Fix hang after ext sigkill

diff --git a/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.c b/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.c
new file mode 100644
index 0000000000..bfce6c3085
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.c
@@ -0,0 +1,40 @@ 
+/* 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>
+
+static void *
+fun (void *dummy)
+{
+  while (1)
+    sleep (1);
+
+  return NULL;
+}
+
+int
+main (void)
+{
+  pthread_t thread;
+  pthread_create (&thread, NULL, fun, NULL);
+
+  while (1)
+    sleep (1);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.exp b/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.exp
new file mode 100644
index 0000000000..3702eb8c13
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.exp
@@ -0,0 +1,73 @@ 
+# Copyright (C) 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/>.
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile \
+	 {pthreads}] == -1} {
+    return -1
+}
+
+set res [runto main no-message temporary]
+if { $res != 1 } {
+    return -1
+}
+
+set pid -1
+gdb_test_multiple "info inferior 1" "get inferior pid" {
+    -re -wrap "process (\[0-9\]*).*" {
+       set pid $expect_out(1,string)
+       pass $gdb_test_name
+    }
+}
+if { $pid == -1 } {
+    return -1
+}
+
+gdb_test_multiple "continue" "" {
+    -re "Continuing" {
+	pass $gdb_test_name
+    }
+}
+
+send_gdb "\003"
+
+gdb_test_multiple "" "get sigint" {
+    -re -wrap "Thread \[0-9\]+ .* received signal SIGINT, Interrupt\..*" {
+       pass $gdb_test_name
+   }
+}
+
+gdb_test_no_output "shell kill -s SIGKILL $pid" "shell kill -s SIGKILL pid"
+
+# Expect the signal to arrive, and the program to terminate.
+# Ideally at the first continue, currently though at the second.
+set killed_msg "Program terminated with signal SIGKILL, Killed\."
+set no_longer_exists_msg "The program no longer exists\."
+set not_being_run_msg "The program is not being run\."
+set msg [join [list $killed_msg $no_longer_exists_msg $not_being_run_msg] \
+	     ".*"]
+
+# Don't expect '$gdb_prompt $', there still may be output after the prompt,
+# f.i.:
+# ...
+# (gdb) [Thread 0x7ffff74c6700 (LWP 19277) exited]^M
+# ...
+gdb_test_multiple "continue" "" {
+    -re ".*\r\n$gdb_prompt " {
+	gdb_test "continue" $msg $gdb_test_name
+    }
+}
+
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 81518b9646..745694df2d 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -571,7 +571,7 @@  proc runto { function args } {
 	    }
 	    return 1
 	}
-	-re "Breakpoint \[0-9\]*, \[0-9xa-f\]* in .*$gdb_prompt $" { 
+	-re "\[Bb\]reakpoint \[0-9\]*, \[0-9xa-f\]* in .*$gdb_prompt $" {
 	    if { $print_pass } {
 		pass $test_name
 	    }
diff --git a/gdb/thread.c b/gdb/thread.c
index 54b59e2244..9876ca3c76 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -1444,6 +1444,8 @@  scoped_restore_current_thread::restore ()
 
 scoped_restore_current_thread::~scoped_restore_current_thread ()
 {
+  if (m_inf == NULL)
+    return;
   if (!m_dont_restore)
     {
       try
@@ -1488,7 +1490,17 @@  scoped_restore_current_thread::scoped_restore_current_thread ()
       else
 	frame = NULL;
 
-      m_selected_frame_id = get_frame_id (frame);
+      try
+       {
+	 m_selected_frame_id = get_frame_id (frame);
+       }
+      catch (const gdb_exception &ex)
+       {
+	 m_inf = NULL;
+	 m_selected_frame_id = null_frame_id;
+	 m_selected_frame_level = -1;
+	 return;
+       }
       m_selected_frame_level = frame_relative_level (frame);
 
       tp->incref ();