Test attaching to a program that constantly spawns short-lived threads

Before the previous fixes, on Linux, this would trigger several
different problems, like:

 [New LWP 27106]
 [New LWP 27047]
 warning: unable to open /proc file '/proc/-1/status'
 [New LWP 27813]
 [New LWP 27869]
 warning: Can't attach LWP 11962: No child processes
 Warning: couldn't activate thread debugging using libthread_db: Cannot find new threads: debugger service failed
 warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.

gdb/testsuite/
2015-01-09  Pedro Alves  <palves@redhat.com>

	* gdb.threads/attach-many-short-lived-threads.c: New file.
	* gdb.threads/attach-many-short-lived-threads.exp: New file.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2015-01-09  Pedro Alves  <palves@redhat.com>
+
+	* gdb.threads/attach-many-short-lived-threads.c: New file.
+	* gdb.threads/attach-many-short-lived-threads.exp: New file.
+
 2014-01-09  Pedro Alves  <palves@redhat.com>
 
 	* gdb.threads/fork-thread-pending.exp: Switch to the main thread

gdb/testsuite/gdb.threads/attach-many-short-lived-threads.c

@@ -0,0 +1,151 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2014-2015 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/>.  */
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <pthread.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <string.h>
+
+pthread_t main_thread;
+pthread_attr_t detached_attr;
+pthread_attr_t joinable_attr;
+
+/* Number of threads we'll create of each variant
+   (joinable/detached).  */
+int n_threads = 50;
+
+/* Mutex used to hold creating detached threads.  */
+pthread_mutex_t dthrds_create_mutex;
+
+/* Wrapper for pthread_create.   */
+
+void
+create_thread (pthread_attr_t *attr,
+	       void *(*start_routine) (void *), void *arg)
+{
+  pthread_t child;
+  int rc;
+
+  while ((rc = pthread_create (&child, attr, start_routine, arg)) != 0)
+    {
+      fprintf (stderr, "unexpected error from pthread_create: %s (%d)\n",
+	       strerror (rc), rc);
+      sleep (1);
+    }
+}
+
+void
+break_fn (void)
+{
+}
+
+/* Data passed to joinable threads on creation.  This is allocated on
+   the heap and ownership transferred from parent to child.  (We do
+   this because it's not portable to cast pthread_t to pointer.)  */
+
+struct thread_arg
+{
+  pthread_t parent;
+};
+
+/* Entry point for joinable threads.  These threads first join their
+   parent before spawning a new child (and exiting).  The parent's tid
+   is passed as pthread_create argument, encapsulated in a struct
+   thread_arg object.  */
+
+void *
+joinable_fn (void *arg)
+{
+  struct thread_arg *p = arg;
+
+  pthread_setname_np (pthread_self (), "joinable");
+
+  if (p->parent != main_thread)
+    assert (pthread_join (p->parent, NULL) == 0);
+
+  p->parent = pthread_self ();
+
+  create_thread (&joinable_attr, joinable_fn, p);
+
+  break_fn ();
+
+  return NULL;
+}
+
+/* Entry point for detached threads.  */
+
+void *
+detached_fn (void *arg)
+{
+  pthread_setname_np (pthread_self (), "detached");
+
+  /* This should throttle threads a bit in case we manage to spawn
+     threads faster than they exit.  */
+  pthread_mutex_lock (&dthrds_create_mutex);
+
+  create_thread (&detached_attr, detached_fn, NULL);
+
+  /* Note this is called before the mutex is unlocked otherwise in
+     non-stop mode, when the breakpoint is hit we'd keep spawning more
+     threads forever while the old threads stay alive (stopped in the
+     breakpoint).  */
+  break_fn ();
+
+  pthread_mutex_unlock (&dthrds_create_mutex);
+
+  return NULL;
+}
+
+int
+main (int argc, char *argv[])
+{
+  int i;
+
+  if (argc > 1)
+    n_threads = atoi (argv[1]);
+
+  pthread_mutex_init (&dthrds_create_mutex, NULL);
+
+  pthread_attr_init (&detached_attr);
+  pthread_attr_setdetachstate (&detached_attr, PTHREAD_CREATE_DETACHED);
+  pthread_attr_init (&joinable_attr);
+  pthread_attr_setdetachstate (&joinable_attr, PTHREAD_CREATE_JOINABLE);
+
+  main_thread = pthread_self ();
+
+  /* Spawn the initial set of test threads.  Some threads are
+     joinable, others are detached.  This exercises different code
+     paths in the runtime.  */
+  for (i = 0; i < n_threads; ++i)
+    {
+      struct thread_arg *p;
+
+      p = malloc (sizeof *p);
+      p->parent = main_thread;
+      create_thread (&joinable_attr, joinable_fn, p);
+
+      create_thread (&detached_attr, detached_fn, NULL);
+    }
+
+  /* Long enough for all the attach/detach sequences done by the .exp
+     file.  */
+  sleep (180);
+  return 0;
+}

