[PATCH v2 5/8] fbsd-nat: Fix thread_alive against a running thread.

[John Baldwin <jhb@FreeBSD.org>]

FreeBSD's ptrace fails requests with EBUSY against a running process.
Report that the thread is alive instead of dead if ptrace fails with
EBUSY.

This fixes an internal error in the gdb.threads/detach-step-over.exp
test where one process was detached while a thread in a second process
was being stepped.  The core incorrectly assumed the stepping thread
had vanished and discarded the pending stepping state.  When the
thread later reported a SIGTRAP from completing the step, this
triggered an assertion.
---
 gdb/fbsd-nat.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/gdb/fbsd-nat.c b/gdb/fbsd-nat.c
index ed69cb33e93..6ce77c5cd16 100644
--- a/gdb/fbsd-nat.c
+++ b/gdb/fbsd-nat.c
@@ -839,7 +839,13 @@ fbsd_nat_target::thread_alive (ptid_t ptid)
 
       if (ptrace (PT_LWPINFO, ptid.lwp (), (caddr_t) &pl, sizeof pl)
 	  == -1)
-	return false;
+	{
+	  /* EBUSY means the associated process is running which means
+	     the LWP does exist and belongs to a running process.  */
+	  if (errno == EBUSY)
+	    return true;
+	  return false;
+	}
 #ifdef PL_FLAG_EXITED
       if (pl.pl_flags & PL_FLAG_EXITED)
 	return false;
-- 
2.40.0