Fix panic when closing conn during cancellable query

fixes #29

internal/ctxwatch/context_watcher_test.go

@@ -59,6 +59,17 @@ func TestContextWatcherMultipleWatchPanics(t *testing.T) {
 	require.Panics(t, func() { cw.Watch(ctx2) }, "Expected panic when Watch called multiple times")
 }
 
+func TestContextWatcherUnwatchIsAlwaysSafe(t *testing.T) {
+	cw := ctxwatch.NewContextWatcher(func() {}, func() {})
+	cw.Unwatch() // unwatch when not / never watching
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	cw.Watch(ctx)
+	cw.Unwatch()
+	cw.Unwatch() // double unwatch
+}
+
 func TestContextWatcherStress(t *testing.T) {
 	var cancelFuncCalls int64
 	var cleanupFuncCalls int64

pgconn.go

@@ -494,6 +494,13 @@ func (pgConn *PgConn) Close(ctx context.Context) error {
 	defer pgConn.conn.Close()
 
 	if ctx != context.Background() {
+		// Close may be called while a cancellable query is in progress. This will most often be triggered by panic when
+		// a defer closes the connection (possibly indirectly via a transaction or a connection pool). Unwatch to end any
+		// previous watch. It is safe to Unwatch regardless of whether a watch is already is progress.
+		//
+		// See https://github.com/jackc/pgconn/issues/29
+		pgConn.contextWatcher.Unwatch()
+
 		pgConn.contextWatcher.Watch(ctx)
 		defer pgConn.contextWatcher.Unwatch()
 	}

pgconn_test.go

@@ -1708,6 +1708,19 @@ func TestHijackAndConstruct(t *testing.T) {
 	ensureConnValid(t, newConn)
 }
 
+func TestConnCloseWhileCancellableQueryInProgress(t *testing.T) {
+	t.Parallel()
+
+	pgConn, err := pgconn.Connect(context.Background(), os.Getenv("PGX_TEST_CONN_STRING"))
+	require.NoError(t, err)
+
+	ctx, _ := context.WithCancel(context.Background())
+	pgConn.Exec(ctx, "select n from generate_series(1,10) n")
+
+	closeCtx, _ := context.WithCancel(context.Background())
+	pgConn.Close(closeCtx)
+}
+
 func Example() {
 	pgConn, err := pgconn.Connect(context.Background(), os.Getenv("PGX_TEST_CONN_STRING"))
 	if err != nil {