[PATCH] 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 the read has the errno=ESRCH, use that as an indication that the
thread has exited between fopen and reading of the status file.
---
 gdb/nat/linux-procfs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
index e2086952ce6..36e0f5bf16a 100644
--- a/gdb/nat/linux-procfs.c
+++ b/gdb/nat/linux-procfs.c
@@ -165,6 +165,9 @@ linux_proc_pid_is_gone (pid_t pid)
     }
   else if (have_state == 0)
     {
+      /* errno is ESRCH "No such process": assume thread has disappeared.  */
+      if (errno == ESRCH)
+	return 1;
       /* No "State:" line, assume thread is alive.  */
       return 0;
     }
-- 
2.39.2