ssh/tailssh: fix race in session termination message delivery

When a recording upload fails mid-session, the recording goroutine
cancels the session context. This triggers two concurrent paths:
exec.CommandContext kills the process (causing cmd.Wait to return),
and killProcessOnContextDone tries to write the termination message
via exitOnce.Do. If cmd.Wait returns first, the main goroutine's
exitOnce.Do(func(){}) steals the once, and the termination message
is never written to the client.

Fix by waiting for killProcessOnContextDone to finish writing the
termination message (via <-ss.exitHandled) before claiming exitOnce,
when the context is already done.

Also fix the fallback path when launchProcess itself fails due to
context cancellation: use SSHTerminationMessage() with the correct
"\r\n\r\n" framing instead of fmt.Fprintf with the internal error
string.

Deflakes TestSSHRecordingCancelsSessionsOnUploadFailure, which was
failing consistently at a low rate due to the exitOnce race. After
this fix, flakestress passes with 8,668 runs, 0 failures.

Fixes #7707 (again. hopefully for good.)

Change-Id: I5ab911c71574db8d3f9d979fb839f273be51ecf9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

0 files changed, 0 insertions, 0 deletions