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fix(agent): Prevent loss of SSH PTY command output #6840

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fix(agent): Prevent loss of SSH PTY command output #6840

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15 changes: 13 additions & 2 deletions agent/agent.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1104,6 +1104,18 @@ func (a *agent) handleSSHSession(session ssh.Session) (retErr error) {
var wg sync.WaitGroup
defer func() {
defer wg.Wait()

// If we call Close() before the output is read, the output
// will be lost. We set a deadline on the read so that we can
// wait for the output to be read before closing the PTY.
// OpenSSH also uses a 100ms timeout for reading from the PTY.
if dlErr := ptty.Output().Reader.SetReadDeadline(time.Now().Add(100 * time.Millisecond)); dlErr != nil {
a.logger.Warn(ctx, "failed to set read deadline, pty output may be lost", slog.Error(dlErr))
} else {
// If we successfully set the deadline, we can immediately
// wait for the output copy goroutine to exit.
wg.Wait()
}
closeErr := ptty.Close()
if closeErr != nil {
a.logger.Warn(ctx, "failed to close tty", slog.Error(closeErr))
Expand Down Expand Up @@ -1131,8 +1143,7 @@ func (a *agent) handleSSHSession(session ssh.Session) (retErr error) {
// output being lost. To avoid this, we wait for the output copy to
// start before waiting for the command to exit. This ensures that the
// output copy goroutine will be scheduled before calling close on the
// pty. There is still a risk of data loss if a command produces a lot
// of output, see TestAgent_Session_TTY_HugeOutputIsNotLost (skipped).
// pty. This is a safety-net in case SetReadDeadline doesn't work.
outputCopyStarted := make(chan struct{})
ptyOutput := func() io.Reader {
defer close(outputCopyStarted)
Expand Down
13 changes: 5 additions & 8 deletions agent/agent_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -410,14 +410,11 @@ func TestAgent_Session_TTY_HugeOutputIsNotLost(t *testing.T) {
// it seems like it could be either.
t.Skip("ConPTY appears to be inconsistent on Windows.")
}
t.Skip("This test proves we have a bug where parts of large output on a PTY can be lost after the command exits, skipped to avoid test failures.")

// This test is here to prevent prove we have a bug where quickly executing
// commands (with TTY) don't flush their output to the SSH session. This is
// due to the pty being closed before all the output has been copied, but
// protecting against this requires a non-trivial rewrite of the output
// processing (or figuring out a way to put the pty in a mode where this
// does not happen).

// This test is here to prevent regressions where the PTY for commands is
// closed before the buffer is consumed.
//
// See: https://github.com/coder/coder/issues/6656
ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
defer cancel()
//nolint:dogsled
Expand Down