[gdb] Fix hang after ext sigkill

Consider the test-case from this patch, compiled with pthread support:
...
$ gcc gdb/testsuite/gdb.threads/killed-outside.c -lpthread -g
...

After running to all_started, we can print pid:
...
$ gdb a.out -ex "b all_started" -ex run -ex "delete 1" -ex "p pid"
...
Reading symbols from a.out...
Breakpoint 1 at 0x40072b: file killed-outside.c, line 29.
Starting program: /data/gdb_versions/devel/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[New Thread 0x7ffff77fc700 (LWP 3155)]

Thread 1 "a.out" hit Breakpoint 1, all_started () at killed-outside.c:29
29      }
$1 = 3151
(gdb)
...

If we then kill the inferior using an external SIGKILL:
...
(gdb) shell kill -9 3151
...
and subsequently continue:
...
(gdb) c
Continuing.
Couldn't get registers: No such process.
Couldn't get registers: No such process.
(gdb) Couldn't get registers: No such process.
(gdb) Couldn't get registers: No such process.
(gdb) Couldn't get registers: No such process.
<repeat>
...
gdb hangs repeating the same warning.  Typing control-C no longer helps,
and we have to kill gdb.

This is a regression since commit 873657b9e8 "Preserve selected thread in
all-stop w/ background execution".  The commit adds a
scoped_restore_current_thread typed variable restore_thread to
fetch_inferior_event, and the hang is caused by the constructor throwing an
exception.

Fix this by catching the exception in the constructor.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-04-21  Tom de Vries  <tdevries@suse.de>

	PR gdb/25471
	* thread.c
	(scoped_restore_current_thread::scoped_restore_current_thread): Catch
	exception in get_frame_id.

gdb/testsuite/ChangeLog:

2020-04-21  Tom de Vries  <tdevries@suse.de>

	PR gdb/25471
	* gdb.threads/killed-outside.c: New test.
	* gdb.threads/killed-outside.exp: New file.

1 files changed, 64 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.threads/killed-outside.c b/gdb/testsuite/gdb.threads/killed-outside.c
new file mode 100644
index 0000000..ac4cf0b
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/killed-outside.c
@@ -0,0 +1,64 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <pthread.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+pid_t pid;
+
+static pthread_barrier_t threads_started_barrier;
+
+static void
+all_started (void)
+{
+}
+
+static void *
+fun (void *dummy)
+{
+  int i;
+
+  pthread_barrier_wait (&threads_started_barrier);
+
+  for (i = 0; i < 180; i++)
+    sleep (1);
+
+  pthread_exit (NULL);
+}
+
+int
+main (void)
+{
+  int i;
+  pthread_t thread;
+
+  pid = getpid ();
+
+  pthread_barrier_init (&threads_started_barrier, NULL, 2);
+
+  pthread_create (&thread, NULL, fun, NULL);
+
+  pthread_barrier_wait (&threads_started_barrier);
+
+  all_started ();
+
+  for (i = 0; i < 180; i++)
+    sleep (1);
+
+  exit (EXIT_SUCCESS);
+}