[Bug gdb/29091] New: Regression in gdb.base/foll-fork.exp and other fork-related tests

[Bug gdb/29091] New: Regression in gdb.base/foll-fork.exp and other fork-related tests

[simark at simark dot ca]

https://sourceware.org/bugzilla/show_bug.cgi?id=29091

            Bug ID: 29091
           Summary: Regression in gdb.base/foll-fork.exp and other
                    fork-related tests
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: simark at simark dot ca
  Target Milestone: ---

Commit 152a1749566 ("gdb: prune inferiors at end of
fetch_inferior_event, fix intermittent failure of
gdb.threads/fork-plus-threads.exp") introduced some follow-fork-related
test failures, such as:

    info inferiors^M
      Num  Description       Connection           Executable        ^M
    * 1    process 634972    1 (native)          
/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork
^M
      2    process 634975    1 (native)          
/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork
^M
    (gdb) PASS: gdb.base/foll-fork.exp: follow-fork-mode=parent:
detach-on-fork=off: cmd=next 2: test_follow_fork: info inferiors
    inferior 2^M
    [Switching to inferior 2 [process 634975]
(/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork)]^M
    [Switching to thread 2.1 (Thread 0x7ffff7c9a740 (LWP 634975))]^M
    #0  0x00007ffff7d7abf7 in _Fork () from /usr/lib/libc.so.6^M
    (gdb) PASS: gdb.base/foll-fork.exp: follow-fork-mode=parent:
detach-on-fork=off: cmd=next 2: test_follow_fork: inferior 2
    continue^M
    Continuing.^M
    [Inferior 2 (process 634975) exited normally]^M
    [Switching to Thread 0x7ffff7c9a740 (LWP 634972)]^M
    (gdb) PASS: gdb.base/foll-fork.exp: follow-fork-mode=parent:
detach-on-fork=off: cmd=next 2: test_follow_fork: continue until exit at
continue unfollowed inferior to end
    break callee^M
    Breakpoint 2 at 0x555555555160: file
/home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/foll-fork.c, line 9.^M
    (gdb) FAIL: gdb.base/foll-fork.exp: follow-fork-mode=parent:
detach-on-fork=off: cmd=next 2: test_follow_fork: break callee

What happens here is:

 - inferior 2 is selected
 - we continue, leading to inferior 2's exit
 - we set breakpoint, expect 2 locations, but only one location is
   resolved

Reading between the lines, we understand that inferior 2 got pruned,
when it shouldn't have been.

The issue can be reproduced by hand with:

