//! Background shadow-write task — persists pipeline items to SQLite asynchronously.
//!
//! `init` opens the database, runs migrations, loads existing content into the
//! in-memory store, and spawns the write loop.  All subsequent writes are sent
//! over an unbounded channel so callers never block on I/O.
//!
//! The opened pool is stored in [`SHARED_POOL`] so that other subsystems
//! (event-trigger store, timer store, scheduled-timer store) can share the
//! same database connection without re-opening the file.
use crate::slog;
use sqlx::SqlitePool;
use sqlx::sqlite::SqliteConnectOptions;
use std::collections::HashMap;
use std::collections::HashSet;
use std::path::Path;
use std::sync::OnceLock;
use tokio::sync::mpsc;

/// One migration row in the live database that is not in the compiled-in set.
///
/// Returned by [`check_schema_drift`] for each unknown migration.
pub struct UnknownMigration {
    /// sqlx migration version number (derived from the filename timestamp).
    pub version: i64,
    /// Human-readable description from the migration filename.
    pub description: String,
    /// When the migration was applied, as stored in `_sqlx_migrations.installed_on`.
    pub installed_on: String,
}

/// The process-global SQLite pool, set once by [`init`].
///
/// Other modules call [`get_shared_pool`] to access the pool without needing
/// to pass it through every call-site.
static SHARED_POOL: OnceLock<SqlitePool> = OnceLock::new();

/// Return a reference to the shared pipeline database pool, if it has been
/// initialised by a prior call to [`init`].
pub fn get_shared_pool() -> Option<&'static SqlitePool> {
    SHARED_POOL.get()
}

/// A pending shadow write for one pipeline item.
pub(super) struct PipelineWriteMsg {
    pub(super) story_id: String,
    pub(super) stage: String,
    pub(super) name: Option<String>,
    pub(super) agent: Option<String>,
    pub(super) retry_count: Option<i64>,
    pub(super) depends_on: Option<String>,
    pub(super) content: Option<String>,
}

/// Handle to the background shadow-write task.
pub struct PipelineDb {
    pub(super) tx: mpsc::UnboundedSender<PipelineWriteMsg>,
}

/// Process-global handle to the background shadow-write task, set once during `init`.
pub(super) static PIPELINE_DB: OnceLock<PipelineDb> = OnceLock::new();

/// Initialise the pipeline database.
///
/// Opens (or creates) the SQLite file at `db_path`, runs embedded migrations,
/// loads existing story content into the in-memory store, and spawns the
/// background write task.  Safe to call only once; subsequent calls are no-ops.
pub async fn init(db_path: &Path) -> Result<(), sqlx::Error> {
    if PIPELINE_DB.get().is_some() {
        return Ok(());
    }

    // Story 1087: before running the migration that splits `stage` into
    // (`pipeline`, `status`), take a timestamped side-car copy of the live DB
    // so the pre-split state is recoverable.  Skip the copy when the file does
    // not yet exist (fresh installs) or when the split-stage migration has
    // already been applied (subsequent restarts).
    backup_pre_pipeline_status(db_path).await;

    let options = SqliteConnectOptions::new()
        .filename(db_path)
        .create_if_missing(true);

    let pool = SqlitePool::connect_with(options).await?;
    sqlx::migrate!("./migrations").run(&pool).await?;

    // Store pool in global static so other subsystems can reuse it.
    let _ = SHARED_POOL.set(pool.clone());

    // Load existing content into the in-memory store.
    let rows: Vec<(String, Option<String>)> =
        sqlx::query_as("SELECT id, content FROM pipeline_items WHERE content IS NOT NULL")
            .fetch_all(&pool)
            .await?;

    let mut content_map = HashMap::new();
    for (id, content) in rows {
        if let Some(c) = content {
            content_map.insert(id, c);
        }
    }
    super::content_store::init_content_store(content_map);

    let (tx, mut rx) = mpsc::unbounded_channel::<PipelineWriteMsg>();

    tokio::spawn(async move {
        while let Some(msg) = rx.recv().await {
            // The "deleted" sentinel means the caller wants the row gone.
            // Issue a real DELETE so the shadow table stays clean and
            // sync_crdt_stages_from_db cannot resurrect a tombstoned item on
            // the next restart.
            if msg.stage == "deleted" {
                let result = sqlx::query("DELETE FROM pipeline_items WHERE id = ?1")
                    .bind(&msg.story_id)
                    .execute(&pool)
                    .await;
                if let Err(e) = result {
                    slog!("[db] Shadow delete failed for '{}': {e}", msg.story_id);
                }
                continue;
            }

