fix(event cache store): always use immediate mode when handling linke…

…d chunk updates

If a linked chunk update starts with a RemoveChunk update, then the
transaction may start with a SELECT query and be considered a read
transaction. Soon enough, it will be upgraded into a write transaction,
because of the next UPDATE/DELETE operations that happen thereafter. If
there's another write transaction already happening, this may result in
a SQLITE_BUSY error, according to
https://www.sqlite.org/lang_transaction.html#deferred_immediate_and_exclusive_transactions

One solution is to always start the transaction in immediate mode. This
may also fail with SQLITE_BUSY according to the documentation, but it's
unclear whether it will happen in general, since we're using WAL mode
too. Let's try it out.

crates/matrix-sdk-sqlite/src/event_cache_store.rs

@@ -27,7 +27,7 @@ use matrix_sdk_base::{
 };
 use matrix_sdk_store_encryption::StoreCipher;
 use ruma::{MilliSecondsSinceUnixEpoch, RoomId};
-use rusqlite::{OptionalExtension, Transaction};
+use rusqlite::{OptionalExtension, Transaction, TransactionBehavior};
 use tokio::fs;
 use tracing::{debug, trace};
 
@@ -378,9 +378,7 @@ impl EventCacheStore for SqliteEventCacheStore {
         let room_id = room_id.to_owned();
         let this = self.clone();
 
-        self.acquire()
-            .await?
-            .with_transaction(move |txn| -> Result<_, Self::Error> {
+        with_immediate_transaction(self.acquire().await?, move |txn| {
                 for up in updates {
                     match up {
                         Update::NewItemsChunk { previous, new, next } => {
@@ -394,7 +392,7 @@ impl EventCacheStore for SqliteEventCacheStore {
                             );
 
                             insert_chunk(
-                                txn,
+                                &txn,
                                 &hashed_room_id,
                                 previous,
                                 new,
@@ -418,7 +416,7 @@ impl EventCacheStore for SqliteEventCacheStore {
 
                             // Insert the chunk as a gap.
                             insert_chunk(
-                                txn,
+                                &txn,
                                 &hashed_room_id,
                                 previous,
                                 new,
@@ -709,6 +707,43 @@ impl EventCacheStore for SqliteEventCacheStore {
     }
 }
 
+/// Like `deadpool::managed::Object::with_transaction`, but starts the
+/// transaction in immediate (write) mode from the beginning, precluding errors
+/// of the kind SQLITE_BUSY from happening, for transactions that may involve
+/// both reads and writes, and start with a write.
+async fn with_immediate_transaction<
+    T: Send + 'static,
+    F: FnOnce(&Transaction<'_>) -> Result<T, Error> + Send + 'static,
+>(
+    conn: SqliteAsyncConn,
+    f: F,
+) -> Result<T, Error> {
+    conn.interact(move |conn| -> Result<T, Error> {
+        // Start the transaction in IMMEDIATE mode since all updates may cause writes,
+        // to avoid read transactions upgrading to write mode and causing
+        // SQLITE_BUSY errors. See also: https://www.sqlite.org/lang_transaction.html#deferred_immediate_and_exclusive_transactions
+        conn.set_transaction_behavior(TransactionBehavior::Immediate);
+
+        let code = || -> Result<T, Error> {
+            let txn = conn.transaction()?;
+            let res = f(&txn)?;
+            txn.commit()?;
+            Ok(res)
+        };
+
+        let res = code();
+
+        // Reset the transaction behavior to use Deferred, after this transaction has
+        // been run, whether it was successful or not.
+        conn.set_transaction_behavior(TransactionBehavior::Deferred);
+
+        res
+    })
+    .await
+    // SAFETY: same logic as in [`deadpool::managed::Object::with_transaction`].`
+    .unwrap()
+}
+
 fn insert_chunk(
     txn: &Transaction<'_>,
     room_id: &Key,