::NotFound { .. } => {}
                        _ => {
                            tracing::error!(
                                path = %path_str,
                                error = %e,
                                "incremental_sync_blocking: delete failed — orphan symbol-index rows may remain",
                            );
                        }
                    }
                }
                removed += 1;
            }
        }
    }

    tracing::info!(
        synced = synced,
        removed = removed,
        new_total = new_paths.len(),
        old_branch = ?old_branch,
        new_branch = %new_branch,
        "incremental_sync_blocking: completed diff-based sync"
    );

    // P24: skip the detection cycle when nothing actually changed in IR.
    // Detection re-aggregates findings across the whole project IR — that's
    // expensive on large codebases and the existing nodes are still valid
    // when no file changed.
    if synced > 0 || removed > 0 {
        let conn = db.connection().clone();
        let file_dates = SqliteFileIRRepository::new(conn.clone())
            .get_file_dates_by_branch(branch_id)
            .unwrap_or_default()
            .into_iter()
            .collect::<HashMap<_, _>>();
        match seshat_graph::run_detection_cycle(
            &conn,
            branch_id,
            detection_config,
            &file_dates,
            &source_map,
        ) {
            Ok(_) => tracing::info!("incremental_sync_blocking: detection cycle complete"),
            Err(e) => {
                tracing::warn!(error = %e, "incremental_sync_blocking: detection cycle failed")
            }
        }
    } else {
        tracing::debug!("incremental_sync_blocking: no IR changes; skipping detection cycle");
    }

    // Record the WORKTREE's HEAD (not the main repo's) so the next startup's
    // freshness check compares against the correct sentinel for this branch.
    let branch_repo = SqliteBranchRepository::new(db.connection().clone());
    record_branch_scan_complete(&branch_repo, project_root, branch_id);
}

fn resolve_branch_tree_paths(
    root: &Path,
    branch_name: &str,
) -> Option<HashMap<String, gix::ObjectId>> {
    let git_root = crate::db::find_git_root(root)?;
    let repo = gix::open(git_root).ok()?;

    let object = {
        let ref_name = format!("refs/heads/{branch_name}");
        if let Some(id) = repo
            .try_find_reference(&ref_name)
            .ok()
            .flatten()
            .and_then(|r| r.into_fully_peeled_id().ok())
        {
            repo.find_object(id.detach()).ok()
        } else {
            gix::ObjectId::from_hex(branch_name.as_bytes())
                .ok()
                .and_then(|oid| repo.find_object(oid).ok())
        }?
    };

    let tree = object.into_commit().tree().ok()?;

    let mut recorder = gix::traverse::tree::Recorder::default();
    tree.traverse().breadthfirst(&mut recorder).ok()?;

    let mut paths = HashMap::new();
    for entry in recorder.records {
        if entry.mode.is_blob() {
            paths.insert(entry.filepath.to_string(), entry.oid);
        }
    }
    Some(paths)
}

/// `project_root` must be the actual working directory of the project (the
/// worktree path for git worktrees) — `scan_project` walks the filesystem
/// from this root, so passing a sibling worktree here would scan the wrong
/// tree.
fn fallback_rescan(
    project_root: &Path,
    db: &Database,
    branch_id: &BranchId,
    scan_config: &ScanConfig,
    _detection_config: &seshat_core::DetectionConfig,
) {
    tracing::info!(root = %project_root.display(), "background_sync: falling back to full rescan");
    // `scan_project` already runs the full detection cycle with the
    // populated source_map — running it again with an empty source_map
    // (the pre-fix behaviour) wiped every snippet. So we only need the
    // scan call here.
    if let Err(e) = scan_project(project_root, scan_config, db, branch_id.clone()) {
        tracing::warn!(error = %e, "background_sync: full rescan scan_project failed");
    }

    // Record the worktree's HEAD as the last scanned commit so the next
    // freshness check compares against the right sentinel.
    let branch_repo = SqliteBranchRepository::new(db.connection().clone());
    record_branch_scan_complete(&branch_repo, project_root, branch_id);
}

/// Run the serve command.
///
/// Discovers the project database (from explicit repo arg, cwd, git root, or
/// single-DB fallback), loads it, displays startup information, and starts the
/// MCP server on stdio transport.
pub fn run_serve(
    repo: Option<&Path>,
    host: Option<String>,
    port: Option<u16>,
    call_log: Option<PathBuf>,
) -> Result<(), CliError> {
    // -- Load config --------------------------------------------------
    let mut config = AppConfig::load().map_err(|e| CliError::CommandFailed {
        command: "serve".to_owned(),
        reason: format!("failed to load config: {e}"),
    })?;

    // CLI flags override config values.
    if let Some(h) = host {
        config.server.host = h;
    }
    if let Some(p) = port {
        config.server.port = p;
    }

    // -- Discover databases or project root --------------------------
    let target =
        crate::db::resolve_serve_db_or_project_root(repo, &config.scan.additional_denylist_paths)?;

    let (db_path, db, mut repo_info, scan_state, auto_scan_project_root, detected_branch) =
        match target {
            ServeTarget::ExistingDb {
                db_path,
                project_root,
            } => {
                let db = Database::open(&db_path).map_err(|e| CliError::CommandFailed {
                    command: "serve".to_owned(),
                    reason: format!("failed to open database: {e}"),
                })?;
                let detected = detect_branch(&project_root);
                let repo_info = load_repo_info(&db, &db_path)?;
                (
                    db_path,
                    db,
                    repo_info,
                    ScanState::not_needed(),
                    None,
                    detected,
                )
            }
            ServeTarget::AutoScan {
                project_root,
                db_path,
            } => {
                // Detect git branch before creating DB.
                let detected = detect_branch(&project_root);

                // Create empty DB (migrations auto-apply).
                let db = Database::open(&db_path).map_err(|e| CliError::CommandFailed {
                    command: "serve".to_owned(),
                    reason: format!("failed to create database: {e}"),
                })?;
                tracing::info!(
                    project_root = %project_root.display(),
                    db_path = %db_path.display(),
                    detected_branch = %detected,
                    "No existing DB found — starting auto-scan"
                );

