Do not execute a query with an already cancelled token

Fixes #3379

src/Npgsql/NpgsqlCommand.cs

@@ -1141,6 +1141,10 @@ internal async ValueTask<NpgsqlDataReader> ExecuteReader(CommandBehavior behavio
             {
                 if (conn.TryGetBoundConnector(out var connector))
                 {
+                    cancellationToken.ThrowIfCancellationRequested();
+                    // We cannot pass a token here, as we'll cancel a non-send query
+                    // Also, we don't pass the cancellation token to StartUserAction, since that would make it scope to the entire action (command execution)
+                    // whereas it should only be scoped to the Execute method.
                     connector.StartUserAction(ConnectorState.Executing, this, CancellationToken.None);
 
                     Task? sendTask = null;

test/Npgsql.Tests/CommandTests.cs

@@ -288,6 +288,28 @@ public async Task Cancel()
             await cancelTask;
         }
 
+        [Test]
+        public async Task CancelAsyncImmediately()
+        {
+            if (IsMultiplexing)
+                return; // Multiplexing, cancellation
+
+            using var cts = new CancellationTokenSource();
+            cts.Cancel();
+
+            await using var conn = await OpenConnectionAsync();
+            using var cmd = conn.CreateCommand();
+            cmd.CommandText = "SELECT 1";
+
+            var t = cmd.ExecuteScalarAsync(cts.Token);
+            Assert.That(t.IsCompleted, Is.True); // checks, if a query has completed synchronously
+            Assert.That(t.Status, Is.EqualTo(TaskStatus.Canceled));
+            Assert.ThrowsAsync<OperationCanceledException>(async () => await t);
+
+            Assert.That(conn.FullState, Is.EqualTo(ConnectionState.Open));
+            Assert.That(await conn.ExecuteScalarAsync("SELECT 1"), Is.EqualTo(1));
+        }
+
         [Test, Description("Cancels an async query with the cancellation token, with successful PG cancellation")]
         public async Task CancelAsyncSoft()
         {
@@ -296,7 +318,7 @@ public async Task CancelAsyncSoft()
 
             await using var conn = await OpenConnectionAsync();
             using var cmd = CreateSleepCommand(conn);
-            var cancellationSource = new CancellationTokenSource();
+            using var cancellationSource = new CancellationTokenSource();
             var t = cmd.ExecuteNonQueryAsync(cancellationSource.Token);
             cancellationSource.Cancel();
 
@@ -322,7 +344,7 @@ public async Task CancelAsyncHard()
 
             var processId = conn.ProcessID;
 
-            var cancellationSource = new CancellationTokenSource();
+            using var cancellationSource = new CancellationTokenSource();
             using var cmd = new NpgsqlCommand("SELECT 1", conn);
             var t = cmd.ExecuteScalarAsync(cancellationSource.Token);
             cancellationSource.Cancel();