[Bug gdb/16779] New: "c &" in all-stop changes current thread

["xdje42 at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16779

            Bug ID: 16779
           Summary: "c &" in all-stop changes current thread
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: xdje42 at gmail dot com

Created attachment 7505
  --> https://sourceware.org/bugzilla/attachment.cgi?id=7505&action=edit
testcase

I noticed this while testing a patch to fix "interrupt".

If I do "c &" in all-stop mode, the current thread is changed.
Not sure this is a bug, per se (given that it's all-stop), but it is
unexpected.

(gdb) set target-async on
(gdb) b all_threads_running
Breakpoint 1 at 0x40079d: file forever-threads.c, line 36.
(gdb) r
Starting program: /home/dje/src/play/forever-threads.x64
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
pid 13681 started
[New Thread 0x7ffff7fca700 (LWP 13685)]
thread tid 13685 started
[New Thread 0x7ffff7779700 (LWP 13686)]
thread tid 13686 started
[New Thread 0x7ffff6f28700 (LWP 13687)]
thread tid 13687 started
[New Thread 0x7ffff66d7700 (LWP 13688)]
thread tid 13688 started

Breakpoint 1, all_threads_running () at forever-threads.c:36
36      }
(gdb) i thr
  Id   Target Id         Frame
  5    Thread 0x7ffff66d7700 (LWP 13688) "forever-threads" 0x000000362ccbd37d
in nanosleep () at ../sysdeps/unix/syscall-template.S:81
  4    Thread 0x7ffff6f28700 (LWP 13687) "forever-threads" 0x000000362ccbd37d
in nanosleep () at ../sysdeps/unix/syscall-template.S:81
  3    Thread 0x7ffff7779700 (LWP 13686) "forever-threads" 0x000000362ccbd37d
in nanosleep () at ../sysdeps/unix/syscall-template.S:81
  2    Thread 0x7ffff7fca700 (LWP 13685) "forever-threads" 0x000000362ccbd37d
in nanosleep () at ../sysdeps/unix/syscall-template.S:81
* 1    Thread 0x7ffff7fcc740 (LWP 13681) "forever-threads" all_threads_running
    () at forever-threads.c:36
(gdb) thr 2
[Switching to thread 2 (Thread 0x7ffff7fca700 (LWP 13685))]
#0  0x000000362ccbd37d in nanosleep ()
    at ../sysdeps/unix/syscall-template.S:81
81      T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
(gdb) i thr
  Id   Target Id         Frame
  5    Thread 0x7ffff66d7700 (LWP 13688) "forever-threads" 0x000000362ccbd37d
in nanosleep () at ../sysdeps/unix/syscall-template.S:81
  4    Thread 0x7ffff6f28700 (LWP 13687) "forever-threads" 0x000000362ccbd37d
in nanosleep () at ../sysdeps/unix/syscall-template.S:81
  3    Thread 0x7ffff7779700 (LWP 13686) "forever-threads" 0x000000362ccbd37d
in nanosleep () at ../sysdeps/unix/syscall-template.S:81
* 2    Thread 0x7ffff7fca700 (LWP 13685) "forever-threads" 0x000000362ccbd37d
in nanosleep () at ../sysdeps/unix/syscall-template.S:81
  1    Thread 0x7ffff7fcc740 (LWP 13681) "forever-threads" all_threads_running
    () at forever-threads.c:36
(gdb) c &
Continuing.
(gdb) i thr
  Id   Target Id         Frame
  5    Thread 0x7ffff66d7700 (LWP 13688) "forever-threads" (running)
  4    Thread 0x7ffff6f28700 (LWP 13687) "forever-threads" (running)
  3    Thread 0x7ffff7779700 (LWP 13686) "forever-threads" (running)
  2    Thread 0x7ffff7fca700 (LWP 13685) "forever-threads" (running)
* 1    Thread 0x7ffff7fcc740 (LWP 13681) "forever-threads" (running)
(gdb)

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