Use find_thread_in_random in select_event_lwp

I noticed that find_thread_in_random duplicates the code in
find_thread_in_random, so this patch changes the latter to use the
former.

There are two other spots in gdb that do this, but to unify all of
them would require switching some code from using the "iterate over"
idiom to using iterators.

Another possible improvement is that find_thread_in_random could be
made single-pass using reservoir sampling.

Tested by the buildbot.

gdb/gdbserver/ChangeLog
2019-04-09  Tom Tromey  <tromey@adacore.com>

	* linux-low.c (select_event_lwp): Use find_thread_in_random.

1 files changed, 4 insertions, 31 deletions

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 94af240..168f4b2 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -2828,7 +2828,6 @@ linux_wait_for_event (ptid_t ptid, int *wstatp, int options)
 static void
 select_event_lwp (struct lwp_info **orig_lp)
 {
-  int random_selector;
   struct thread_info *event_thread = NULL;
 
   /* In all-stop, give preference to the LWP that is being
@@ -2862,39 +2861,13 @@ select_event_lwp (struct lwp_info **orig_lp)
       /* No single-stepping LWP.  Select one at random, out of those
          which have had events.  */
 
-      /* First see how many events we have.  */
-      int num_events = 0;
-      for_each_thread ([&] (thread_info *thread)
-	{
-	  lwp_info *lp = get_thread_lwp (thread);
-
-	  /* Count only resumed LWPs that have an event pending. */
-	  if (thread->last_status.kind == TARGET_WAITKIND_IGNORE
-	      && lp->status_pending_p)
-	    num_events++;
-	});
-      gdb_assert (num_events > 0);
-
-      /* Now randomly pick a LWP out of those that have had
-	 events.  */
-      random_selector = (int)
-	((num_events * (double) rand ()) / (RAND_MAX + 1.0));
-
-      if (debug_threads && num_events > 1)
-	debug_printf ("SEL: Found %d SIGTRAP events, selecting #%d\n",
-		      num_events, random_selector);
-
-      event_thread = find_thread ([&] (thread_info *thread)
+      event_thread = find_thread_in_random ([&] (thread_info *thread)
 	{
 	  lwp_info *lp = get_thread_lwp (thread);
 
-	  /* Select only resumed LWPs that have an event pending.  */
-	  if (thread->last_status.kind == TARGET_WAITKIND_IGNORE
-	      && lp->status_pending_p)
-	    if (random_selector-- == 0)
-	      return true;
-
-	  return false;
+	  /* Only resumed LWPs that have an event pending. */
+	  return (thread->last_status.kind == TARGET_WAITKIND_IGNORE
+		  && lp->status_pending_p);
 	});
     }