database/sql: don't hang if the driver Exec method panics

Fixes #13677.
Fixes #15901.

Change-Id: Idffb82cdcba4985954d061bdb021217f47ff4984
Reviewed-on: https://go-review.googlesource.com/23576
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql_test.go

@@ -2299,6 +2299,53 @@ func TestConnectionLeak(t *testing.T) {
 	wg.Wait()
 }
 
+// badConn implements a bad driver.Conn, for TestBadDriver.
+// The Exec method panics.
+type badConn struct{}
+
+func (bc badConn) Prepare(query string) (driver.Stmt, error) {
+	return nil, errors.New("badConn Prepare")
+}
+
+func (bc badConn) Close() error {
+	return nil
+}
+
+func (bc badConn) Begin() (driver.Tx, error) {
+	return nil, errors.New("badConn Begin")
+}
+
+func (bc badConn) Exec(query string, args []driver.Value) (driver.Result, error) {
+	panic("badConn.Exec")
+}
+
+// badDriver is a driver.Driver that uses badConn.
+type badDriver struct{}
+
+func (bd badDriver) Open(name string) (driver.Conn, error) {
+	return badConn{}, nil
+}
+
+// Issue 15901.
+func TestBadDriver(t *testing.T) {
+	Register("bad", badDriver{})
+	db, err := Open("bad", "ignored")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() {
+		if r := recover(); r == nil {
+			t.Error("expected panic")
+		} else {
+			if want := "badConn.Exec"; r.(string) != want {
+				t.Errorf("panic was %v, expected %v", r, want)
+			}
+		}
+	}()
+	defer db.Close()
+	db.Exec("ignored")
+}
+
 func BenchmarkConcurrentDBExec(b *testing.B) {
 	b.ReportAllocs()
 	ct := new(concurrentDBExecTest)