[gdb] Handle vfork in thread with follow-fork-mode child

When debugging any of the testcases added by this commit, which do a
vfork in a thread with "set follow-fork-mode child" + "set
detach-on-fork on", we run into this assertion:

...
src/gdb/nat/x86-linux-dregs.c:146: internal-error: \
  void x86_linux_update_debug_registers(lwp_info*): \
  Assertion `lwp_is_stopped (lwp)' failed.
...

The assert is caused by the following: the vfork-child exit or exec
event is handled by handle_vfork_child_exec_or_exit, which calls
target_detach to detach from the vfork parent.  During target_detach
we call linux_nat_target::detach, which:

#1 - stops all the threads
#2 - waits for all the threads to be stopped
#3 - detaches all the threads

However, during the second step we run into this code in
stop_wait_callback:

...
  /* If this is a vfork parent, bail out, it is not going to report
     any SIGSTOP until the vfork is done with.  */
  if (inf->vfork_child != NULL)
    return 0;
...

and we don't wait for the threads to be stopped, which results in this
assert in x86_linux_update_debug_registers triggering during the third
step:

...
  gdb_assert (lwp_is_stopped (lwp));
...

The fix is to reset the vfork parent's vfork_child field before
calling target_detach in handle_vfork_child_exec_or_exit.  There's
already similar code for the other paths handled by
handle_vfork_child_exec_or_exit, so this commit refactors the code a
bit so that all paths share the same code.

The new tests cover both a vfork child exiting, and a vfork child
execing, since both cases would trigger the assertion.

The new testcases also exercise following the vfork children with "set
detach-on-fork off", since it doesn't seem to be tested anywhere.

Tested on x86_64-linux, using native and native-gdbserver.

gdb/ChangeLog:
2019-04-18  Tom de Vries  <tdevries@suse.de>
	    Pedro Alves  <palves@redhat.com>

	PR gdb/24454
	* infrun.c (handle_vfork_child_exec_or_exit): Reset vfork parent's
	vfork_child field before calling target_detach.

gdb/testsuite/ChangeLog:
2019-04-18  Tom de Vries  <tdevries@suse.de>
	    Pedro Alves  <palves@redhat.com>

	PR gdb/24454
	* gdb.threads/vfork-follow-child-exec.c: New file.
	* gdb.threads/vfork-follow-child-exec.exp: New file.
	* gdb.threads/vfork-follow-child-exit.c: New file.
	* gdb.threads/vfork-follow-child-exit.exp: New file.