                // Create scan state before the discovery check so that any early
                // error paths can still transition it to Failed.
                let scan_state = ScanState::in_progress();

                // File count pre-check: abort auto-scan if project is too large.
                let scan_config = config.scan.clone();
                let auto_scan_limit = scan_config.auto_scan_limit;
                match seshat_scanner::discover_files(&project_root, &scan_config) {
                    Ok(discovery_result) => {
                        let file_count = discovery_result.files.len();

                        if file_count > auto_scan_limit {
                            scan_state.mark_failed(format!(
                            "Project too large for auto-scan ({} files). Run: seshat scan --verbose",
                            file_count
                        ));
                            let repo_info = load_repo_info(&db, &db_path)?;
                            (db_path, db, repo_info, scan_state, None, detected)
                        } else {
                            let repo_info = load_repo_info(&db, &db_path)?;
                            (
                                db_path,
                                db,
                                repo_info,
                                scan_state,
                                Some(project_root),
                                detected,
                            )
                        }
                    }
                    Err(e) => {
                        // Discovery failed — continue with empty DB.
                        // MCP calls will get AUTO_SCAN_FAILED error.
                        scan_state.mark_failed(format!("auto-scan discovery failed: {e}"));
                        let repo_info = load_repo_info(&db, &db_path)?;
                        (db_path, db, repo_info, scan_state, None, detected)
                    }
                }
            }
        };

    // -- Handle branch switching / snapshots --------------------------
    let is_auto_scan = auto_scan_project_root.is_some();
    let old_branch_for_sync = if is_auto_scan {
        None
    } else {
        Some(repo_info.branch.0.clone())
    };

    let final_branch = if is_auto_scan {
        handle_auto_scan_snapshot(&db, &detected_branch)?
    } else {
        handle_branch_switch(&db, &detected_branch, &repo_info.branch, is_auto_scan)?
    };

    // Update repo_info.branch to reflect the actual branch after any switch.
    repo_info.branch = final_branch.clone();

    // -- Resolve the project root used for git operations and sync --------
    // Auto-scan owns its own root; otherwise use the shared sync_root_for
    // helper from cwd. This routes through the same fallback semantics as
    // ResolvedProject::sync_root (git common-dir, else cwd verbatim).
    let sync_root = match &auto_scan_project_root {
        Some(root) => root.clone(),
        None => crate::db::sync_root_for(&std::env::current_dir().unwrap_or_default()),
    };
    // For git worktrees `sync_root` points at the main repo (so we can read
    // shared refs), while the actual files live under the worktree checkout
    // dir — which is what `current_dir()` returns and what `scan_project`
    // walks. `project_root` therefore stays cwd-rooted for the no-auto-scan
    // case, matching the watcher path below.
    let sync_project_root: PathBuf = match &auto_scan_project_root {
        Some(root) => root.clone(),
        None => std::env::current_dir().unwrap_or_else(|_| PathBuf::from(".")),
    };

    // -- Detect HEAD change since last scan (US-010) ----------------------
    // For the auto-scan path, scan_project below is the scan; running an
    // additional sync on top would race with it. For the existing-DB path,
    // compare branches.last_scanned_commit against git rev-parse HEAD;
    // git-unavailable is treated as "no change" per AC#2.
    let head_change_hint: Option<String> = if is_auto_scan {
        None
    } else {
        let branch_repo = SqliteBranchRepository::new(db.connection().clone());
        match seshat_scanner::check_branch_freshness(
            &branch_repo,
            &sync_project_root,
            &final_branch,
        ) {
            seshat_scanner::FreshnessCheck::UpToDate
            | seshat_scanner::FreshnessCheck::GitUnavailable => None,
            seshat_scanner::FreshnessCheck::Stale {
                old_commit,
                new_commit,
            } => {
                let old_short = old_commit
                    .as_deref()
                    .map(|c| c.chars().take(7).collect::<String>())
                    .unwrap_or_else(|| "(none)".to_owned());
                let new_short: String = new_commit.chars().take(7).collect();
                tracing::info!(
                    branch = %final_branch.0,
                    old_head = %old_short,
                    new_head = %new_short,
                    "serve: detected HEAD change since last scan — triggering background sync"
                );
                old_commit
            }
        }
    };

    // -- Shared sync flag for MCP metadata ---------------------------------
    let sync_in_progress = Arc::new(AtomicBool::new(false));
    // -- Concurrent switch guard (prevents multiple parallel branch switches) --
    let switch_in_progress = Arc::new(AtomicBool::new(false));

    // -- Background diff-based sync (branch switch and/or HEAD change) ----
    let sync_old_branch = old_branch_for_sync.filter(|b| *b != final_branch.0);
    let needs_sync = sync_old_branch.is_some() || head_change_hint.is_some();
    // Resolve the old-side hint for tree-diff. Branch-switch wins (it carries
    // a refs/heads/<name> that gix resolves directly); on a same-branch HEAD
    // change the last_scanned_commit hash works as a commit-ish via the
    // ObjectId fallback in `resolve_branch_tree_paths`. None on the HEAD-change
    // path means there was no recorded sentinel — background_sync will fall
    // through to a full rescan when the new-branch tree itself is unresolvable.
    let sync_old_hint: Option<String> =
        sync_old_branch.clone().or_else(|| head_change_hint.clone());

    if needs_sync {
        let sync_root_clone = sync_root.clone();
        let project_root_clone = sync_project_root.clone();
        let sync_db_path = db_path.clone();
        let sync_branch = final_branch.clone();
        let sync_scan_config = config.scan.clone();
        let sync_detection_config = config.detection.clone();
        let sync_flag = sync_in_progress.clone();
        std::thread::spawn(move || {
            struct ClearOnDrop(Arc<AtomicBool>);
            impl Drop for ClearOnDrop {
                fn drop(&mut self) {
                    self.0.store(false, Ordering::Relaxed);
                }
            }
            sync_flag.store(true, Ordering::Relaxed);
            let _guard = ClearOnDrop(sync_flag);
            let sync_db = match Database::open(&sync_db_path) {
                Ok(d) => d,
                Err(e) => {
                    tracing::error!(error = %e, "background_sync: failed to open DB");
                    return;
                }
            };
            background_sync(
                &project_root_clone,
                &sync_root_clone,
                sync_old_hint.as_deref(),
                &sync_branch.0,
                &sync_db,
                &sync_branch,
                &sync_scan_config,
                &sync_detection_config,
            );
        });
    }