            let now = chrono::Utc::now().to_rfc3339();
            let result = sqlx::query(
                "INSERT INTO pipeline_items \
                     (id, name, stage, agent, retry_count, depends_on, content, created_at, updated_at) \
                 VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?8) \
                 ON CONFLICT(id) DO UPDATE SET \
                     name        = excluded.name, \
                     stage       = excluded.stage, \
                     agent       = excluded.agent, \
                     retry_count = excluded.retry_count, \
                     depends_on  = excluded.depends_on, \
                     content     = COALESCE(excluded.content, pipeline_items.content), \
                     updated_at  = excluded.updated_at",
            )
            .bind(&msg.story_id)
            .bind(&msg.name)
            .bind(&msg.stage)
            .bind(&msg.agent)
            .bind(msg.retry_count)
            .bind(&msg.depends_on)
            .bind(&msg.content)
            .bind(&now)
            .execute(&pool)
            .await;

            if let Err(e) = result {
                slog!("[db] Shadow write failed for '{}': {e}", msg.story_id);
            }
        }
    });

    let _ = PIPELINE_DB.set(PipelineDb { tx });
    Ok(())
}

/// Story 1087: file name of the split-stage migration.  The version prefix is
/// the same `i64` sqlx assigns to that migration on `installed_on` rows in
/// `_sqlx_migrations`.
const SPLIT_STAGE_MIGRATION_VERSION: i64 = 20260515000000;

/// Story 1087: take a timestamped side-car copy of `pipeline.db` if and only if
/// the split-stage migration has not yet been applied.  This is the AC1 backup
/// — `pipeline.db.pre-pipeline-status.<unix-ts>.bak` next to the live file.
///
/// Failures are logged but never propagated: a missing backup must not block
/// the server from starting (a corrupt source file or a read-only directory
/// will be surfaced by the migration step itself).
pub(crate) async fn backup_pre_pipeline_status(db_path: &Path) {
    if !db_path.exists() {
        return;
    }

    // Cheap pre-check: open the DB read-only and see whether the split-stage
    // migration version is recorded in `_sqlx_migrations`.  If it is, the
    // backup has already been taken on a previous start and there is nothing
    // to do.
    let options = SqliteConnectOptions::new()
        .filename(db_path)
        .read_only(true)
        .create_if_missing(false);

    let probe = SqlitePool::connect_with(options).await;
    if let Ok(pool) = probe {
        let already_split: Result<Option<(i64,)>, _> =
            sqlx::query_as("SELECT version FROM _sqlx_migrations WHERE version = ?1 LIMIT 1")
                .bind(SPLIT_STAGE_MIGRATION_VERSION)
                .fetch_optional(&pool)
                .await;
        pool.close().await;
        if let Ok(Some(_)) = already_split {
            return;
        }
    }

    let ts = chrono::Utc::now().timestamp();
    let mut backup = db_path.as_os_str().to_owned();
    backup.push(format!(".pre-pipeline-status.{ts}.bak"));
    let backup_path = std::path::PathBuf::from(backup);

    match tokio::fs::copy(db_path, &backup_path).await {
        Ok(_) => slog!(
            "[db] Wrote pre-pipeline-status backup of {} to {}",
            db_path.display(),
            backup_path.display(),
        ),
        Err(e) => slog!(
            "[db] Failed to write pre-pipeline-status backup of {}: {e}",
            db_path.display(),
        ),
    }
}

/// Compare the live `_sqlx_migrations` table against the compiled-in migration
/// set and return any rows whose version is not known to this binary.
///
/// A non-empty result means the database was previously opened by a newer
/// binary that applied additional migrations.  The server must refuse to start
/// in that state because the schema may contain tables or columns that this
/// binary does not understand.
pub async fn check_schema_drift(pool: &SqlitePool) -> Vec<UnknownMigration> {
    let migrator = sqlx::migrate!("./migrations");
    let known: HashSet<i64> = migrator.migrations.iter().map(|m| m.version).collect();

    let rows: Vec<(i64, String, String)> = sqlx::query_as(
        "SELECT version, description, installed_on FROM _sqlx_migrations ORDER BY version",
    )
    .fetch_all(pool)
    .await
    .unwrap_or_default();

    rows.into_iter()
        .filter(|(v, _, _)| !known.contains(v))
        .map(|(version, description, installed_on)| UnknownMigration {
            version,
            description,
            installed_on,
        })
        .collect()
}