follow-exec: delete all non-execing threads

This fixes invalid reads Valgrind first caught when debugging against
a GDBserver patched with a series that adds exec events to the remote
protocol.  Like these, using the gdb.threads/thread-execl.exp test:

$ valgrind ./gdb -data-directory=data-directory ./testsuite/gdb.threads/thread-execl  -ex "tar extended-remote :9999" -ex "b thread_execler" -ex "c" -ex "set scheduler-locking on"
...
Breakpoint 1, thread_execler (arg=0x0) at src/gdb/testsuite/gdb.threads/thread-execl.c:29
29        if (execl (image, image, NULL) == -1)
(gdb) n
Thread 32509.32509 is executing new program: build/gdb/testsuite/gdb.threads/thread-execl
[New Thread 32509.32532]
==32510== Invalid read of size 4
==32510==    at 0x5AA7D8: delete_breakpoint (breakpoint.c:13989)
==32510==    by 0x6285D3: delete_thread_breakpoint (thread.c:100)
==32510==    by 0x628603: delete_step_resume_breakpoint (thread.c:109)
==32510==    by 0x61622B: delete_thread_infrun_breakpoints (infrun.c:2928)
==32510==    by 0x6162EF: for_each_just_stopped_thread (infrun.c:2958)
==32510==    by 0x616311: delete_just_stopped_threads_infrun_breakpoints (infrun.c:2969)
==32510==    by 0x616C96: fetch_inferior_event (infrun.c:3267)
==32510==    by 0x63A2DE: inferior_event_handler (inf-loop.c:57)
==32510==    by 0x4E0E56: remote_async_serial_handler (remote.c:11877)
==32510==    by 0x4AF620: run_async_handler_and_reschedule (ser-base.c:137)
==32510==    by 0x4AF6F0: fd_event (ser-base.c:182)
==32510==    by 0x63806D: handle_file_event (event-loop.c:762)
==32510==  Address 0xcf333e0 is 16 bytes inside a block of size 200 free'd
==32510==    at 0x4A07577: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==32510==    by 0x77CB74: xfree (common-utils.c:98)
==32510==    by 0x5AA954: delete_breakpoint (breakpoint.c:14056)
==32510==    by 0x5988BD: update_breakpoints_after_exec (breakpoint.c:3765)
==32510==    by 0x61360F: follow_exec (infrun.c:1091)
==32510==    by 0x6186FA: handle_inferior_event (infrun.c:4061)
==32510==    by 0x616C55: fetch_inferior_event (infrun.c:3261)
==32510==    by 0x63A2DE: inferior_event_handler (inf-loop.c:57)
==32510==    by 0x4E0E56: remote_async_serial_handler (remote.c:11877)
==32510==    by 0x4AF620: run_async_handler_and_reschedule (ser-base.c:137)
==32510==    by 0x4AF6F0: fd_event (ser-base.c:182)
==32510==    by 0x63806D: handle_file_event (event-loop.c:762)
==32510==
[Switching to Thread 32509.32532]

Breakpoint 1, thread_execler (arg=0x0) at src/gdb/testsuite/gdb.threads/thread-execl.c:29
29        if (execl (image, image, NULL) == -1)
(gdb)

The breakpoint in question is the step-resume breakpoint of the
non-main thread, the one that was "next"ed.

The exact same issue can be seen on mainline with native debugging, by
running the thread-execl.exp test in non-stop mode, because the kernel
doesn't report a thread exit event for the execing thread.

Tested on x86_64 Fedora 20.

gdb/ChangeLog:
2015-03-02  Pedro Alves  <palves@redhat.com>

	* infrun.c (follow_exec): Delete all threads of the process except
	the event thread.  Extended comments.

gdb/testsuite/ChangeLog:
2015-03-02  Pedro Alves  <palves@redhat.com>

	* gdb.threads/thread-execl.exp (do_test): Handle non-stop.
	(top level): Call do_test with non-stop as well.

2 files changed, 26 insertions, 3 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 76ed5e2..3b7bf7d 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2015-03-02  Pedro Alves  <palves@redhat.com>
 
+	* gdb.threads/thread-execl.exp (do_test): Handle non-stop.
+	(top level): Call do_test with non-stop as well.
+
+2015-03-02  Pedro Alves  <palves@redhat.com>
+
 	* lib/gdb.exp (gdb_test_multiple) <internal error>: Set result to
 	-1.
 
diff --git a/gdb/testsuite/gdb.threads/thread-execl.exp b/gdb/testsuite/gdb.threads/thread-execl.exp
index 4a8016c..a598ad0 100644
--- a/gdb/testsuite/gdb.threads/thread-execl.exp
+++ b/gdb/testsuite/gdb.threads/thread-execl.exp
@@ -34,9 +34,18 @@ if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" \
 proc do_test { schedlock } {
     global binfile
 
-    with_test_prefix "schedlock $schedlock" {
+    if {$schedlock == "non-stop"} {
+	set prefix $schedlock
+    } else {
+	set prefix "schedlock $schedlock"
+    }
+    with_test_prefix "$prefix" {
 	clean_restart ${binfile}
 
+	if {$schedlock == "non-stop"} {
+	    gdb_test_no_output "set non-stop 1"
+	}
+
 	if ![runto_main] {
 	    return 0
 	}
@@ -45,16 +54,25 @@ proc do_test { schedlock } {
 	gdb_breakpoint "thread_execler"
 	gdb_test "continue" ".*thread_execler.*" "continue to thread start"
 
+	if {$schedlock == "non-stop"} {
+	    gdb_test "thread 2" \
+		"Switching to .*thread_execler.*" \
+		"switch to event thread"
+	}
+
 	# Now set a breakpoint at `main', and step over the execl call.  The
 	# breakpoint at main should be reached.  GDB should not try to revert
 	# back to the old thread from the old image and resume stepping it
 	# (since it is gone).
 	gdb_breakpoint "main"
-	gdb_test_no_output "set scheduler-locking $schedlock"
+
+	if {$schedlock != "non-stop"} {
+	    gdb_test_no_output "set scheduler-locking $schedlock"
+	}
 	gdb_test "next" ".*main.*" "get to main in new image"
     }
 }
 
-foreach schedlock {"off" "step" "on"} {
+foreach schedlock {"off" "step" "on" "non-stop"} {
     do_test $schedlock
 }