feat: wal_checkpoint(): Do wal_checkpoint(PASSIVE) and wal_checkpoint(FULL) before wal_checkpoint(TRUNCATE)

This way the subsequent `wal_checkpoint(TRUNCATE)` is faster. We don't want to block writers and
readers for a long period.

src/sql.rs

@@ -3,7 +3,7 @@
 use std::collections::{HashMap, HashSet};
 use std::path::{Path, PathBuf};
 
-use anyhow::{Context as _, Result, bail};
+use anyhow::{Context as _, Result, bail, ensure};
 use rusqlite::{Connection, OpenFlags, Row, config::DbConfig, types::ValueRef};
 use tokio::sync::RwLock;
 
@@ -13,7 +13,6 @@ use crate::config::Config;
 use crate::constants::DC_CHAT_ID_TRASH;
 use crate::context::Context;
 use crate::debug_logging::set_debug_logging_xdc;
-use crate::ensure_and_debug_assert;
 use crate::ephemeral::start_ephemeral_timers;
 use crate::imex::BLOBS_BACKUP_NAME;
 use crate::location::delete_orphaned_poi_locations;
@@ -24,7 +23,7 @@ use crate::net::http::http_cache_cleanup;
 use crate::net::prune_connection_history;
 use crate::param::{Param, Params};
 use crate::stock_str;
-use crate::tools::{SystemTime, delete_file, time};
+use crate::tools::{SystemTime, Time, delete_file, time, time_elapsed};
 
 /// Extension to [`rusqlite::ToSql`] trait
 /// which also includes [`Send`] and [`Sync`].
@@ -180,7 +179,7 @@ impl Sql {
 
     /// Creates a new connection pool.
     fn new_pool(dbfile: &Path, passphrase: String) -> Result<Pool> {
-        let mut connections = Vec::new();
+        let mut connections = Vec::with_capacity(Self::N_DB_CONNECTIONS);
         for _ in 0..Self::N_DB_CONNECTIONS {
             let connection = new_connection(dbfile, &passphrase)?;
             connections.push(connection);
@@ -642,28 +641,74 @@ impl Sql {
     }
 
     /// Runs a checkpoint operation in TRUNCATE mode, so the WAL file is truncated to 0 bytes.
-    pub(crate) async fn wal_checkpoint(&self) -> Result<()> {
-        let lock = self.pool.read().await;
-        let pool = lock.as_ref().context("No SQL connection pool")?;
-        let mut conns = Vec::new();
+    pub(crate) async fn wal_checkpoint(context: &Context) -> Result<()> {
+        let t_start = Time::now();
+        let lock = context.sql.pool.read().await;
+        let Some(pool) = lock.as_ref() else {
+            // No db connections, nothing to checkpoint.
+            return Ok(());
+        };
+
+        // Do as much work as possible without blocking anybody.
         let query_only = true;
+        let conn = pool.get(query_only).await?;
+        tokio::task::block_in_place(|| {
+            // Execute some transaction causing the WAL file to be opened so that the
+            // `wal_checkpoint()` can proceed, otherwise it fails when called the first time,
+            // see https://sqlite.org/forum/forumpost/7512d76a05268fc8.
+            conn.query_row("PRAGMA table_list", [], |_| Ok(()))?;
+            conn.query_row("PRAGMA wal_checkpoint(PASSIVE)", [], |_| Ok(()))
+        })?;
+
+        // Kick out writers.
+        const _: () = assert!(Sql::N_DB_CONNECTIONS > 1, "Deadlock possible");
+        let _write_lock = pool.write_lock().await;
+        let t_writers_blocked = Time::now();
+        // Ensure that all readers use the most recent database snapshot (are at the end of WAL) so
+        // that `wal_checkpoint(FULL)` isn't blocked. We could use `PASSIVE` as well, but it's
+        // documented poorly, https://www.sqlite.org/pragma.html#pragma_wal_checkpoint and
+        // https://www.sqlite.org/c3ref/wal_checkpoint_v2.html don't tell how it interacts with new
+        // readers.
+        let mut read_conns = Vec::with_capacity(Self::N_DB_CONNECTIONS - 1);
+        for _ in 0..(Self::N_DB_CONNECTIONS - 1) {
+            read_conns.push(pool.get(query_only).await?);
+        }
+        read_conns.clear();
+        // Checkpoint the remaining WAL pages without blocking readers.
+        let (pages_total, pages_checkpointed) = tokio::task::block_in_place(|| {
+            conn.query_row("PRAGMA wal_checkpoint(FULL)", [], |row| {
+                let pages_total: i64 = row.get(1)?;
+                let pages_checkpointed: i64 = row.get(2)?;
+                Ok((pages_total, pages_checkpointed))
+            })
+        })?;
+        if pages_checkpointed < pages_total {
+            warn!(
+                context,
+                "Cannot checkpoint whole WAL. Pages total: {pages_total}, checkpointed: {pages_checkpointed}. Make sure there are no external connections running transactions.",
+            );
+        }
         // Kick out readers to avoid blocking/SQLITE_BUSY.
         for _ in 0..(Self::N_DB_CONNECTIONS - 1) {
-            conns.push(pool.get(query_only).await?);
+            read_conns.push(pool.get(query_only).await?);
         }
-        let conn = pool.get(query_only).await?;
-        tokio::task::block_in_place(move || {
-            // Execute some transaction causing the WAL file to be opened so that the
-            // `wal_checkpoint()` can proceed, otherwise it fails when called the first time, see
-            // https://sqlite.org/forum/forumpost/7512d76a05268fc8.
-            conn.query_row("PRAGMA table_list", [], |_row| Ok(()))?;
+        let t_readers_blocked = Time::now();
+        tokio::task::block_in_place(|| {
             let blocked = conn.query_row("PRAGMA wal_checkpoint(TRUNCATE)", [], |row| {
                 let blocked: i64 = row.get(0)?;
                 Ok(blocked)
             })?;
-            ensure_and_debug_assert!(blocked == 0,);
+            ensure!(blocked == 0);
             Ok(())
-        })
+        })?;
+        info!(
+            context,
+            "wal_checkpoint: Total time: {:?}. Writers blocked for: {:?}. Readers blocked for: {:?}.",
+            time_elapsed(&t_start),
+            time_elapsed(&t_writers_blocked),
+            time_elapsed(&t_readers_blocked),
+        );
+        Ok(())
     }
 }
 
