database/sql: add Pinger interface to driver Conn

Change-Id: If6eb3a7c9ad48a517e584567b1003479c1df6cca
Reviewed-on: https://go-review.googlesource.com/32136
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/driver/driver.go

@@ -69,6 +69,17 @@ var ErrSkip = errors.New("driver: skip fast-path; continue as if unimplemented")
 // you shouldn't return ErrBadConn.
 var ErrBadConn = errors.New("driver: bad connection")
 
+// Pinger is an optional interface that may be implemented by a Conn.
+//
+// If a Conn does not implement Pinger, the sql package's DB.Ping and
+// DB.PingContext will check if there is at least one Conn available.
+//
+// If Conn.Ping returns ErrBadConn, DB.Ping and DB.PingContext will remove
+// the Conn from pool.
+type Pinger interface {
+	Ping(ctx context.Context) error
+}
+
 // Execer is an optional interface that may be implemented by a Conn.
 //
 // If a Conn does not implement Execer, the sql package's DB.Exec will

src/database/sql/sql.go

@@ -553,15 +553,27 @@ func Open(driverName, dataSourceName string) (*DB, error) {
 // PingContext verifies a connection to the database is still alive,
 // establishing a connection if necessary.
 func (db *DB) PingContext(ctx context.Context) error {
-	// TODO(bradfitz): give drivers an optional hook to implement
-	// this in a more efficient or more reliable way, if they
-	// have one.
-	dc, err := db.conn(ctx, cachedOrNewConn)
+	var dc *driverConn
+	var err error
+
+	for i := 0; i < maxBadConnRetries; i++ {
+		dc, err = db.conn(ctx, cachedOrNewConn)
+		if err != driver.ErrBadConn {
+			break
+		}
+	}
+	if err == driver.ErrBadConn {
+		dc, err = db.conn(ctx, alwaysNewConn)
+	}
 	if err != nil {
 		return err
 	}
-	db.putConn(dc, nil)
-	return nil
+
+	if pinger, ok := dc.ci.(driver.Pinger); ok {
+		err = pinger.Ping(ctx)
+	}
+	db.putConn(dc, err)
+	return err
 }
 
 // Ping verifies a connection to the database is still alive,

src/database/sql/sql_test.go

@@ -2581,6 +2581,50 @@ func TestBadDriver(t *testing.T) {
 	db.Exec("ignored")
 }
 
+type pingDriver struct {
+	fails bool
+}
+
+type pingConn struct {
+	badConn
+	driver *pingDriver
+}
+
+var pingError = errors.New("Ping failed")
+
+func (pc pingConn) Ping(ctx context.Context) error {
+	if pc.driver.fails {
+		return pingError
+	}
+	return nil
+}
+
+var _ driver.Pinger = pingConn{}
+
+func (pd *pingDriver) Open(name string) (driver.Conn, error) {
+	return pingConn{driver: pd}, nil
+}
+
+func TestPing(t *testing.T) {
+	driver := &pingDriver{}
+	Register("ping", driver)
+
+	db, err := Open("ping", "ignored")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := db.Ping(); err != nil {
+		t.Errorf("err was %#v, expected nil", err)
+		return
+	}
+
+	driver.fails = true
+	if err := db.Ping(); err != pingError {
+		t.Errorf("err was %#v, expected pingError", err)
+	}
+}
+
 func BenchmarkConcurrentDBExec(b *testing.B) {
 	b.ReportAllocs()
 	ct := new(concurrentDBExecTest)