fix(agent): guard against multiple rpty race for same id (#7998)

* fix(agent): guard against multiple rpty race for same id
* fix(agent): ensure pty is closed on error

agent/agent.go

@@ -1025,16 +1025,32 @@ func (a *agent) handleReconnectingPTY(ctx context.Context, logger slog.Logger, m
 	}()
 
 	var rpty *reconnectingPTY
-	rawRPTY, ok := a.reconnectingPTYs.Load(msg.ID)
+	sendConnected := make(chan *reconnectingPTY, 1)
+	// On store, reserve this ID to prevent multiple concurrent new connections.
+	waitReady, ok := a.reconnectingPTYs.LoadOrStore(msg.ID, sendConnected)
 	if ok {
+		close(sendConnected) // Unused.
 		logger.Debug(ctx, "connecting to existing session")
-		rpty, ok = rawRPTY.(*reconnectingPTY)
+		c, ok := waitReady.(chan *reconnectingPTY)
 		if !ok {
-			return xerrors.Errorf("found invalid type in reconnecting pty map: %T", rawRPTY)
+			return xerrors.Errorf("found invalid type in reconnecting pty map: %T", waitReady)
 		}
+		rpty, ok = <-c
+		if !ok || rpty == nil {
+			return xerrors.Errorf("reconnecting pty closed before connection")
+		}
+		c <- rpty // Put it back for the next reconnect.
 	} else {
 		logger.Debug(ctx, "creating new session")
 
+		connected := false
+		defer func() {
+			if !connected && retErr != nil {
+				a.reconnectingPTYs.Delete(msg.ID)
+				close(sendConnected)
+			}
+		}()
+
 		// Empty command will default to the users shell!
 		cmd, err := a.sshServer.CreateCommand(ctx, msg.Command, nil)
 		if err != nil {
@@ -1055,7 +1071,7 @@ func (a *agent) handleReconnectingPTY(ctx context.Context, logger slog.Logger, m
 			return xerrors.Errorf("start command: %w", err)
 		}
 
-		ctx, cancelFunc := context.WithCancel(ctx)
+		ctx, cancel := context.WithCancel(ctx)
 		rpty = &reconnectingPTY{
 			activeConns: map[string]net.Conn{
 				// We have to put the connection in the map instantly otherwise
@@ -1064,10 +1080,9 @@ func (a *agent) handleReconnectingPTY(ctx context.Context, logger slog.Logger, m
 			},
 			ptty: ptty,
 			// Timeouts created with an after func can be reset!
-			timeout:        time.AfterFunc(a.reconnectingPTYTimeout, cancelFunc),
+			timeout:        time.AfterFunc(a.reconnectingPTYTimeout, cancel),
 			circularBuffer: circularBuffer,
 		}
-		a.reconnectingPTYs.Store(msg.ID, rpty)
 		// We don't need to separately monitor for the process exiting.
 		// When it exits, our ptty.OutputReader() will return EOF after
 		// reading all process output.
@@ -1115,8 +1130,12 @@ func (a *agent) handleReconnectingPTY(ctx context.Context, logger slog.Logger, m
 			rpty.Close()
 			a.reconnectingPTYs.Delete(msg.ID)
 		}); err != nil {
+			_ = process.Kill()
+			_ = ptty.Close()
 			return xerrors.Errorf("start routine: %w", err)
 		}
+		connected = true
+		sendConnected <- rpty
 	}
 	// Resize the PTY to initial height + width.
 	err := rpty.ptty.Resize(msg.Height, msg.Width)