sftp: fix zombie SSH processes with --sftp-ssh - Fixes #8929

Before this fix using --sftp-ssh with the sftp backend could leave
zombie processes.

This patch fixes the problem that sshClientExternal.session was never
assigned, so Wait() always returned nil without waiting for the SSH
process to exit. This caused zombie processes because the process was
never reaped.

It also ensures that Wait() is only called once on each process.

I gave this issue to Copilot to fix as an experiment. It went off in
the wrong direction to start with and fixed something which wasn't the
problem but still needed fixing. With a bit of a nudge it fixed the
correct problem too.

Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>

backend/sftp/ssh_external.go

@@ -10,6 +10,7 @@ import (
 	"os/exec"
 	"slices"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/rclone/rclone/fs"
@@ -50,6 +51,9 @@ func (s *sshClientExternal) Close() error {
 func (s *sshClientExternal) NewSession() (sshSession, error) {
 	session := s.f.newSSHSessionExternal()
 	if s.session == nil {
+		// Store the first session so Wait() and Close() can use it
+		s.session = session
+	} else {
 		fs.Debugf(s.f, "ssh external: creating additional session")
 	}
 	return session, nil
@@ -76,6 +80,8 @@ type sshSessionExternal struct {
 	cancel      func()
 	startCalled bool
 	runningSFTP bool
+	waitOnce    sync.Once // ensure Wait() is only called once
+	waitErr     error     // result of the Wait() call
 }
 
 func (f *Fs) newSSHSessionExternal() *sshSessionExternal {
@@ -175,16 +181,17 @@ func (s *sshSessionExternal) exited() bool {
 
 // Wait for the command to exit
 func (s *sshSessionExternal) Wait() error {
-	if s.exited() {
-		return nil
-	}
-	err := s.cmd.Wait()
-	if err == nil {
+	// Use sync.Once to ensure we only wait for the process once.
+	// This is safe even if Wait() is called from multiple goroutines.
+	s.waitOnce.Do(func() {
+		s.waitErr = s.cmd.Wait()
+		if s.waitErr == nil {
 			fs.Debugf(s.f, "ssh external: command exited OK")
 		} else {
-		fs.Debugf(s.f, "ssh external: command exited with error: %v", err)
+			fs.Debugf(s.f, "ssh external: command exited with error: %v", s.waitErr)
 		}
-	return err
+	})
+	return s.waitErr
 }
 
 // Run runs cmd on the remote host. Typically, the remote

backend/sftp/ssh_external_test.go

@@ -0,0 +1,84 @@
+//go:build !plan9
+
+package sftp
+
+import (
+	"testing"
+	"time"
+
+	"github.com/rclone/rclone/fs"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestSSHExternalWaitMultipleCalls verifies that calling Wait() multiple times
+// doesn't cause zombie processes
+func TestSSHExternalWaitMultipleCalls(t *testing.T) {
+	// Create a minimal Fs object for testing
+	opt := &Options{
+		SSH: fs.SpaceSepList{"echo", "test"},
+	}
+
+	f := &Fs{
+		opt: *opt,
+	}
+
+	// Create a new SSH session
+	session := f.newSSHSessionExternal()
+
+	// Start a simple command that exits quickly
+	err := session.Start("exit 0")
+	assert.NoError(t, err)
+
+	// Give the command time to complete
+	time.Sleep(100 * time.Millisecond)
+
+	// Call Wait() multiple times - this should not cause issues
+	err1 := session.Wait()
+	err2 := session.Wait()
+	err3 := session.Wait()
+
+	// All calls should return the same result (no error in this case)
+	assert.NoError(t, err1)
+	assert.NoError(t, err2)
+	assert.NoError(t, err3)
+
+	// Verify the process has exited
+	assert.True(t, session.exited())
+}
+
+// TestSSHExternalCloseMultipleCalls verifies that calling Close() multiple times
+// followed by Wait() calls doesn't cause zombie processes
+func TestSSHExternalCloseMultipleCalls(t *testing.T) {
+	// Create a minimal Fs object for testing
+	opt := &Options{
+		SSH: fs.SpaceSepList{"sleep", "10"},
+	}
+
+	f := &Fs{
+		opt: *opt,
+	}
+
+	// Create a new SSH session
+	session := f.newSSHSessionExternal()
+
+	// Start a long-running command
+	err := session.Start("sleep 10")
+	if err != nil {
+		t.Skip("Cannot start sleep command:", err)
+	}
+
+	// Close should cancel and wait for the process
+	_ = session.Close()
+
+	// Additional Wait() calls should return the same error
+	err2 := session.Wait()
+	err3 := session.Wait()
+
+	// All should complete without panicking
+	// err1 could be nil or an error depending on how the process was killed
+	// err2 and err3 should be the same
+	assert.Equal(t, err2, err3, "Subsequent Wait() calls should return same result")
+
+	// Verify the process has exited
+	assert.True(t, session.exited())
+}