    // -- Run branch snapshot garbage collection -----------------------
    // Same resolution as `sync_root` above — branch GC reads git refs to
    // decide which DB-side branches no longer exist on disk.
    let gc_repo_path = match &auto_scan_project_root {
        Some(root) => root.clone(),
        None => crate::db::sync_root_for(&std::env::current_dir().unwrap_or_default()),
    };
    if let Ok(deleted) = gc_branch_snapshots(&db, &gc_repo_path) {
        if !deleted.is_empty() {
            tracing::info!(
                deleted_count = deleted.len(),
                deleted_branches = ?deleted,
                "Garbage collected orphan branch snapshots on startup"
            );
        }
    }

    // -- Load submodule connections -----------------------------------
    let submodule_rows = load_submodule_rows(&db);
    let submodules = open_submodule_connections(&submodule_rows, &repo_info.name);

    // -- Resolve call log path ----------------------------------------
    let call_log_path = resolve_call_log_path(call_log, config.server.call_log.as_deref());

    // -- Create embedding provider (optional) -------------------------
    let embedding_provider: Option<Arc<dyn seshat_embedding::EmbeddingProvider>> =
        config.embedding.as_ref().and_then(|emb_config| {
            match seshat_embedding::create_provider(emb_config) {
                Ok(provider) => {
                    tracing::info!("Embedding provider enabled: {emb_config}");
                    Some(Arc::from(provider))
                }
                Err(e) => {
                    tracing::warn!("Failed to create embedding provider: {e}");
                    eprintln!("  Warning: embedding provider unavailable: {e}");
                    None
                }
            }
        });

    // -- Start MCP server (async via tokio) ---------------------------
    let server_config = config.server.clone();
    let _start = Instant::now();

    let runtime = tokio::runtime::Runtime::new().map_err(|e| CliError::CommandFailed {
        command: "serve".to_owned(),
        reason: format!("failed to create tokio runtime: {e}"),
    })?;

    let root = ProjectConnection::new(
        db.connection().clone(),
        repo_info.name.clone(),
        detected_branch.clone(),
    );

    // Derive project root: use the auto-scan root if available.
    // Otherwise use the current working directory — for git worktrees, cwd is
    // the worktree checkout directory, which is what we need for file diffing.
    // find_git_root would walk up to the main repo root, which is wrong for
    // worktrees (they live under a different path than the main .git dir).
    let project_root = match &auto_scan_project_root {
        Some(root) => root.clone(),
        None => std::env::current_dir().unwrap_or_else(|_| PathBuf::from(".")),
    };

    let watcher_enabled = config.watcher.enabled;
    let watcher_params = WatcherParams {
        enabled: watcher_enabled,
        debounce_ms: config.watcher.debounce_ms,
        ignore_patterns: config.watcher.ignore_patterns.clone(),
        warm_tier_interval_seconds: config.watcher.warm_tier_interval_seconds,
        bulk_change_threshold: config.watcher.bulk_change_threshold,
    };
    let watcher_scan_config = config.scan.clone();
    let watcher_detection_config = config.detection.clone();

    let has_auto_scan = auto_scan_project_root.is_some();
    let auto_scan_root = auto_scan_project_root.clone();

    runtime
        .block_on(async {
            let scan_state_clone = scan_state.clone();

            // -- Launch background scan (if auto-scan) ------------------
            if let Some(scan_root) = auto_scan_root.clone() {
                let scan_config = config.scan.clone();
                let scan_db = db.clone();
                let scan_branch = detected_branch.clone();
                tokio::spawn(async move {
                    let branch = seshat_core::BranchId::from(scan_branch);
                    let result = tokio::task::spawn_blocking(move || {
                        scan_project(&scan_root, &scan_config, &scan_db, branch)
                    })
                    .await;
                    match result {
                        Ok(Ok(_scan_result)) => {
                            tracing::info!("Auto-scan completed successfully");
                            scan_state_clone.mark_complete();
                        }
                        Ok(Err(scan_err)) => {
                            tracing::error!("Auto-scan failed: {scan_err}");
                            scan_state_clone.mark_failed(scan_err.to_string());
                        }
                        Err(join_err) => {
                            tracing::error!("Auto-scan task panicked: {join_err}");
                            scan_state_clone.mark_failed(join_err.to_string());
                        }
                    }
                });
            }

            // -- Launch periodic GC background task -------------------
            let gc_db = db.clone();
            let gc_repo_path = gc_repo_path.clone();
            let (gc_shutdown_tx, mut gc_shutdown_rx) = oneshot::channel();
            let gc_task = tokio::spawn(async move {
                let mut interval = tokio::time::interval(std::time::Duration::from_secs(3600));
                loop {
                    tokio::select! {
                        _ = interval.tick() => {
                            let db_clone = gc_db.clone();
                            let path_clone = gc_repo_path.clone();
                            match tokio::task::spawn_blocking(move || {
                                gc_branch_snapshots(&db_clone, &path_clone)
                            })
                            .await
                            {
                                Ok(Ok(deleted_list)) => {
                                    if !deleted_list.is_empty() {
                                        tracing::info!(
                                            deleted_count = deleted_list.len(),
                                            deleted_branches = ?deleted_list,
                                            "Periodic branch snapshot garbage collection"
                                        );
                                    }
                                }
                                Ok(Err(e)) => {
                                    tracing::error!(error = %e, "Periodic GC failed");
                                }
                                Err(join_err) => {
                                    tracing::error!(error = %join_err, "Periodic GC task panicked");
                                }
                            }
                        }
                        _ = &mut gc_shutdown_rx => {
                            tracing::debug!("GC background task shutting down");
                            break;
                        }
                    }
                }
            });
            let gc_handle = GcHandle {
                shutdown_tx: gc_shutdown_tx,
                task: gc_task,
            };

