[gdb/testsuite] Fix gdb.threads/access-mem-running-thread-exit.exp

In OBS (Open Build Service), with a 15.2 based gdb package, occasionally I run
into:
...
(gdb) inferior 2
[Switching to inferior 2 [process 31372] (access-mem-running-thread-exit)]
[Switching to thread 2.1 (Thread 0xf7db9700 (LWP 31372))](running)
(gdb) print global_var = 555
$1 = 555
(gdb) print global_var
$2 = 556
(gdb) FAIL: $exp: all-stop: access mem \
  (print global_var after writing, inf=2, iter=1)
...

I managed to reproduce this on current trunk using a reproducer patch (posted
in the PR).

The problem is due to commit 31c21e2c13d ("[gdb/testsuite] Fix
gdb.threads/access-mem-running-thread-exit.exp with clang"), which introduced
an increment of global_var at the start of main.

This created a race between:
- gdb modifying global_var, and
- the inferior modifying global_var.

Fix this by:
- adding a new empty function setup_done,
- adding a call to setup_done after the increment of global_var, and
- rather than running to main, running to setup_done.

Tested on x86_64-linux.

PR testsuite/32822
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32822

1 files changed, 7 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.threads/access-mem-running-thread-exit.c b/gdb/testsuite/gdb.threads/access-mem-running-thread-exit.c
index af05b13..e22bf12 100644
--- a/gdb/testsuite/gdb.threads/access-mem-running-thread-exit.c
+++ b/gdb/testsuite/gdb.threads/access-mem-running-thread-exit.c
@@ -97,6 +97,11 @@ thread_fn (void *arg)
   return NULL;
 }
 
+static void
+setup_done (void)
+{
+}
+
 int
 main (void)
 {
@@ -104,6 +109,8 @@ main (void)
 
   global_var++;
 
+  setup_done ();
+
   for (i = 0; i < 4; i++)
     {
       struct thread_arg *p;