[PATCH v3] Fix sporadic XFAILs in gdb.threads/attach-many-short-lived-threads.exp

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

This is about random test failures like those:

XFAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 6: attach (EPERM)
XFAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 7: attach (EPERM)
XFAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: attach (EPERM)
XFAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 9: attach (EPERM)
XFAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 10: attach (EPERM)

The reason for this effect is apparently as follows:

There is a race condition when gdb tries to attach a thread but the
thread exits at the same time.  Normally when that happens the return
code of ptrace(PTRACE_ATTACH, x) is EPERM, which could also have other
reasons.  To detect the true reason, we try to open /proc/<pid>/status
which normally fails in that situation, but it may happen that the
fopen succeeds, and the thread disappears while reading the content,
then linux_proc_pid_get_state fails to find the "State:" line.
Since there is no other possible reason why this can happen,
use that as an indication that the thread has most likely exited.
---
 gdb/nat/linux-procfs.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

v2: from kernel code review, it seems the missing "State:"
 can only happen if the thread disappeared, so no need to
 look at errno at all here.

v3: update commit message.

diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
index e2086952ce6..8d46d5bf289 100644
--- a/gdb/nat/linux-procfs.c
+++ b/gdb/nat/linux-procfs.c
@@ -157,17 +157,12 @@ linux_proc_pid_is_gone (pid_t pid)
   enum proc_state state;
 
   have_state = linux_proc_pid_get_state (pid, 0, &state);
-  if (have_state < 0)
+  if (have_state <= 0)
     {
-      /* If we can't open the status file, assume the thread has
-	 disappeared.  */
+      /* If we can't open the status file or there is no "State:" line,
+	 assume the thread has disappeared.  */
       return 1;
     }
-  else if (have_state == 0)
-    {
-      /* No "State:" line, assume thread is alive.  */
-      return 0;
-    }
   else
     return (state == PROC_STATE_ZOMBIE || state == PROC_STATE_DEAD);
 }
-- 
2.39.2