Fix canceled context broke mysqlConn (#862)

Fix #858

Author: methane

Diff for: connection_go18.go

@@ -149,22 +149,21 @@ func (mc *mysqlConn) watchCancel(ctx context.Context) error {
 		mc.cleanup()
 		return nil
 	}
+	// When ctx is already cancelled, don't watch it.
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+	// When ctx is not cancellable, don't watch it.
 	if ctx.Done() == nil {
 		return nil
 	}
-
-	mc.watching = true
-	select {
-	default:
-	case <-ctx.Done():
-		return ctx.Err()
-	}
+	// When watcher is not alive, can't watch it.
 	if mc.watcher == nil {
 		return nil
 	}
 
+	mc.watching = true
 	mc.watcher <- ctx
-
 	return nil
 }
 

Diff for: connection_go18_test.go

@@ -11,6 +11,7 @@
 package mysql
 
 import (
+	"context"
 	"database/sql/driver"
 	"testing"
 )
@@ -28,3 +29,31 @@ func TestCheckNamedValue(t *testing.T) {
 		t.Fatalf("uint64 high-bit not converted, got %#v %T", value.Value, value.Value)
 	}
 }
+
+// TestCleanCancel tests passed context is cancelled at start.
+// No packet should be sent.  Connection should keep current status.
+func TestCleanCancel(t *testing.T) {
+	mc := &mysqlConn{
+		closech: make(chan struct{}),
+	}
+	mc.startWatcher()
+	defer mc.cleanup()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	for i := 0; i < 3; i++ { // Repeat same behavior
+		err := mc.Ping(ctx)
+		if err != context.Canceled {
+			t.Errorf("expected context.Canceled, got %#v", err)
+		}
+
+		if mc.closed.IsSet() {
+			t.Error("expected mc is not closed, closed actually")
+		}
+
+		if mc.watching {
+			t.Error("expected watching is false, but true")
+		}
+	}
+}