refactor: move WAL checkpointing into sql::pool submodule

This change is mainly to avoid exposing the write lock outside the pool module.
To avoid deadlocks, outside code should work only with the pooled connections
and use no more than one connection per thread.

src/sql/pool.rs

@@ -51,6 +51,9 @@ use anyhow::{Context, Result};
 use rusqlite::Connection;
 use tokio::sync::{Mutex, OwnedMutexGuard, OwnedSemaphorePermit, Semaphore};
 
+mod wal_checkpoint;
+pub(crate) use wal_checkpoint::WalCheckpointStats;
+
 /// Inner connection pool.
 #[derive(Debug)]
 struct InnerPool {
@@ -196,11 +199,8 @@ impl Pool {
         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
+    /// Truncates the WAL file.
+    pub(crate) async fn wal_checkpoint(&self) -> Result<WalCheckpointStats> {
+        wal_checkpoint::wal_checkpoint(self).await
     }
 }