            // -- Start watcher (delayed if auto-scan) ------------------
            // When auto-scan is in progress, watcher must wait for scan to
            // complete before starting (it needs a populated DB).
            //
            // P0 guardrail (see PRD US-004): refuse to spawn the watcher
            // task when the auto-scan has already failed. `notify-debouncer-full`
            // recursively walks the project root on init, which is what
            // blew up to 91.8 GB on a dangerous cwd in the original report.
            let watcher_rx = if watcher_should_start(watcher_enabled, &scan_state) {
                let (watcher_tx, watcher_rx) = tokio::sync::oneshot::channel();
                let params = watcher_params;
                let root = project_root.clone();
                let db_p = db_path.clone();
                let conn = db.connection().clone();
                let branch = BranchId::from(detected_branch.as_str());
                let wait_scan = scan_state.clone();

                let on_branch_switch: Arc<dyn Fn() + Send + Sync + 'static> = {
                    let root_clone = project_root.clone();
                    let sync_root_clone = sync_root.clone();
                    let db_path_clone = db_path.clone();
                    let scan_cfg_clone = watcher_scan_config.clone();
                    let detect_cfg_clone = watcher_detection_config.clone();
                    let sync_flag = sync_in_progress.clone();
                    let switch_guard = switch_in_progress.clone();
                    Arc::new(move || {
                        // CAS guard: skip if another switch is already in progress.
                        if switch_guard
                            .compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)
                            .is_err()
                        {
                            tracing::debug!("Branch switch already in progress — skipping duplicate event");
                            return;
                        }
                        let root = root_clone.clone();
                        let sync_root = sync_root_clone.clone();
                        let db_path = db_path_clone.clone();
                        let scan_cfg = scan_cfg_clone.clone();
                        let detect_cfg = detect_cfg_clone.clone();
                        let sync_flag = sync_flag.clone();
                        let switch_guard = switch_guard.clone();
                        std::thread::spawn(move || {
                            struct ClearOnDrop(Arc<AtomicBool>);
                            impl Drop for ClearOnDrop {
                                fn drop(&mut self) {
                                    self.0.store(false, Ordering::Relaxed);
                                }
                            }
                            let _guard = ClearOnDrop(switch_guard);
                            sync_flag.store(true, Ordering::Relaxed);
                            let _flag_guard = ClearOnDrop(sync_flag);
                            let start = Instant::now();
                            let new_branch = detect_branch(&root);
                            let db = match Database::open(&db_path) {
                                Ok(d) => d,
                                Err(e) => {
                                    tracing::error!(error = %e, "Failed to open DB for branch switch");
                                    return;
                                }
                            };
                            let branch_repo = SqliteBranchRepository::new(db.connection().clone());
                            let current_branch = branch_repo
                                .get_current_branch()
                                .map(|b| b.0.clone())
                                .unwrap_or_else(|e| {
                                    tracing::debug!(error = %e, "Could not read current branch from DB, defaulting to 'main'");
                                    "main".to_string()
                                });

                            tracing::info!(
                                old_branch = %current_branch,
                                new_branch = %new_branch,
                                "Branch switch detected by watcher"
                            );
                            if new_branch == current_branch {
                                tracing::debug!("Branch unchanged, no switch needed");
                                return;
                            }
                            let new_id = BranchId::from(new_branch.as_str());
                            let old_id = BranchId::from(current_branch.as_str());

                            let branches = match branch_repo.list_branches() {
                                Ok(b) => b,
                                Err(e) => {
                                    tracing::error!(error = %e, "Failed to list branches for switch");
                                    return;
                                }
                            };
                            let snapshot_exists = branches.iter().any(|b| b.0 == new_branch);
                            if snapshot_exists {
                                match branch_repo.switch_branch(&new_id) {
                                    Ok(()) => {
                                        let elapsed = start.elapsed();
                                        tracing::info!(
                                            to = %new_branch,
                                            elapsed_ms = elapsed.as_millis(),
                                            "Branch switch completed (instant, snapshot existed)"
                                        );
                                    }
                                    Err(e) => {
                                        tracing::error!(error = %e, "Failed to switch branch");
                                        return;
                                    }
                                }
                            } else {
                                tracing::info!(
                                    source = %current_branch,
                                    target = %new_branch,
                                    "No snapshot for target — creating"
                                );
                                match branch_repo.create_snapshot(&old_id, &new_id) {
                                    Ok(()) => {
                                        match branch_repo.switch_branch(&new_id) {
                                            Ok(()) => {
                                                let elapsed = start.elapsed();
                                                tracing::info!(
                                                    to = %new_branch,
                                                    elapsed_ms = elapsed.as_millis(),
                                                    "Branch switch completed (snapshot created)"
                                                );
                                            }
                                            Err(e) => {
                                                tracing::error!(error = %e, "Failed to switch after snapshot");
                                                return;
                                            }
                                        }
                                    }
                                    Err(e) => {
                                        tracing::error!(error = %e, "Failed to create snapshot");
                                        return;
                                    }
                                }
                            }

                            let old_b = current_branch;
                            background_sync(
                                &root,
                                &sync_root,
                                Some(&old_b),
                                &new_branch,
                                &db,
                                &new_id,
                                &scan_cfg,
                                &detect_cfg,
                            );
                        });
                    })
                };

                tokio::spawn(async move {
                    // If auto-scan is in progress, wait for it to complete
                    // before starting the watcher.
                    wait_scan.wait_for_scan();

                    // Race guard: at the time of the outer `watcher_should_start`
                    // check, the auto-scan was still running. It may have
                    // failed during the wait; re-check before constructing
                    // the OS watcher (which recursively walks the tree).
                    if let Some(msg) = wait_scan.error_message() {
                        tracing::info!(
                            error_message = %msg,
                            "Auto-scan failed during watcher wait; not starting file watcher",
                        );
                        let _ = watcher_tx.send(Err(WatcherError::ScanFailed(msg)));
                        return;
                    }

                    let result = start_watcher(
                        params,
                        root,
                        db_p,
                        conn,
                        branch,
                        watcher_scan_config,
                        watcher_detection_config,
                        on_branch_switch,
                    )
                    .await;
                    if let Err(ref e) = result {
                        tracing::warn!(
                            "File watcher failed to start: {e}. \
                             Serving without incremental updates."
                        );
                    }
                    let _ = watcher_tx.send(result);
                });
                Some(watcher_rx)
            } else {
                None
            };

