Merge pull request #1013 from dhermes/handle-broken-pipe

Mark `net.Conn` failed writes as recoverable when 0 bytes were written.

conn.go

@@ -891,9 +891,20 @@ func (cn *conn) Exec(query string, args []driver.Value) (res driver.Result, err
 	return r, err
 }
 
+type safeRetryError struct {
+	Err error
+}
+
+func (se *safeRetryError) Error() string {
+	return se.Err.Error()
+}
+
 func (cn *conn) send(m *writeBuf) {
-	_, err := cn.c.Write(m.wrap())
+	n, err := cn.c.Write(m.wrap())
 	if err != nil {
+		if n == 0 {
+			err = &safeRetryError{Err: err}
+		}
 		panic(err)
 	}
 }

error.go

@@ -495,6 +495,9 @@ func (cn *conn) errRecover(err *error) {
 	case *net.OpError:
 		cn.bad = true
 		*err = v
+	case *safeRetryError:
+		cn.bad = true
+		*err = driver.ErrBadConn
 	case error:
 		if v == io.EOF || v.(error).Error() == "remote error: handshake failure" {
 			*err = driver.ErrBadConn