gdb/testsuite/gdb.threads/attach-many-short-lived-threads.exp

@@ -0,0 +1,132 @@
+# Copyright 2008-2015 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 attaching to a program that is constantly spawning short-lived
+# threads.  The stresses the edge cases of attaching to threads that
+# have just been created or are in process of dying.  In addition, the
+# test attaches, debugs, detaches, reattaches in a loop a few times,
+# to stress the behavior of the debug API around detach (some systems
+# end up leaving stale state behind that confuse the following
+# attach).
+
+if {![can_spawn_for_attach]} {
+    return 0
+}
+
+standard_testfile
+
+# The test proper.  See description above.
+
+proc test {} {
+    global binfile
+    global gdb_prompt
+    global decimal
+
+    clean_restart ${binfile}
+
+    set testpid [spawn_wait_for_attach $binfile]
+
+    set attempts 10
+    for {set attempt 1} { $attempt <= $attempts } { incr attempt } {
+	with_test_prefix "iter $attempt" {
+	    set attached 0
+	    set eperm 0
+	    set test "attach"
+	    gdb_test_multiple "attach $testpid" $test {
+		-re "new threads in iteration" {
+		    # Seen when "set debug libthread_db" is on.
+		    exp_continue
+		}
+		-re "warning: Cannot attach to lwp $decimal: Operation not permitted" {
+		    # On Linux, PTRACE_ATTACH sometimes fails with
+		    # EPERM, even though /proc/PID/status indicates
+		    # the thread is running.
+		    set eperm 1
+		    exp_continue
+		}
+		-re "debugger service failed.*$gdb_prompt $" {
+		    fail $test
+		}
+		-re "$gdb_prompt $" {
+		    if {$eperm} {
+			xfail "$test (EPERM)"
+		    } else {
+			pass $test
+		    }
+		}
+		-re "Attaching to program.*process $testpid.*$gdb_prompt $" {
+		    pass $test
+		}
+	    }
+
+	    # Sleep a bit and try updating the thread list.  We should
+	    # know about all threads already at this point.  If we see
+	    # "New Thread" or similar being output, then "attach" is
+	    # failing to actually attach to all threads in the process,
+	    # which would be a bug.
+	    sleep 1
+
+	    set test "no new threads"
+	    gdb_test_multiple "info threads" $test {
+		-re "New .*$gdb_prompt $" {
+		    fail $test
+		}
+		-re "$gdb_prompt $" {
+		    pass $test
+		}
+	    }
+
+	    # Force breakpoints always inserted, so that threads we might
+	    # have failed to attach to hit them even when threads we do
+	    # know about are stopped.
+	    gdb_test_no_output "set breakpoint always-inserted on"
+
+	    # Run to a breakpoint a few times.  A few threads should spawn
+	    # and die meanwhile.  This checks that thread creation/death
+	    # events carry on correctly after attaching.  Also, be
+	    # detaching from the program and reattaching, we check that
+	    # the program doesn't die due to gdb leaving a pending
+	    # breakpoint hit on a new thread unprocessed.
+	    gdb_test "break break_fn" "Breakpoint.*" "break break_fn"
+
+	    # Wait a bit, to give time for most threads to hit the
+	    # breakpoint, including threads we might have failed to
+	    # attach.
+	    sleep 2
+
+	    set bps 3
+	    for {set bp 1} { $bp <= $bps } { incr bp } {
+		gdb_test "continue" "Breakpoint.*" "break at break_fn: $bp"
+	    }
+
+	    if {$attempt < $attempts} {
+		gdb_test "detach" "Detaching from.*"
+	    } else {
+		gdb_test "kill" "" "kill process" "Kill the program being debugged.*y or n. $" "y"
+	    }
+
+	    gdb_test_no_output "set breakpoint always-inserted off"
+	    delete_breakpoints
+	}
+    }
+
+    remote_exec target "kill -9 ${testpid}"
+}
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug pthreads}] == -1} {
+    return -1
+}
+
+test