            // -- Print startup banner ------------------------------------
            // Branch order (guards against confusing messaging when a user
            // disables the watcher in config AND auto-scan also fails):
            //
            //   1. `!watcher_enabled` → "disabled" (config says so)
            //   2. scan failed         → "disabled (auto-scan failed: …)"
            //   3. scan still running  → "starting (after scan)"
            //   4. otherwise           → "starting"
            //
            // The scan-failure branch matches on `scan_error.is_some()`
            // alone (without requiring `has_auto_scan`) because the
            // AutoScan failure path sets `auto_scan_project_root = None`,
            // i.e. `has_auto_scan` flips to `false` precisely on failure.
            // `error_message().is_some()` only ever becomes true on the
            // AutoScan failure path — encoded as a `debug_assert!` below
            // so the invariant breaks loudly in tests if anything in
            // `ScanState` evolves to violate it.
            let watcher_status: std::borrow::Cow<'_, str> = if !watcher_enabled {
                std::borrow::Cow::Borrowed("disabled")
            } else if let Some(msg) = scan_state.error_message() {
                debug_assert!(
                    !has_auto_scan,
                    "scan_state.error_message().is_some() should imply has_auto_scan=false \
                     (the AutoScan failure branch sets auto_scan_project_root=None)"
                );
                std::borrow::Cow::Owned(format!("disabled (auto-scan failed: {msg})"))
            } else if has_auto_scan && !scan_state.auto_scanned() {
                std::borrow::Cow::Borrowed("starting (after scan)")
            } else {
                std::borrow::Cow::Borrowed("starting")
            };
            print_startup(
                &repo_info,
                &submodules,
                &config,
                call_log_path.as_deref(),
                &watcher_status,
                is_auto_scan,
                &detected_branch,
            );

            // -- Run MCP server -----------------------------------------
            let detached_head = final_branch.0.len() >= 7
                && final_branch.0.chars().all(|c| c.is_ascii_hexdigit());

            let shutdown = async {
                tokio::signal::ctrl_c()
                    .await
                    .expect("failed to listen for Ctrl+C");
                eprintln!();
                eprintln!("Shutting down...");
            };

            let result = seshat_mcp::start_stdio_with_shutdown(
                server_config,
                root,
                submodules,
                call_log_path,
                embedding_provider,
                scan_state,
                sync_in_progress.clone(),
                true,
                detached_head,
                project_root.clone(),
                shutdown,
                std::time::Duration::from_secs(5),
            )
            .await;

            // -- Shutdown GC background task ------------------------------
            drop(gc_handle);

            // -- Shutdown watcher ---------------------------------------
            if let Some(mut rx) = watcher_rx {
                if let Ok(Ok(handle)) = rx.try_recv() {
                    handle.shutdown().await;
                }
            }

            result
        })
        .map_err(|e| CliError::CommandFailed {
            command: "serve".to_owned(),
            reason: format!("MCP server error: {e}"),
        })
}

/// Load repository metadata from the database for startup display.
fn load_repo_info(db: &Database, db_path: &Path) -> Result<RepoInfo, CliError> {
    let name = db_path
        .file_stem()
        .map(|s| s.to_string_lossy().to_string())
        .unwrap_or_else(|| "unknown".to_owned());

    let info = crate::db::load_project_info(db);

    Ok(RepoInfo {
        name,
        db_path: db_path.to_path_buf(),
        branch: info.branch,
        file_count: info.file_count,
        convention_count: info.convention_count,
    })
}

/// Load the list of submodule rows from the root database.
///
/// Returns an empty `Vec` if the query fails (e.g. empty DB, no submodules
/// table data).
fn load_submodule_rows(db: &Database) -> Vec<SubmoduleRow> {
    let sub_repo = SqliteSubmoduleRepository::new(db.connection().clone());
    match sub_repo.list() {
        Ok(rows) => rows,
        Err(e) => {
            eprintln!(
                "  Warning: could not read submodules table: {e}. Continuing without submodules."
            );
            Vec::new()
        }
    }
}

/// Open database connections for each submodule and build the `ProjectConnection` map.
///
/// For each submodule row, resolves the DB path, opens the database, reads its
/// branch, and wraps it in a `ProjectConnection`. If a submodule DB is missing
/// or fails to open, a warning is logged and that submodule is skipped.
fn open_submodule_connections(
    rows: &[SubmoduleRow],
    root_project_name: &str,
) -> HashMap<String, ProjectConnection> {
    let mut submodules = HashMap::new();

    for row in rows {
        let db_path =
            match crate::db::resolve_submodule_db_path(root_project_name, &row.relative_path) {
                Ok(p) => p,
                Err(e) => {
                    eprintln!(
                        "  Warning: could not resolve DB path for submodule '{}': {e}. Skipping.",
                        row.relative_path
                    );
                    continue;
                }
            };

        if !db_path.exists() {
            eprintln!(
                "  Warning: submodule DB not found at '{}'. Skipping '{}'.",
                db_path.display(),
                row.relative_path
            );
            continue;
        }

        let db = match Database::open(&db_path) {
            Ok(d) => d,
            Err(e) => {
                eprintln!(
                    "  Warning: failed to open submodule DB '{}': {e}. Skipping '{}'.",
                    db_path.display(),
                    row.relative_path
                );
                continue;
            }
        };

