Multi-arch exec, more register reading avoidance

As mentioned in commit bf93d7ba99 ("Add thread after updating
gdbarch when exec'ing"), we should avoid doing register reads after a
process does an exec and before we've updated that inferior's gdbarch.
Otherwise, we may interpret the registers using the wrong
architecture.

There's still (at least) one case where we still read registers
post-exec with the pre-exec architecture.  That's when infrun decides
it needs to switch context to the exec'ing thread.  I.e., if the exec
event is processed at a time when the current thread is not already
the exec'ing thread, then we get (with the test added by this commit):

  continue
  Continuing.
  Truncated register 50 in remote 'g' packet
  Truncated register 50 in remote 'g' packet
  (gdb) FAIL: gdb.multi/multi-arch-exec.exp: selected_thread=2: follow_exec_mode=same: continue across exec that changes architecture

The fix is to avoid reading registers when switching context in this
case.

(I'd be nice to get rid of the constant stop_pc reading when switching
threads, but that'd be a deeper change.)

gdb/ChangeLog:
2017-10-09  Pedro Alves  <palves@redhat.com>

	* infrun.c (handle_inferior_event_1) <TARGET_WAITKIND_EXECD>: Skip
	reading registers when switching context.

gdb/testsuite/ChangeLog:
2017-10-09  Pedro Alves  <palves@redhat.com>

	* gdb.multi/multi-arch-exec.c: Include <pthread.h> and <assert.h>.
	(barrier): New.
	(thread_start, all_started): New functions.
	(main): Spawn new thread and wait until it is scheduled.
	* gdb.multi/multi-arch-exec.exp: Build $srcfile1 with the pthreads
	option.
	(do_test): Add 'selected_thread' parameter.  Run to all_started
	instead of main.  Explicitly set the breakpoint at main.  Switch
	to the SELECTED_THREAD thread.
	(top level): Test handling the exec event with either the main
	thread or the second thread selected.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2017-10-09  Pedro Alves  <palves@redhat.com>
+
+	* infrun.c (handle_inferior_event_1) <TARGET_WAITKIND_EXECD>: Skip
+	reading registers when switching context.
+
 2017-10-09  John Baldwin  <jhb@FreeBSD.org>
 
 	* fbsd-nat.c (fbsd_siginfo_size): Use gdbarch_long_bit.

gdb/infrun.c

@@ -5307,8 +5307,11 @@ Cannot fill $_exitsignal with the correct signal number.\n"));
       if (debug_infrun)
         fprintf_unfiltered (gdb_stdlog, "infrun: TARGET_WAITKIND_EXECD\n");
 
+      /* Note we can't read registers yet (the stop_pc), because we
+	 don't yet know the inferior's post-exec architecture.
+	 'stop_pc' is explicitly read below instead.  */
       if (!ptid_equal (ecs->ptid, inferior_ptid))
-	context_switch (ecs->ptid);
+	switch_to_thread_no_regs (ecs->event_thread);
 
       /* Do whatever is necessary to the parent branch of the vfork.  */
       handle_vfork_child_exec_or_exit (1);

gdb/testsuite/ChangeLog

@@ -1,3 +1,17 @@
+2017-10-09  Pedro Alves  <palves@redhat.com>
+
+	* gdb.multi/multi-arch-exec.c: Include <pthread.h> and <assert.h>.
+	(barrier): New.
+	(thread_start, all_started): New functions.
+	(main): Spawn new thread and wait until it is scheduled.
+	* gdb.multi/multi-arch-exec.exp: Build $srcfile1 with the pthreads
+	option.
+	(do_test): Add 'selected_thread' parameter.  Run to all_started
+	instead of main.  Explicitly set the breakpoint at main.  Switch
+	to the SELECTED_THREAD thread.
+	(top level): Test handling the exec event with either the main
+	thread or the second thread selected.
+
 2017-10-09  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/print-file-var-main.c: Fix get_version_2 value check

gdb/testsuite/gdb.multi/multi-arch-exec.c

@@ -20,11 +20,32 @@
 #include <unistd.h>
 #include <limits.h>
 #include <string.h>
+#include <pthread.h>
+
+#define NUM_THREADS 1
+
+static pthread_barrier_t barrier;
+
+static void *
+thread_start (void *arg)
+{
+  pthread_barrier_wait (&barrier);
+
+  while (1)
+    sleep (1);
+  return NULL;
+}
+
+static void
+all_started (void)
+{
+}
 
 int
 main (int argc, char ** argv)
 {
   char prog[PATH_MAX];
+  pthread_t thread;
   int len;
 
   strcpy (prog, argv[0]);
@@ -33,6 +54,12 @@ main (int argc, char ** argv)
   memcpy (prog + len - 15, "multi-arch-exec-hello", 21);
   prog[len + 6] = 0;
 
+  pthread_barrier_init (&barrier, NULL, NUM_THREADS + 1);
+  pthread_create (&thread, NULL, thread_start, NULL);
+
+  pthread_barrier_wait (&barrier);
+  all_started ();
+
   execl (prog,
          prog,
          (char *) NULL);

gdb/testsuite/gdb.multi/multi-arch-exec.exp

@@ -53,7 +53,7 @@ if [istarget "s390*-*-*"] {
 }
 
 if { [prepare_for_testing "failed to prepare" ${exec1} "${srcfile1}" \
-	  [list debug \
+	  [list debug pthreads \
 	       additional_flags=${march1}]] } {
     return -1
 }
@@ -76,22 +76,38 @@ if { [prepare_for_testing "failed to prepare" ${exec2} "${srcfile2}" \
     return -1
 }
 
-proc do_test { mode } {
+proc do_test { mode selected_thread } {
 	global exec1
 
 	clean_restart ${exec1}
-	if ![runto_main] then {
-	    fail "couldn't run to main"
+	if ![runto all_started] then {
+	    fail "couldn't run to all_started"
 	    return -1
 	}
 
+	# Delete the breakpoint at 'all_started' otherwise GDB may
+	# switch context back to thread 1 to step over the breakpoint.
+	delete_breakpoints
+
+	# A location for this breakpoint should be found in the new
+	# post-exec image too.
+	gdb_breakpoint main
+
+	gdb_test "thread $selected_thread" "Switching to thread $selected_thread .*"
+
 	gdb_test_no_output "set follow-exec-mode $mode"
 
 	# Test that GDB updates the target description / arch successfuly
 	# after the exec.
-	gdb_test "continue" "Breakpoint 1, main.*" "continue across exec that changes architecture"
+	gdb_test "continue" "Breakpoint 2, main.*" "continue across exec that changes architecture"
 }
 
-foreach follow_exec_mode {"same" "new"} {
-    do_test $follow_exec_mode
+# Test handling the exec event with either the main thread or the
+# second thread selected.  This tries to ensure that GDB doesn't read
+# registers off of the execing thread before figuring out its
+# architecture.
+foreach_with_prefix selected_thread {1 2} {
+    foreach_with_prefix follow_exec_mode {"same" "new"} {
+	do_test $follow_exec_mode $selected_thread
+    }
 }