4 files changed, 242 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.threads/vfork-follow-child-exec.c b/gdb/testsuite/gdb.threads/vfork-follow-child-exec.c
new file mode 100644
index 0000000..80632d1
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/vfork-follow-child-exec.c
@@ -0,0 +1,66 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2019 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 <stdio.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <string.h>
+#include <stdlib.h>
+
+static char *program_name;
+
+static void *
+f (void *arg)
+{
+  int res = vfork ();
+
+  if (res == -1)
+    {
+      perror ("vfork");
+      return NULL;
+    }
+  else if (res == 0)
+    {
+      /* Child.  */
+      execl (program_name, program_name, "1", NULL);
+      perror ("exec");
+      abort ();
+    }
+  else
+    {
+      /* Parent.  */
+      return NULL;
+    }
+}
+
+int
+main (int argc, char **argv)
+{
+  pthread_t tid;
+
+  if (argc > 1)
+    {
+      /* Getting here via execl.  */
+      return 0;
+    }
+
+  program_name = argv[0];
+
+  pthread_create (&tid, NULL, f, NULL);
+  pthread_join (tid, NULL);
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/vfork-follow-child-exec.exp b/gdb/testsuite/gdb.threads/vfork-follow-child-exec.exp
new file mode 100644
index 0000000..5a28715
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/vfork-follow-child-exec.exp
@@ -0,0 +1,64 @@
+# Copyright (C) 2019 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/>.  */
+
+# Test following a vfork child that execs, when the vfork parent is a
+# threaded program, and it's a non-main thread that vforks.
+
+standard_testfile
+
+if {[build_executable "failed to prepare" $testfile $srcfile {debug pthreads}]} {
+    return -1
+}
+
+# DETACH indicates whether "set detach-on-fork" is enabled.  It is
+# either "on" or "off".
+
+proc test_vfork {detach} {
+    global binfile
+
+    clean_restart $binfile
+
+    if ![runto_main] then {
+	fail "can't run to main"
+	return 0
+    }
+
+    delete_breakpoints
+
+    gdb_test_no_output "set follow-fork-mode child"
+    gdb_test_no_output "set detach-on-fork $detach"
+
+    if {$detach == "off"} {
+	gdb_test "continue" \
+	    [multi_line \
+		 "Attaching after .* vfork to child .*" \
+		 ".*New inferior 2 .*" \
+		 ".* is executing new program: .*" \
+		 ".*Inferior 2 .* exited normally.*"]
+    } else {
+	gdb_test "continue" \
+	    [multi_line \
+		 "Attaching after .* vfork to child .*" \
+		 ".*New inferior 2 .*" \
+		 ".*Detaching vfork parent process .* after child exec.*" \
+		 ".*Inferior 1 .* detached.*" \
+		 ".*is executing new program: .*" \
+		 ".*Inferior 2 .*exited normally.*"]
+    }
+}
+
+foreach_with_prefix detach-on-fork {"off" "on"} {
+    test_vfork ${detach-on-fork}
+}
diff --git a/gdb/testsuite/gdb.threads/vfork-follow-child-exit.c b/gdb/testsuite/gdb.threads/vfork-follow-child-exit.c
new file mode 100644
index 0000000..6ae254c
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/vfork-follow-child-exit.c
@@ -0,0 +1,52 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2019 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 <stdio.h>
+#include <unistd.h>
+#include <pthread.h>
+
+static void *
+f (void *arg)
+{
+  int res = vfork ();
+
+  if (res == -1)
+    {
+      perror ("vfork");
+      return NULL;
+    }
+  else if (res == 0)
+    {
+      /* Child.  */
+      _exit (0);
+    }
+  else
+    {
+      /* Parent.  */
+      return NULL;
+    }
+}
+
+int
+main (void)
+{
+  pthread_t tid;
+
+  pthread_create (&tid, NULL, f, NULL);
+  pthread_join (tid, NULL);
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/vfork-follow-child-exit.exp b/gdb/testsuite/gdb.threads/vfork-follow-child-exit.exp
new file mode 100644
index 0000000..f07215d
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/vfork-follow-child-exit.exp
@@ -0,0 +1,60 @@
+# Copyright (C) 2019 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/>.  */
+
+# Test following a vfork child that exits, when the vfork parent is a
+# threaded program, and it's a non-main thread that vforks.
+
+standard_testfile
+
+if {[build_executable "failed to prepare" $testfile $srcfile {debug pthreads}]} {
+    return -1
+}
+
+# DETACH indicates whether "set detach-on-fork" is enabled.  It is
+# either "on" or "off".
+
+proc test_vfork {detach} {
+    global binfile
+
+    clean_restart $binfile
+
+    if ![runto_main] then {
+	fail "can't run to main"
+	return 0
+    }
+
+    gdb_test_no_output "set follow-fork-mode child"
+    gdb_test_no_output "set detach-on-fork $detach"
+
+    if {$detach == "off"} {
+	gdb_test "continue" \
+	    [multi_line \
+		 "Attaching after .* vfork to child .*" \
+		 ".*New inferior 2 .*" \
+		 ".*Inferior 2 .*exited normally.*"]
+    } else {
+	gdb_test "continue" \
+	    [multi_line \
+		 "Attaching after .* vfork to child .*" \
+		 ".*New inferior 2 .*" \
+		 ".*Detaching vfork parent process .* after child exit.*" \
+		 ".*Inferior 1 .* detached.*" \
+		 ".*Inferior 2 .*exited normally.*"]
+    }
+}
+
+foreach_with_prefix detach-on-fork {"off" "on"} {
+    test_vfork ${detach-on-fork}
+}