        // Read the submodule's branch (default to "main" if not set).
        let branch_repo = SqliteBranchRepository::new(db.connection().clone());
        let branch = branch_repo.get_current_branch().unwrap_or_else(|_| {
            tracing::debug!("Could not detect submodule branch from DB, defaulting to 'main'");
            BranchId::from("main")
        });

        let pc = ProjectConnection::new(
            db.connection().clone(),
            row.relative_path.clone(),
            branch.to_string(),
        );

        submodules.insert(row.relative_path.clone(), pc);
    }

    submodules
}

/// Print the startup information block to stderr.
fn print_startup(
    info: &RepoInfo,
    submodules: &HashMap<String, ProjectConnection>,
    config: &AppConfig,
    call_log_path: Option<&Path>,
    watcher_status: &str,
    auto_scanning: bool,
    detected_branch: &str,
) {
    eprintln!("seshat v{}", env!("CARGO_PKG_VERSION"));
    eprintln!();
    eprintln!("  Repo:         {}", info.name);
    eprintln!("  Branch:       {}", detected_branch);
    if auto_scanning {
        eprintln!("  Files:        0 (auto-scanning...)");
    } else {
        eprintln!("  Files:        {}", info.file_count);
    }
    eprintln!("  Conventions:  {}", info.convention_count);
    eprintln!("  Database:     {}", info.db_path.display());
    eprintln!("  Watcher:      {watcher_status}");

    if submodules.is_empty() {
        eprintln!("  Submodules:   none");
    } else {
        eprintln!("  Submodules:   {}", submodules.len());
        let mut names: Vec<&String> = submodules.keys().collect();
        names.sort();
        for name in names {
            eprintln!("    - {name}");
        }
    }

    if let Some(path) = call_log_path {
        eprintln!("  Call log:     {}", path.display());
    }

    eprintln!();
    eprintln!(
        "  Transport:    stdio ({}:{})",
        config.server.host, config.server.port
    );
    eprintln!();
    eprintln!("Ready. Waiting for MCP client connection...");
}

#[cfg(test)]
mod tests {
    use super::*;
    use seshat_core::DetectionConfig;
    use std::collections::HashMap;

    #[test]
    fn load_repo_info_empty_db() {
        // Verify that load_repo_info works with an empty in-memory DB.
        let db = Database::open(":memory:").expect("in-memory db");
        let path = PathBuf::from("/tmp/test-seshat-project.db");
        let info = load_repo_info(&db, &path).expect("should succeed with empty db");
        assert_eq!(info.name, "test-seshat-project");
        assert_eq!(info.file_count, 0);
        assert_eq!(info.convention_count, 0);
        assert_eq!(info.branch, BranchId::from("main"));
    }

    #[test]
    fn load_submodule_rows_empty_db() {
        let db = Database::open(":memory:").expect("in-memory db");
        let rows = load_submodule_rows(&db);
        assert!(rows.is_empty());
    }

    #[test]
    fn load_submodule_rows_with_data() {
        use seshat_storage::{SqliteSubmoduleRepository, SubmoduleInput, SubmoduleRepository};

        let db = Database::open(":memory:").expect("in-memory db");
        let sub_repo = SqliteSubmoduleRepository::new(db.connection().clone());
        sub_repo
            .insert(&SubmoduleInput {
                relative_path: "vendor/libfoo".to_string(),
                name: "libfoo".to_string(),
                db_path: "/data/seshat/repos/proj/vendor/libfoo.db".to_string(),
                commit_hash: Some("abc123".to_string()),
            })
            .expect("insert");
        sub_repo
            .insert(&SubmoduleInput {
                relative_path: "libs/core".to_string(),
                name: "core".to_string(),
                db_path: "/data/seshat/repos/proj/libs/core.db".to_string(),
                commit_hash: Some("def456".to_string()),
            })
            .expect("insert");

        let rows = load_submodule_rows(&db);
        assert_eq!(rows.len(), 2);
        // list() returns sorted by relative_path
        assert_eq!(rows[0].relative_path, "libs/core");
        assert_eq!(rows[1].relative_path, "vendor/libfoo");
    }

    #[test]
    fn open_submodule_connections_empty_rows() {
        let submodules = open_submodule_connections(&[], "test-project");
        assert!(submodules.is_empty());
    }

    #[test]
    fn open_submodule_connections_missing_db_skipped() {
        let project_name = "serve-test-missing-db";

        let row = SubmoduleRow {
            id: 1,
            relative_path: "vendor/nonexistent".to_string(),
            name: "nonexistent".to_string(),
            db_path: "/no/such/path.db".to_string(),
            commit_hash: Some("abc123".to_string()),
            created_at: "2026-04-03T00:00:00".to_string(),
            updated_at: "2026-04-03T00:00:00".to_string(),
        };

        let submodules = open_submodule_connections(&[row], project_name);
        // Should be empty since the DB file doesn't exist.
        assert!(submodules.is_empty());

        // Clean up directories created as side effect of resolve_submodule_db_path.
        if let Ok(repos) = crate::db::xdg_repos_dir() {
            let _ = std::fs::remove_dir_all(repos.join(project_name));
        }
    }

    #[test]
    fn resolve_call_log_bare_flag_uses_default_path() {
        // --call-log with no value → default_missing_value="" → empty PathBuf
        let result = resolve_call_log_path(Some(PathBuf::from("")), None);
        let path = result.expect("should resolve to default path");
        // Normalize path separators so the assertion holds on Windows where
        // PathBuf renders as `…\seshat\call-log.jsonl`.
        let normalized = path.to_string_lossy().replace('\\', "/");
        assert!(
            normalized.ends_with("seshat/call-log.jsonl"),
            "expected default path to end with seshat/call-log.jsonl, got {normalized}"
        );
    }

    #[test]
    fn resolve_call_log_explicit_path() {
        let result = resolve_call_log_path(Some(PathBuf::from("/tmp/my-log.jsonl")), None);
        assert_eq!(result, Some(PathBuf::from("/tmp/my-log.jsonl")));
    }

    #[test]
    fn resolve_call_log_from_config() {
        let result = resolve_call_log_path(None, Some("/config/path.jsonl"));
        assert_eq!(result, Some(PathBuf::from("/config/path.jsonl")));
    }

    #[test]
    fn resolve_call_log_cli_overrides_config() {
        let result = resolve_call_log_path(
            Some(PathBuf::from("/cli/path.jsonl")),
            Some("/config/path.jsonl"),
        );
        assert_eq!(result, Some(PathBuf::from("/cli/path.jsonl")));
    }

