Add PgConn.SyncConn

This provides a way to ensure it is safe to directly read or write to
the underlying net.Conn.

https://github.com/jackc/pgx/issues/1673

pgconn/internal/bgreader/bgreader.go

@@ -9,18 +9,18 @@ import (
 )
 
 const (
-	bgReaderStatusStopped = iota
-	bgReaderStatusRunning
-	bgReaderStatusStopping
+	StatusStopped = iota
+	StatusRunning
+	StatusStopping
 )
 
 // BGReader is an io.Reader that can optionally buffer reads in the background. It is safe for concurrent use.
 type BGReader struct {
 	r io.Reader
 
-	cond           *sync.Cond
-	bgReaderStatus int32
-	readResults    []readResult
+	cond        *sync.Cond
+	status      int32
+	readResults []readResult
 }
 
 type readResult struct {
@@ -34,14 +34,14 @@ func (r *BGReader) Start() {
 	r.cond.L.Lock()
 	defer r.cond.L.Unlock()
 
-	switch r.bgReaderStatus {
-	case bgReaderStatusStopped:
-		r.bgReaderStatus = bgReaderStatusRunning
+	switch r.status {
+	case StatusStopped:
+		r.status = StatusRunning
 		go r.bgRead()
-	case bgReaderStatusRunning:
+	case StatusRunning:
 		// no-op
-	case bgReaderStatusStopping:
-		r.bgReaderStatus = bgReaderStatusRunning
+	case StatusStopping:
+		r.status = StatusRunning
 	}
 }
 
@@ -51,16 +51,23 @@ func (r *BGReader) Stop() {
 	r.cond.L.Lock()
 	defer r.cond.L.Unlock()
 
-	switch r.bgReaderStatus {
-	case bgReaderStatusStopped:
+	switch r.status {
+	case StatusStopped:
 		// no-op
-	case bgReaderStatusRunning:
-		r.bgReaderStatus = bgReaderStatusStopping
-	case bgReaderStatusStopping:
+	case StatusRunning:
+		r.status = StatusStopping
+	case StatusStopping:
 		// no-op
 	}
 }
 
+// Status returns the current status of the background reader.
+func (r *BGReader) Status() int32 {
+	r.cond.L.Lock()
+	defer r.cond.L.Unlock()
+	return r.status
+}
+
 func (r *BGReader) bgRead() {
 	keepReading := true
 	for keepReading {
@@ -70,8 +77,8 @@ func (r *BGReader) bgRead() {
 
 		r.cond.L.Lock()
 		r.readResults = append(r.readResults, readResult{buf: buf, err: err})
-		if r.bgReaderStatus == bgReaderStatusStopping || err != nil {
-			r.bgReaderStatus = bgReaderStatusStopped
+		if r.status == StatusStopping || err != nil {
+			r.status = StatusStopped
 			keepReading = false
 		}
 		r.cond.L.Unlock()
@@ -89,7 +96,7 @@ func (r *BGReader) Read(p []byte) (int, error) {
 	}
 
 	// There are no unread background read results and the background reader is stopped.
-	if r.bgReaderStatus == bgReaderStatusStopped {
+	if r.status == StatusStopped {
 		return r.r.Read(p)
 	}
 

pgconn/pgconn.go

@@ -556,7 +556,8 @@ func (pgConn *PgConn) receiveMessage() (pgproto3.BackendMessage, error) {
 	return msg, nil
 }
 
-// Conn returns the underlying net.Conn. This rarely necessary.
+// Conn returns the underlying net.Conn. This rarely necessary. If the connection will be directly used for reading or
+// writing then SyncConn should usually be called before Conn.
 func (pgConn *PgConn) Conn() net.Conn {
 	return pgConn.conn
 }
@@ -1740,6 +1741,30 @@ func (pgConn *PgConn) flushWithPotentialWriteReadDeadlock() error {
 	return err
 }
 
+// SyncConn prepares the underlying net.Conn for direct use. PgConn may internally buffer reads or use goroutines for
+// background IO. This means that any direct use of the underlying net.Conn may be corrupted if a read is already
+// buffered or a read is in progress. SyncConn drains read buffers and stops background IO. In some cases this may
+// require sending a ping to the server. ctx can be used to cancel this operation. This should be called before any
+// operation that will use the underlying net.Conn directly. e.g. Before Conn() or Hijack().
+//
+// This should not be confused with the PostgreSQL protocol Sync message.
+func (pgConn *PgConn) SyncConn(ctx context.Context) error {
+	for i := 0; i < 10; i++ {
+		if pgConn.bgReader.Status() == bgreader.StatusStopped && pgConn.frontend.ReadBufferLen() == 0 {
+			return nil
+		}
+
+		err := pgConn.Ping(ctx)
+		if err != nil {
+			return fmt.Errorf("SyncConn: Ping failed while syncing conn: %w", err)
+		}
+	}
+
+	// This should never happen. Only way I can imagine this occuring is if the server is constantly sending data such as
+	// LISTEN/NOTIFY or log notifications such that we never can get an empty buffer.
+	return errors.New("SyncConn: conn never synchronized")
+}
+
 // HijackedConn is the result of hijacking a connection.
 //
 // Due to the necessary exposure of internal implementation details, it is not covered by the semantic versioning
@@ -1754,9 +1779,9 @@ type HijackedConn struct {
 	Config            *Config
 }
 
-// Hijack extracts the internal connection data. pgConn must be in an idle state. pgConn is unusable after hijacking.
-// Hijacking is typically only useful when using pgconn to establish a connection, but taking complete control of the
-// raw connection after that (e.g. a load balancer or proxy).
+// Hijack extracts the internal connection data. pgConn must be in an idle state. SyncConn should be called immediately
+// before Hijack. pgConn is unusable after hijacking. Hijacking is typically only useful when using pgconn to establish
+// a connection, but taking complete control of the raw connection after that (e.g. a load balancer or proxy).
 //
 // Due to the necessary exposure of internal implementation details, it is not covered by the semantic versioning
 // compatibility.

pgconn/pgconn_test.go

@@ -2319,6 +2319,9 @@ func TestHijackAndConstruct(t *testing.T) {
 	origConn, err := pgconn.Connect(ctx, os.Getenv("PGX_TEST_DATABASE"))
 	require.NoError(t, err)
 
+	err = origConn.SyncConn(ctx)
+	require.NoError(t, err)
+
 	hc, err := origConn.Hijack()
 	require.NoError(t, err)
 

pgproto3/frontend.go

@@ -361,3 +361,7 @@ func (f *Frontend) findAuthenticationMessageType(src []byte) (BackendMessage, er
 func (f *Frontend) GetAuthType() uint32 {
 	return f.authType
 }
+
+func (f *Frontend) ReadBufferLen() int {
+	return f.cr.wp - f.cr.rp
+}