[RFC][gdb] Handle pending ^C after rl_callback_read_char

Tom de Vries via Gdb-patches
2022-09-13T12:56:44Z

Hi,

In completion tests in various test-cases, we've been running into these
"clearing input line" timeouts:
...
(gdb) $cmd^GPASS: gdb.gdb/unittest.exp: tab complete "$cmd"
FAIL: gdb.gdb/unittest.exp: tab complete "$cmd" (clearing input line) (timeout)
...
where $cmd == "maintenance selftest name_that_does_not_exist".

AFAIU, the following scenario happens:
- expect sends "$cmd\t"
- gdb detects the stdin event, and calls rl_callback_read_char until it
comes to handle \t
- readline interprets the \t as completion, tries to complete, fails to do so,
outputs a bell (^G)
- expect sees the bell, and proceeds to send ^C
- readline is still in the call to rl_callback_read_char, and stores the
signal in _rl_caught_signal
- readline returns from the call to rl_callback_read_char, without having
handled _rl_caught_signal
- gdb goes to wait for the next event
- expect times out waiting for "Quit", the expected reaction for ^C

Fix this by handling pending signals after each call to rl_callback_read_char.

The fix is only available for readline 8.x, if --with-system-readline provides
an older version, then the fix is disabled due to missing function
rl_check_signals.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27813

Any comments?

Thanks,
- Tom

[gdb] Handle pending ^C after rl_callback_read_char

---
gdb/event-top.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/gdb/event-top.c b/gdb/event-top.c
index 290c3d87744..00adbe8fb4f 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -186,6 +186,22 @@ gdb_rl_callback_read_char_wrapper_noexcept () noexcept
TRY_SJLJ
{
rl_callback_read_char ();
+#if RL_VERSION_MAJOR >= 8
+ /* It can happen that readline (while in rl_callback_read_char)
+ received a signal, but didn't handle it yet. Make sure it's handled
+ now. If we don't do that we run into two related problems:
+ - we have to wait for another event triggering
+ rl_callback_read_char before the signal is handled
+ - there's no guarantee that the signal will be processed before the
+ event. */
+ while (rl_pending_signal () != 0)
+ /* Do this in a while loop, in case rl_check_signals also leaves a
+ pending signal. I'm not sure if that's possible, but it seems
+ better to handle the scenario than to assert. */
+ rl_check_signals ();
+#else
+ /* Unfortunately, rl_check_signals is not available. */
+#endif
if (after_char_processing_hook)
(*after_char_processing_hook) ();
}