    #[test]
    fn resolve_call_log_disabled_when_no_flag_and_no_config() {
        let result = resolve_call_log_path(None, None);
        assert!(result.is_none());
    }

    #[test]
    fn open_submodule_connections_with_real_dbs() {
        use std::fs;

        let project_name = "serve-test-submod";
        let mount_path = "vendor/testlib";

        // resolve_submodule_db_path creates the DB in the real XDG data dir
        // (required because open_submodule_connections resolves paths itself).
        let db_path =
            crate::db::resolve_submodule_db_path(project_name, mount_path).expect("resolve path");

        // RAII guard: clean up the XDG directory on drop (even on panic).
        struct Cleanup(PathBuf);
        impl Drop for Cleanup {
            fn drop(&mut self) {
                let _ = fs::remove_dir_all(&self.0);
            }
        }
        let repos_dir = crate::db::xdg_repos_dir().expect("xdg repos dir");
        let _guard = Cleanup(repos_dir.join(project_name));

        let db = Database::open(&db_path).expect("create submodule DB");
        drop(db);

        let row = SubmoduleRow {
            id: 1,
            relative_path: mount_path.to_string(),
            name: "testlib".to_string(),
            db_path: db_path.to_string_lossy().to_string(),
            commit_hash: Some("abc123".to_string()),
            created_at: "2026-04-03T00:00:00".to_string(),
            updated_at: "2026-04-03T00:00:00".to_string(),
        };

        let submodules = open_submodule_connections(&[row], project_name);
        assert_eq!(submodules.len(), 1);
        assert!(submodules.contains_key(mount_path));

        let pc = &submodules[mount_path];
        assert_eq!(pc.name, mount_path);
        assert_eq!(pc.branch, "main"); // default branch for empty DB
        // _guard drops here, cleaning up the project dir.
    }

    // ── handle_auto_scan_snapshot ─────────────────────────────────────

    #[test]
    fn handle_auto_scan_snapshot_main_branch_no_op() {
        let db = Database::open(":memory:").expect("in-memory db");
        let result = handle_auto_scan_snapshot(&db, "main").expect("should succeed");
        assert_eq!(result, BranchId::from("main"));
    }

    // ── print_startup ─────────────────────────────────────────────────

    #[test]
    fn print_startup_does_not_panic() {
        let repos_dir = crate::db::xdg_repos_dir().expect("xdg repos dir");
        let _ = std::fs::create_dir_all(&repos_dir);
        let info = RepoInfo {
            name: "test-project".to_string(),
            db_path: PathBuf::from("/tmp/test.db"),
            file_count: 5,
            convention_count: 42,
            branch: BranchId::from("main"),
        };
        let config = AppConfig::load().unwrap_or_default();
        print_startup(
            &info,
            &HashMap::new(),
            &config,
            None,
            "running",
            false,
            "main",
        );
    }

    // ── RepoInfo ──────────────────────────────────────────────────────

    #[test]
    fn repo_info_default_name_extraction() {
        let info = RepoInfo {
            name: "my-awesome-project".to_string(),
            db_path: PathBuf::from("/tmp/test.db"),
            file_count: 10,
            convention_count: 20,
            branch: BranchId::from("feat/bar"),
        };
        assert_eq!(info.name, "my-awesome-project");
        assert_eq!(info.file_count, 10);
        assert_eq!(info.convention_count, 20);
        assert_eq!(info.branch, BranchId::from("feat/bar"));
    }

    // ── fallback_rescan ───────────────────────────────────────────────

    #[test]
    fn fallback_rescan_empty_dir_handles_gracefully() {
        use tempfile::tempdir;
        let dir = tempdir().expect("tempdir");
        let db = Database::open(":memory:").expect("in-memory db");
        let branch = BranchId::from("main");
        // Empty dir — fallback_rescan should log warnings but not panic.
        fallback_rescan(
            dir.path(),
            &db,
            &branch,
            &ScanConfig::default(),
            &DetectionConfig::default(),
        );
    }

    // ── resolve_branch_tree_paths ─────────────────────────────────────

    #[test]
    fn resolve_branch_tree_paths_not_a_git_repo_returns_none() {
        use tempfile::tempdir;
        let dir = tempdir().expect("tempdir");
        let result = resolve_branch_tree_paths(dir.path(), "main");
        assert!(result.is_none());
    }

    // ── handle_branch_switch ───────────────────────────────────────────

    fn seed_branch(db: &Database, branch_name: &str) -> BranchId {
        let branch = BranchId::from(branch_name);
        let br = SqliteBranchRepository::new(db.connection().clone());
        br.switch_branch(&branch).unwrap();
        // Insert a node so list_branches returns this branch.
        let c = db.connection().lock().unwrap();
        c.execute(
            "INSERT INTO nodes (branch_id, nature, weight, confidence, adoption_count, total_count, description, ext_data)
             VALUES (?1, 'convention', 'strong', 0.9, 5, 10, 'test', '{\"source\":\"auto_detected\"}')",
            rusqlite::params![branch_name],
        ).unwrap();
        branch
    }

    #[test]
    fn handle_branch_switch_same_branch_returns_current() {
        let db = Database::open(":memory:").expect("in-memory db");
        let current = BranchId::from("main");
        let result = handle_branch_switch(&db, "main", &current, false).unwrap();
        assert_eq!(result, current);
    }

    #[test]
    fn handle_branch_switch_target_has_data_no_snapshot() {
        let db = Database::open(":memory:").expect("in-memory db");
        let current = BranchId::from("main");
        seed_branch(&db, "feat/test");
        let result = handle_branch_switch(&db, "feat/test", &current, false).unwrap();
        assert_eq!(result, BranchId::from("feat/test"));
    }

    #[test]
    fn handle_branch_switch_source_no_data_still_switches() {
        let db = Database::open(":memory:").expect("in-memory db");
        let current = BranchId::from("main");
        let result = handle_branch_switch(&db, "feat/empty", &current, false).unwrap();
        assert_eq!(result, BranchId::from("feat/empty"));
    }

