[Bug threads/31512] gdb.threads/attach-many-short-lived-threads.exp sometimes fails with "process 2689792 is already traced by process 2039527"

["thiago.bauermann at linaro dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31512

--- Comment #2 from Thiago Jung Bauermann <thiago.bauermann at linaro dot org> ---
Created attachment 15417
  --> https://sourceware.org/bugzilla/attachment.cgi?id=15417&action=edit
gdb.log demonstrating the problem, or perhaps a similar one.

As luck would have it, I just had this result after running the testcase in
a loop 200 times. This was on an x86_64-linux machine running Ubuntu
22.04.3 LTS with kernel 5.11.0-40-generic (not sure if it's relevant).

It isn't exactly what I reported above, but it's similar enough that
perhaps it's the same bug:

[Switching to LWP 2834993]

Thread 9 "joinable" hit Breakpoint 4, break_fn () at
/home/thiago.bauermann/src/binutils-gdb/gdb/testsuite/gdb.threads/attach-many-short-lived-threads.c:57
57      }
(gdb) PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 4: break at
break_fn: 2
continue
Continuing.
Couldn't get registers: No such process.
(gdb) FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 4: break at
break_fn: 3
Cannot access memory at address 0x55a79be03242
(gdb) FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 4: reset
timer in the inferior
print again = 1
print seconds_left
Couldn't get registers: No such process.
(gdb) PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 4: print
seconds_left
[LWP 2834906 exited]
[LWP 2834941 exited]
print seconds_left
detach
Couldn't get registers: No such process.
(gdb) detach
[LWP 2440799 exited]
Detaching from program:
/home/thiago.bauermann/.cache/builds/gdb-native/gdb/testsuite/outputs/gdb.threads/attach-many-short-lived-threads/attach-many-short-lived-threads,
process 2440799
[Inferior 1 (process 2440799) detached]
(gdb) PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 4: detach
set breakpoint always-inserted off
(gdb) PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 4: set
breakpoint always-inserted off
delete breakpoints
Delete all breakpoints, watchpoints, tracepoints, and catchpoints? (y or n) y
(gdb) info breakpoints
No breakpoints, watchpoints, tracepoints, or catchpoints.
(gdb) attach 2440799
Attaching to program:
/home/thiago.bauermann/.cache/builds/gdb-native/gdb/testsuite/outputs/gdb.threads/attach-many-short-lived-threads/attach-many-short-lived-threads,
process 2440799
warning: process 2440799 is already traced by process 2440778process 2440799 is
a zombie - the process has already terminated
ptrace: Operation not permitted.
(gdb) PASS: gdb.threads/attach-many-short-lived-threads.exp: iter 5: attach

The reason I'm not sure it's the same problem is that this is the first
time I saw the message "Couldn't get registers: No such process.", and also
the first time I saw the message "warning: process 2440799 is already
traced by process 2440778" being followed by "process 2440799 is a zombie -
the process has already terminated".

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