From edb219ecfece6af744ec0dedd17326b46f5fec45 Mon Sep 17 00:00:00 2001
From: Simon Binder <oss@simonbinder.eu>
Date: Wed, 31 Mar 2021 23:09:23 +0200
Subject: [PATCH] Use Future.delayed in cancellation zones only

---
 moor/CHANGELOG.md                                  |  2 +-
 moor/lib/src/runtime/cancellation_zone.dart        |  3 +++
 moor/lib/src/runtime/executor/helpers/engines.dart | 11 +++++++++--
 moor/lib/src/runtime/remote/server_impl.dart       |  5 -----
 4 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/moor/CHANGELOG.md b/moor/CHANGELOG.md
index c7d46afb..97cd8eea 100644
--- a/moor/CHANGELOG.md
+++ b/moor/CHANGELOG.md
@@ -1,7 +1,7 @@
 ## 4.3.0-dev
 
 - On supported platforms, cancel ongoing stream selects when the stream is disposed
-  - At the moment, `moor/ffi` on a background isolate supports this feature
+  - At the moment, `moor/ffi` and isolate databases are supported
 
 ## 4.2.1
 
diff --git a/moor/lib/src/runtime/cancellation_zone.dart b/moor/lib/src/runtime/cancellation_zone.dart
index b5996d45..11bea0d8 100644
--- a/moor/lib/src/runtime/cancellation_zone.dart
+++ b/moor/lib/src/runtime/cancellation_zone.dart
@@ -65,6 +65,9 @@ class CancellationException implements Exception {
   }
 }
 
+/// Returns true if we're currently in a zone that can be cancelled.
+bool get isInCancellationZone => Zone.current[_key] != null;
+
 /// Checks whether the active zone is a cancellation zone that has been
 /// cancelled. If it is, a [CancellationException] will be thrown.
 void checkIfCancelled() {
diff --git a/moor/lib/src/runtime/executor/helpers/engines.dart b/moor/lib/src/runtime/executor/helpers/engines.dart
index 0cd47eb8..3bb5c68d 100644
--- a/moor/lib/src/runtime/executor/helpers/engines.dart
+++ b/moor/lib/src/runtime/executor/helpers/engines.dart
@@ -22,8 +22,15 @@ mixin _ExecutorWithQueryDelegate on QueryExecutor {
 
   Future<T> _synchronized<T>(Future<T> Function() action) {
     if (isSequential) {
-      return _lock.synchronized(() {
-        checkIfCancelled();
+      return _lock.synchronized(() async {
+        if (isInCancellationZone) {
+          // Popular sequential database implementations (especially moor/ffi)
+          // are synchronous. Wait for one event loop iteration to give
+          // cancellations time to come in.
+          await Future.delayed(Duration.zero);
+          checkIfCancelled();
+        }
+
         return action();
       });
     } else {
diff --git a/moor/lib/src/runtime/remote/server_impl.dart b/moor/lib/src/runtime/remote/server_impl.dart
index 78d5195e..c010bf10 100644
--- a/moor/lib/src/runtime/remote/server_impl.dart
+++ b/moor/lib/src/runtime/remote/server_impl.dart
@@ -121,11 +121,6 @@ class ServerImplementation implements MoorServer {
 
   Future<dynamic> _runQuery(StatementMethod method, String sql,
       List<Object?> args, int? transactionId) async {
-    // Wait for one event loop iteration until we run the request. This helps
-    // with cancellations for synchronous database implementations like moor/ffi
-    // since we don't have a chance to react to cancellation requests otherwise.
-    await Future.delayed(Duration.zero);
-
     final executor = await _loadExecutor(transactionId);
 
     switch (method) {