    $ ./gdb -q --data-directory=data-directory
testsuite/outputs/gdb.base/foll-fork/foll-fork -ex "set detach-on-fork off" -ex
start -ex "next 2" -ex "inferior 2" -ex "set debug infrun"
    ...
    Temporary breakpoint 1, main () at
/home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/foll-fork.c:14
    14        int  v = 5;
    [New inferior 2 (process 637627)]
    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".
    17        if (pid == 0) /* set breakpoint here */
    [Switching to inferior 2 [process 637627]
(/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork)]
    [Switching to thread 2.1 (Thread 0x7ffff7c9a740 (LWP 637627))]
    #0  0x00007ffff7d7abf7 in _Fork () from /usr/lib/libc.so.6
    (gdb) continue
    Continuing.
    [infrun] clear_proceed_status_thread: 637627.637627.0
    [infrun] proceed: enter
      [infrun] proceed: addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT
      [infrun] scoped_disable_commit_resumed: reason=proceeding
      [infrun] start_step_over: enter
        [infrun] start_step_over: stealing global queue of threads to step,
length = 0
        [infrun] operator(): step-over queue now empty
      [infrun] start_step_over: exit
      [infrun] proceed: start: resuming threads, all-stop-on-top-of-non-stop
        [infrun] proceed: resuming 637627.637627.0
        [infrun] resume_1: step=0, signal=GDB_SIGNAL_0, trap_expected=0,
current thread [637627.637627.0] at 0x7ffff7d7abf7
        [infrun] do_target_resume: resume_ptid=637627.637627.0, step=0,
sig=GDB_SIGNAL_0
        [infrun] infrun_async: enable=1
        [infrun] prepare_to_wait: prepare_to_wait
      [infrun] proceed: end: resuming threads, all-stop-on-top-of-non-stop
      [infrun] reset: reason=proceeding
      [infrun] maybe_set_commit_resumed_all_targets: enabling commit-resumed
for target native
      [infrun] maybe_call_commit_resumed_all_targets: calling commit_resumed
for target native
      [infrun] maybe_call_commit_resumed_all_targets: calling commit_resumed
for target native
    [infrun] proceed: exit
    [infrun] fetch_inferior_event: enter
      [infrun] scoped_disable_commit_resumed: reason=handling event
      [infrun] do_target_wait: Found 2 inferiors, starting at #1
      [infrun] random_pending_event_thread: None found.
      [infrun] print_target_wait_results: target_wait (-1.0.0 [process -1],
status) =
      [infrun] print_target_wait_results:   637627.637627.0 [process 637627],
      [infrun] print_target_wait_results:   status->kind = EXITED, exit_status
= 0
      [infrun] handle_inferior_event: status->kind = EXITED, exit_status = 0
    [Inferior 2 (process 637627) exited normally]
      [infrun] stop_waiting: stop_waiting
      [infrun] stop_all_threads: start: reason=presenting stop to user in
all-stop, inf=-1
        [infrun] stop_all_threads: pass=0, iterations=0
        [infrun] stop_all_threads:   637624.637624.0 not executing
        [infrun] stop_all_threads: pass=1, iterations=1
        [infrun] stop_all_threads:   637624.637624.0 not executing
        [infrun] stop_all_threads: done
      [infrun] stop_all_threads: end: reason=presenting stop to user in
all-stop, inf=-1
    [Switching to Thread 0x7ffff7c9a740 (LWP 637624)]
      [infrun] infrun_async: enable=0
      [infrun] reset: reason=handling event
      [infrun] maybe_set_commit_resumed_all_targets: not requesting
commit-resumed for target native, no resumed threads
    (gdb) [infrun] fetch_inferior_event: exit
    (gdb) info inferiors
      Num  Description       Connection           Executable
    * 1    process 637624    1 (native)          
/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork
    (gdb) i th
      Id   Target Id                                      Frame
    * 1    Thread 0x7ffff7c9a740 (LWP 637624) "foll-fork" main () at
/home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/foll-fork.c:17

After handling the EXITED event for inferior 2, inferior 2 should have
stayed the current inferior, which should have prevented it from getting
pruned.  When debugging, we find that when getting at the
prune_inferiors call, the current inferior is inferior 1.  Further
debugging shows that prior to the call to
clean_up_just_stopped_threads_fsms, the current inferior is inferior 2,
and after, it's inferior 1.  Then, back in fetch_inferior_event, the
restore_thread object is disabled, due to:

            /* If we got a TARGET_WAITKIND_NO_RESUMED event, then the
               previously selected thread is gone.  We have two
               choices - switch to no thread selected, or restore the
               previously selected thread (now exited).  We chose the
               later, just because that's what GDB used to do.  After
               this, "info threads" says "The current thread <Thread
               ID 2> has terminated." instead of "No thread
               selected.".  */
            if (!non_stop
                && cmd_done
                && ecs->ws.kind () != TARGET_WAITKIND_NO_RESUMED)
              restore_thread.dont_restore ();

So in the end, inferior 1 stays current, and inferior 2 gets wrongfully
pruned.

Proposed patch here:
https://sourceware.org/pipermail/gdb-patches/2022-April/188309.html

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[Bug gdb/29091] Regression in gdb.base/foll-fork.exp and other fork-related tests

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=29091

Tom de Vries <vries at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vries at gcc dot gnu.org

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
With this committed (
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=225170409b47bc96b62b2ecfcc0d9d5ae1ef8949
) can we close this?

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