    #[test]
    fn handle_branch_switch_source_has_data_creates_snapshot() {
        let db = Database::open(":memory:").expect("in-memory db");
        let current = BranchId::from("main");
        seed_branch(&db, "main");
        let result = handle_branch_switch(&db, "feat/snap", &current, false).unwrap();
        assert_eq!(result, BranchId::from("feat/snap"));
        // Snapshot created — verify feat/snap now has nodes.
        let br = SqliteBranchRepository::new(db.connection().clone());
        let branches = br.list_branches().unwrap();
        assert!(branches.iter().any(|b| b.0 == "feat/snap"));
    }

    // ── handle_auto_scan_snapshot ───────────────────────────────────────

    #[test]
    fn auto_scan_snapshot_non_main_no_main_data_still_switches() {
        let db = Database::open(":memory:").expect("in-memory db");
        let result = handle_auto_scan_snapshot(&db, "feat/bar").unwrap();
        assert_eq!(result, BranchId::from("feat/bar"));
    }

    #[test]
    fn auto_scan_snapshot_non_main_with_main_data_creates_snapshot() {
        let db = Database::open(":memory:").expect("in-memory db");
        seed_branch(&db, "main");
        let result = handle_auto_scan_snapshot(&db, "feat/baz").unwrap();
        assert_eq!(result, BranchId::from("feat/baz"));
        let br = SqliteBranchRepository::new(db.connection().clone());
        let branches = br.list_branches().unwrap();
        assert!(branches.iter().any(|b| b.0 == "feat/baz"));
    }

    // ── watcher_should_start (P0 guardrail, US-004) ───────────────────

    #[test]
    fn watcher_should_start_disabled_returns_false_regardless_of_scan_state() {
        // Even with a healthy scan_state, a disabled config blocks the watcher.
        let state_ok = ScanState::not_needed();
        assert!(!watcher_should_start(false, &state_ok));

        let state_complete = ScanState::in_progress();
        state_complete.mark_complete();
        assert!(!watcher_should_start(false, &state_complete));
    }

    #[test]
    fn watcher_should_start_enabled_with_no_scan_returns_true() {
        // ExistingDb path: ScanState::not_needed() — no auto-scan ran,
        // watcher should start as before this guardrail existed.
        let state = ScanState::not_needed();
        assert!(watcher_should_start(true, &state));
    }

    #[test]
    fn watcher_should_start_enabled_with_completed_scan_returns_true() {
        // AutoScan happy path: scan finished successfully → watcher starts.
        let state = ScanState::in_progress();
        state.mark_complete();
        assert!(watcher_should_start(true, &state));
    }

    #[test]
    fn watcher_should_start_enabled_with_in_progress_scan_returns_true() {
        // AutoScan in-progress path: outer gate decides to spawn the
        // watcher task NOW; the spawned task waits for completion via
        // wait_for_scan() and re-checks error_message() before walking.
        let state = ScanState::in_progress();
        assert!(watcher_should_start(true, &state));
    }

    #[test]
    fn watcher_should_start_enabled_with_failed_scan_returns_false() {
        // P0: this is the bug class US-004 closes. A failed auto-scan
        // means we must NOT construct notify-debouncer-full / walk the tree.
        let state = ScanState::in_progress();
        state.mark_failed("project too large".to_owned());
        assert!(!watcher_should_start(true, &state));
    }

    #[test]
    fn watcher_should_start_disabled_with_failed_scan_returns_false() {
        // Belt-and-suspenders: both gates closed.
        let state = ScanState::in_progress();
        state.mark_failed("scan timeout".to_owned());
        assert!(!watcher_should_start(false, &state));
    }

    // ── Race-guard re-check inside spawned watcher task (FR-5) ─────────
    //
    // The outer `watcher_should_start` gate may pass while scan is still
    // `InProgress` — the spawned task then `wait_for_scan()`s. If the scan
    // transitions to `Failed` during that wait, the inner re-check
    // (`error_message().is_some()`) must catch it BEFORE `start_watcher`
    // gets to construct `notify-debouncer-full` and walk the tree.
    //
    // We can't drive the actual `tokio::spawn` block from here without
    // standing up the full serve flow, so we exercise the underlying
    // `ScanState` synchronisation pattern directly.

    #[test]
    fn race_guard_pattern_detects_pre_wait_failure() {
        // Failure was set BEFORE wait_for_scan returns: the post-wait
        // error_message() check must surface it.
        let state = ScanState::in_progress();
        state.mark_failed("simulated pre-wait failure".to_owned());
        state.wait_for_scan(); // returns immediately — not InProgress anymore
        assert_eq!(
            state.error_message(),
            Some("simulated pre-wait failure".to_owned())
        );
    }

    #[test]
    fn race_guard_pattern_returns_none_for_normal_completion() {
        let state = ScanState::in_progress();
        state.mark_complete();
        state.wait_for_scan();
        assert_eq!(state.error_message(), None);
    }

    #[test]
    fn race_guard_pattern_observes_failure_set_during_wait() {
        // Honest race test: thread A enters wait_for_scan while state is
        // InProgress; thread B then mark_fails. A must wake up, observe
        // the failure via error_message(), and return Some(reason).
        use std::sync::Arc;
        use std::thread;
        use std::time::Duration;

        let state = ScanState::in_progress();
        let waiter_state = state.clone();
        let observed: Arc<std::sync::Mutex<Option<String>>> = Arc::new(std::sync::Mutex::new(None));
        let observed_for_thread = Arc::clone(&observed);
        let waiter = thread::spawn(move || {
            waiter_state.wait_for_scan();
            *observed_for_thread.lock().expect("lock") = waiter_state.error_message();
        });

        // Give the waiter time to enter `wait_for_scan` and park on the
        // condvar. A short sleep is fine because the waiter blocks until
        // we notify via mark_failed.
        thread::sleep(Duration::from_millis(50));
        state.mark_failed("simulated late failure".to_owned());

        waiter.join().expect("waiter thread join");
        let captured = observed.lock().expect("lock").clone();
        assert_eq!(captured, Some("simulated late failure".to_owned()));
    }
}