fix ssh subsystem request invocation

client.go

@@ -34,8 +34,9 @@ type clientConn struct {
 	reqid atomic.Uint32
 	rd    io.Reader
 
-	resPool *sync.WorkPool[result]
+	wait func() error
 
+	resPool *sync.WorkPool[result]
 	bufPool *sync.SlicePool[[]byte, byte]
 	pktPool *sync.Pool[sshfx.RawPacket]
 
@@ -107,6 +108,26 @@ func (c *clientConn) getChan(reqid uint32) (chan<- result, bool) {
 
 func (c *clientConn) Wait() error {
 	<-c.closed
+
+	if c.wait == nil {
+		return c.err
+	}
+
+	if err := c.wait(); err != nil {
+
+		// TODO: when when https://github.com/golang/go/issues/35025 is fixed,
+		// we can remove this if block entirely.
+		// Right now, it's always going to return this, so it is not useful.
+		// But we have this code here so that as soon as the ssh library is updated,
+		// we can return a possibly more useful error.
+		if err.Error() == "ssh: session not started" {
+			return c.err
+		}
+
+		return err
+	}
+
+	// c.wait returned no error; so let's return something maybe more useful.
 	return c.err
 }
 
@@ -114,7 +135,8 @@ func (c *clientConn) disconnect(err error) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	c.err = err
+	c.err = cmp.Or(c.err, err)
+
 	select {
 	case <-c.closed:
 		// already closed
@@ -365,11 +387,20 @@ func WithMaxPacketLength(length int) ClientOption {
 	}
 }
 
+// CopyStderrTo specifies a writer to which the standard error of the remote sftp-server command should be written.
+func CopyStderrTo(wr io.WriteCloser) ClientOption {
+	return func(cl *Client) error {
+		cl.stderrTo = wr
+		return nil
+	}
+}
+
 // Client represents an SFTP session on a *ssh.ClientConn SSH connection.
 // Multiple clients can be active on a single SSH connection,
 // and a client may be called concurrently from multiple goroutines.
 type Client struct {
-	conn clientConn
+	conn     clientConn
+	stderrTo io.WriteCloser
 
 	maxPacket   uint32
 	maxDataLen  int
@@ -554,24 +585,30 @@ func NewClient(ctx context.Context, conn *ssh.Client, opts ...ClientOption) (*Cl
 		return nil, err
 	}
 
+	pw, err := s.StdinPipe()
+	if err != nil {
+		s.Close()
+		return nil, err
+	}
+
+	pr, err := s.StdoutPipe()
+	if err != nil {
+		s.Close()
+		return nil, err
+	}
+
+	perr, err := s.StderrPipe()
+	if err != nil {
+		s.Close()
+		return nil, err
+	}
+
 	if err := s.RequestSubsystem("sftp"); err != nil {
 		s.Close()
 		return nil, err
 	}
 
-	w, err := s.StdinPipe()
-	if err != nil {
-		s.Close()
-		return nil, err
-	}
-
-	r, err := s.StdoutPipe()
-	if err != nil {
-		s.Close()
-		return nil, err
-	}
-
-	return NewClientPipe(ctx, r, w, opts...)
+	return newClientPipe(ctx, pr, perr, pw, s.Wait, opts)
 }
 
 // NewClientPipe creates a new SFTP client given a Reader and WriteCloser.
@@ -579,10 +616,16 @@ func NewClient(ctx context.Context, conn *ssh.Client, opts ...ClientOption) (*Cl
 //
 // The given context is only used for the negotiation of init and version packets.
 func NewClientPipe(ctx context.Context, rd io.Reader, wr io.WriteCloser, opts ...ClientOption) (*Client, error) {
+	return newClientPipe(ctx, rd, nil, wr, nil, opts)
+}
+
+func newClientPipe(ctx context.Context, rd, stderr io.Reader, wr io.WriteCloser, wait func() error, opts []ClientOption) (*Client, error) {
 	cl := &Client{
 		conn: clientConn{
-			rd:     rd,
-			wr:     wr,
+			rd: rd,
+			wr: wr,
+
+			wait:   wait,
 			closed: make(chan struct{}),
 		},
 
@@ -597,6 +640,27 @@ func NewClientPipe(ctx context.Context, rd io.Reader, wr io.WriteCloser, opts ..
 		}
 	}
 
+	if stderr != nil {
+		wr := io.Discard
+		if cl.stderrTo != nil {
+			wr = cl.stderrTo
+		}
+
+		go func() {
+			defer func() {
+				if closer, ok := wr.(io.Closer); ok {
+					if err := closer.Close(); err != nil {
+						cl.conn.disconnect(fmt.Errorf("error closing stderrTo: %w", err))
+					}
+				}
+			}()
+
+			if _, err := io.Copy(wr, stderr); err != nil {
+				cl.conn.disconnect(fmt.Errorf("error copying stderr: %w", err))
+			}
+		}()
+	}
+
 	exts, err := cl.conn.handshake(ctx, cl.maxPacket)
 	if err != nil {
 		return nil, err
@@ -605,7 +669,6 @@ func NewClientPipe(ctx context.Context, rd io.Reader, wr io.WriteCloser, opts ..
 	cl.exts = exts
 
 	cl.conn.resPool = sync.NewWorkPool[result](cl.maxInflight)
-
 	cl.conn.bufPool = sync.NewSlicePool[[]byte](cl.maxInflight, int(cl.maxPacket))
 	cl.conn.pktPool = sync.NewPool[sshfx.RawPacket](cl.maxInflight)
 

localfs/localfs_integration_test.go

@@ -213,6 +213,7 @@ func TestMain(m *testing.M) {
 
 func withOpenSSHImpl(m *testing.M) error {
 	sftpServerLocations := []string{
+		"/usr/libexec/ssh/sftp-server",
 		"/usr/libexec/sftp-server",
 		"/usr/lib/openssh/sftp-server",
 		"/usr/lib/ssh/sftp-server",