gdb/linux-nat: remove check based on current_inferior in linux_handle_extended_wait

The check removed by this patch, using current_inferior, looks wrong.
When debugging multiple inferiors with the Linux native target and
linux_handle_extended_wait is called, there's no guarantee about which
is the current inferior.  The vfork-done event we receive could be for
any inferior.  If the vfork-done event is for a non-current inferior, we
end up wrongfully ignoring it.  As a result, the core never processes a
TARGET_WAITKIND_VFORK_DONE event, program_space::breakpoints_not_allowed
is never cleared, and breakpoints are never reinserted.  However,
because the Linux native target decided to ignore the event, it resumed
the thread - while breakpoints out.  And that's bad.

The proposed fix is to remove this check.  Always report vfork-done
events and let infrun's logic decide if it should be ignored.  We don't
save much cycles by filtering the event here.

Add a test that replicates the situation described above.  See comments
in the test for more details.

Change-Id: Ibe33c1716c3602e847be6c2093120696f2286fbf

gdb/linux-nat.c

@@ -1990,20 +1990,11 @@ linux_handle_extended_wait (struct lwp_info *lp, int status)
 
   if (event == PTRACE_EVENT_VFORK_DONE)
     {
-      if (current_inferior ()->waiting_for_vfork_done)
-	{
-	  linux_nat_debug_printf
-	    ("Got expected PTRACE_EVENT_VFORK_DONE from LWP %ld: stopping",
-	     lp->ptid.lwp ());
-
-	  ourstatus->set_vfork_done ();
-	  return 0;
-	}
-
       linux_nat_debug_printf
-	("Got PTRACE_EVENT_VFORK_DONE from LWP %ld: ignoring", lp->ptid.lwp ());
-
-      return 1;
+	("Got PTRACE_EVENT_VFORK_DONE from LWP %ld",
+	 lp->ptid.lwp ());
+	ourstatus->set_vfork_done ();
+	return 0;
     }
 
   internal_error (__FILE__, __LINE__,

gdb/testsuite/gdb.threads/vfork-multi-inferior-sleep.c

@@ -0,0 +1,25 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022 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>
+
+int
+main (void)
+{
+  sleep (30);
+  return 0;
+}

gdb/testsuite/gdb.threads/vfork-multi-inferior.c

@@ -0,0 +1,55 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022 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 <sys/wait.h>
+#include <assert.h>
+
+static void
+should_break_here (void)
+{
+}
+
+int
+main (void)
+{
+  int i;
+
+  for (i = 0; i < NR_LOOPS; i++)
+    {
+      int pid = vfork ();
+
+      if (pid != 0)
+	{
+	  /* Parent */
+	  int stat;
+	  int ret = waitpid (pid, &stat, 0);
+	  assert (ret == pid);
+	  assert (WIFEXITED (stat));
+	  assert (WEXITSTATUS (stat) == 12);
+
+	  should_break_here ();
+	}
+      else
+	{
+	  /* Child */
+	  _exit (12);
+	}
+    }
+
+  return 0;
+}

gdb/testsuite/gdb.threads/vfork-multi-inferior.exp

@@ -0,0 +1,115 @@
+# Copyright 2020-2022 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 handling a vfork while another inferior is running.  The bug that
+# prompted writing this test case was in the Linux native target.  The target
+# assumed that the vfork-done event it received was for the current inferior
+# (an invalid assumption, the current inferior is the one randomly selected by
+# do_target_wait (at the time of writing).  This caused the target to drop the
+# vfork-done event, because it was seen as unneeded and to restart the thread
+# as if nothing happened.  This however resulted in the thread running with
+# breakpoints not inserted.
+#
+# To catch the bug, this test verifies that we can hit a breakpoint after a
+# vfork call, while a second inferior runs in the background.
+
+if [use_gdb_stub] {
+    unsupported "test uses multiple inferiors"
+    return
+}
+
+standard_testfile .c -sleep.c
+
+set srcfile_sleep $srcfile2
+set binfile_sleep ${binfile}-sleep
+
+# The reproducibility of the bug depends on which inferior randomly selects in
+# do_target_wait when consuming the vfork-done event.  Since GDB doesn't call
+# srand(), we are likely to always see the same sequence of inferior selected by
+# do_target_wait, which can hide the bug if you are not "lucky".  To work
+# around that, call vfork and hit the breakpoint in a loop, it makes it
+# somewhat likely that the wrong inferior will be selected eventually.
+set nr_loops 20
+
+# Compile the main program that calls vfork and hits a breakpoint.
+set opts [list debug additional_flags=-DNR_LOOPS=$nr_loops]
+if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable \
+	$opts] != "" } {
+    untested "failed to compile"
+    return -1
+}
+
+# Compile the secondary program, which just sleeps.
+if  { [gdb_compile "${srcdir}/${subdir}/${srcfile_sleep}" "${binfile_sleep}" executable \
+	{debug}] != "" } {
+    untested "failed to compile"
+    return -1
+}
+
+# We exercise two methods of getting a second inferior to execute while the
+# first one vforks.  METHOD can be:
+#
+#   - non-stop: start GDB with non-stop on and run the second inferior in
+#               background.
+#   - schedule-multiple: set "schedule-multiple on", this will make "continue"
+#                        resume both inferiors.
+proc do_test {method} {
+    save_vars { ::GDBFLAGS } {
+	if { $method == "non-stop" } {
+	    append ::GDBFLAGS " -ex \"set non-stop on\""
+	}
+	clean_restart
+    }
+
+    # Start the second inferior in background.
+    gdb_test "add-inferior" "Added inferior 2.*"
+    gdb_test "inferior 2" "Switching to inferior 2 .*"
+    gdb_file_cmd ${::binfile_sleep}
+    if { $method == "non-stop" } {
+	gdb_test "run &" "Starting program: .*" "run inferior 2"
+    } else {
+	gdb_test "start" "Temporary breakpoint $::decimal, main .*" \
+		"start inferior 2"
+    }
+
+    # Start the first inferior.
+    gdb_test "inferior 1" "Switching to inferior 1 .*"
+    gdb_file_cmd ${::binfile}
+    gdb_test "break should_break_here" "Breakpoint $::decimal at .*"
+    gdb_test "start" "Thread 1.1 .* hit Temporary breakpoint.*" \
+	"start inferior 1"
+
+    # Only enable schedule-multiple this late, because of:
+    # https://sourceware.org/bugzilla/show_bug.cgi?id=28777
+    if { $method == "schedule-multiple" } {
+	gdb_test_no_output "set schedule-multiple on"
+    }
+
+
+    # Continue over vfork and until the breakpoint.  The number of loops here
+    # matches the number of loops in the program.  So if a breakpoint is missed
+    # at some point, a "continue" will wrongfully continue until the end of the
+    # program, which will fail the test.
+    for {set i 0} {$i < $::nr_loops} {incr i} {
+	with_test_prefix "i=$i" {
+	    gdb_test "continue" \
+		"Thread 1.1 .* hit Breakpoint $::decimal, should_break_here.*"
+	}
+    }
+}
+
+foreach_with_prefix method {schedule-multiple non-stop} {
+    do_test $method
+}