[Bug gdb/31440] [gdb] FAIL: gdb.python/py-progspace-events.exp: inferior 1 (timeout)

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31440

--- Comment #5 from Tom de Vries <vries at gcc dot gnu.org> ---
I added some debug printfs, and got the following output.

Passing case:
...
(gdb) PASS: gdb.python/py-progspace-events.exp: continue to breakpoint: run to
breakpt in inferior 2
continue^M
Continuing.^M
not removable: inf 1^M
not deletable: inf 2^M
[Inferior 2 (process 4549) exited normally]^M
not removable: inf 1^M
not deletable: inf 2^M
(gdb) not removable: inf 1^M
not deletable: inf 2^M
PASS: gdb.python/py-progspace-events.exp: continue until inferior 2 exits
inferior 1^M
[Switching to inferior 1 [process 4546] (py-progspace-events)]^M
[Switching to thread 1.1 (process 4546)]^M
#0  breakpt () at py-progspace-events.c:28^M
28      { /* Nothing.  */ }^M
(gdb) PASS: gdb.python/py-progspace-events.exp: inferior 1
step^M
not deletable: inf 1^M
delete_inferior: inf 2^M
FreeProgspaceEvent: <gdb.Progspace object at 0x7efd1faf0ee0>^M
...

Failing case:
...
(gdb) PASS: gdb.python/py-progspace-events.exp: continue to breakpoint: run to
breakpt in inferior 2
continue^M
Continuing.^M
not removable: inf 1^M
not deletable: inf 2^M
[Inferior 2 (process 4752) exited normally]^M
not removable: inf 1^M
not deletable: inf 2^M
(gdb) PASS: gdb.python/py-progspace-events.exp: continue until inferior 2 exits
inferior 1^M
[Switching to inferior 1 [process 4742] (py-progspace-events)]^M
[Switching to thread 1.1 (process 4742)]^M
#0  breakpt () at py-progspace-events.c:28^M
28      { /* Nothing.  */ }^M
(gdb) not deletable: inf 1^M
delete_inferior: inf 2^M
FreeProgspaceEvent: <gdb.Progspace object at 0x7f3a6faf0ee0>^M
FAIL: gdb.python/py-progspace-events.exp: inferior 1 (timeout)
step^M
...

So, it seems there's a race between some event and the "inferior 1" command.

In the passing case, the event is handled first.  During handling,
prune_inferiors is called, but it can't remove inferior 1, because it's still
the current one.

In the failing case, the "inferior 1" is handled first. Then during handling of
the event, prune_inferiors is called, and it can remove inferior 1 because it's
no longer the current one.

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