darwin: warning resuming inferior after attach

This patch avoids a warning when resuming the execution of a program
that we attached to. For instance:

    (gdb) attach 29989
    (gdb) continue
    Continuing.
    warning: Mach error at "/[...]/darwin-nat.c:738" in function
             "darwin_resume_thread": (os/kern) failure (0x5)

The warning is relatively harmless, but worrisome for the average
user.  The warning comes from the fact that darwin_wait recorded
that the fake stop event corresponding to the suspend action was
caused by the main thread being suspended on a SIGTRAP. As a result
of this, when resuming the program's execution, we try to resume
the thread's execution as well.  This is where the issue occurs,
as the thread was not in fact suspended, thus triggering the warning.

gdb/ChangeLog:

        * darwin-nat.c (darwin_setup_fake_stop_event): New function.
        (darwin_attach): Adjust using darwin_setup_fake_stop_event.

1 files changed, 28 insertions, 2 deletions

diff --git a/gdb/darwin-nat.c b/gdb/darwin-nat.c
index 6dedc40..acdbf36 100644
--- a/gdb/darwin-nat.c
+++ b/gdb/darwin-nat.c
@@ -1586,6 +1586,32 @@ darwin_create_inferior (struct target_ops *ops, char *exec_file,
 }
 
 
+/* Set things up such that the next call to darwin_wait will immediately
+   return a fake stop event for inferior INF.
+
+   This assumes that the inferior's thread list has been initialized,
+   as it will suspend the inferior's first thread.  */
+
+static void
+darwin_setup_fake_stop_event (struct inferior *inf)
+{
+  darwin_thread_t *thread;
+  kern_return_t kret;
+
+  gdb_assert (darwin_inf_fake_stop == NULL);
+  darwin_inf_fake_stop = inf;
+
+  /* When detecting a fake pending stop event, darwin_wait returns
+     an event saying that the first thread is in a DARWIN_STOPPED
+     state.  To make that accurate, we need to suspend that thread
+     as well.  Otherwise, we'll try resuming it when resuming the
+     inferior, and get a warning because the thread's suspend count
+     is already zero, making the resume request useless.  */
+  thread = VEC_index (darwin_thread_t, inf->private->threads, 0);
+  kret = thread_suspend (thread->gdb_port);
+  MACH_CHECK_ERROR (kret);
+}
+
 /* Attach to process PID, then initialize for debugging it
    and wait for the trace-trap that results from attaching.  */
 static void
@@ -1637,8 +1663,8 @@ darwin_attach (struct target_ops *ops, char *args, int from_tty)
 
   darwin_check_osabi (inf->private, ptid_get_tid (inferior_ptid));
 
-  gdb_assert (darwin_inf_fake_stop == NULL);
-  darwin_inf_fake_stop = inf;
+  darwin_setup_fake_stop_event (inf);
+
   inf->private->no_ptrace = 1;
 }