gdb: Fix assert for extended-remote target (PR gdb/18050)

Consider the following GDB session:

   (gdb) target extended-remote :2347
   (gdb) file /path/to/exe
   (gdb) set remote exec-file /path/to/exe
   (gdb) set detach-on-fork off
   (gdb) break breakpt
   (gdb) run
   # ... hits breakpoint
   (gdb) info inferiors
     Num  Description       Executable
   * 1    process 17001     /path/to/exe
     2    process 17002     /path/to/exe
   (gdb) kill
   (gdb) info inferiors
     Num  Description       Executable
   * 1    <null>            /path/to/exe
     2    process 17002     /path/to/exe
   (gdb) target extended-remote :2348
   ../../src/gdb/thread.c:660: internal-error: thread_info* any_thread_of_process(int): Assertion `pid != 0' failed.
   A problem internal to GDB has been detected,
   further debugging may prove unreliable.

Or, from bug PR gdb/18050:

   (gdb) start
   (gdb) add-inferior -exec /path/to/exe
   (gdb) target extended-remote :2347
   ../../src/gdb/thread.c:660: internal-error: thread_info* any_thread_of_process(int): Assertion `pid != 0' failed.
   A problem internal to GDB has been detected,
   further debugging may prove unreliable.

The issue is calling target.c:dispose_inferior with a killed inferior in
the inferior list.  This assertion is fixed in this commit.

The new test for this issue only runs on platforms that support
'detach-on-fork', and when using
'--target_board=native-extended-gdbserver'.

gdb/ChangeLog:

	PR gdb/18050:
	* target.c (dispose_inferior): Don't dispose of inferiors that are
	already killed.

gdb/testsuite/ChangeLog:

	PR gdb/18050:
	* gdb.server/extended-remote-restart.c: New file.
	* gdb.server/extended-remote-restart.exp: New file.

1 files changed, 60 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.server/extended-remote-restart.c b/gdb/testsuite/gdb.server/extended-remote-restart.c
new file mode 100644
index 0000000..e195f48
--- /dev/null
+++ b/gdb/testsuite/gdb.server/extended-remote-restart.c
@@ -0,0 +1,60 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2018 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 <unistd.h>
+#include <stdlib.h>
+
+static void
+breakpt ()
+{
+  asm ("" ::: "memory");
+}
+
+static void
+go_child ()
+{
+  breakpt ();
+
+  while (1)
+    sleep (1);
+}
+
+static void
+go_parent ()
+{
+  breakpt ();
+
+  while (1)
+    sleep (1);
+}
+
+int
+main ()
+{
+  pid_t pid;
+
+  pid = fork ();
+  if (pid == -1)
+    abort ();
+
+  if (pid == 0)
+    go_child ();
+  else
+    go_parent ();
+
+  exit (EXIT_SUCCESS);
+}