@@ -792,8 +837,9 @@ pub async fn housekeeping(context: &Context) -> Result<()> {
     // bigger than 200M) and also make sure we truncate the WAL periodically. Auto-checkponting does
     // not normally truncate the WAL (unless the `journal_size_limit` pragma is set), see
     // https://www.sqlite.org/wal.html.
-    if let Err(err) = context.sql.wal_checkpoint().await {
+    if let Err(err) = Sql::wal_checkpoint(context).await {
         warn!(context, "wal_checkpoint() failed: {err:#}.");
+        debug_assert!(false);
     }
 
     context

src/sql/pool.rs

@@ -67,7 +67,7 @@ struct InnerPool {
     ///
     /// This mutex is locked when write connection
     /// is outside the pool.
-    write_mutex: Arc<Mutex<()>>,
+    pub(crate) write_mutex: Arc<Mutex<()>>,
 }
 
 impl InnerPool {
@@ -96,13 +96,13 @@ impl InnerPool {
                     .pop()
                     .context("Got a permit when there are no connections in the pool")?
             };
-            conn.pragma_update(None, "query_only", "1")?;
             let conn = PooledConnection {
                 pool: Arc::downgrade(&self),
                 conn: Some(conn),
                 _permit: permit,
                 _write_mutex_guard: None,
             };
+            conn.pragma_update(None, "query_only", "1")?;
             Ok(conn)
         } else {
             // We get write guard first to avoid taking a permit
@@ -119,13 +119,13 @@ impl InnerPool {
                     "Got a permit and write lock when there are no connections in the pool",
                 )?
             };
-            conn.pragma_update(None, "query_only", "0")?;
             let conn = PooledConnection {
                 pool: Arc::downgrade(&self),
                 conn: Some(conn),
                 _permit: permit,
                 _write_mutex_guard: Some(write_mutex_guard),
             };
+            conn.pragma_update(None, "query_only", "0")?;
             Ok(conn)
         }
     }
@@ -195,4 +195,12 @@ impl Pool {
     pub async fn get(&self, query_only: bool) -> Result<PooledConnection> {
         Arc::clone(&self.inner).get(query_only).await
     }
+
+    /// Returns a mutex guard guaranteeing that there are no concurrent write connections.
+    ///
+    /// NB: Make sure you're not holding all connections when calling this, otherwise it deadlocks
+    /// if there is a concurrent writer waiting for available connection.
+    pub(crate) async fn write_lock(&self) -> OwnedMutexGuard<()> {
+        Arc::clone(&self.inner.write_mutex).lock_owned().await
+    }
 }