Fix PR gdb/19828: gdb -p <process from a container>: internal error

When GDB attaches to a process, it looks at the /proc/PID/task/ dir
for all clone threads of that process, and attaches to each of them.

Usually, if there is more than one clone thread, it means the program
is multi threaded and linked with pthreads.  Thus when GDB soon after
attaching finds and loads a libthread_db matching the process, it'll
add a thread to the thread list for each of the initially found
lower-level LWPs.

If, however, GDB fails to find/load a matching libthread_db, nothing
is adding the LWPs to the thread list.  And because of that, "detach"
hits an internal error:

  (gdb) PASS: gdb.threads/clone-attach-detach.exp: fg attach 1: attach
  info threads
    Id   Target Id         Frame
  * 1    LWP 6891 "clone-attach-de" 0x00007f87e5fd0790 in __nanosleep_nocancel () at ../sysdeps/unix/syscall-template.S:84
  (gdb) FAIL: gdb.threads/clone-attach-detach.exp: fg attach 1: info threads shows two LWPs
  detach
  .../src/gdb/thread.c:1010: internal-error: is_executing: Assertion `tp' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n)
  FAIL: gdb.threads/clone-attach-detach.exp: fg attach 1: detach (GDB internal error)

From here:

  ...
  #8  0x00000000007ba7cc in internal_error (file=0x98ea68 ".../src/gdb/thread.c", line=1010, fmt=0x98ea30 "%s: Assertion `%s' failed.")
      at .../src/gdb/common/errors.c:55
  #9  0x000000000064bb83 in is_executing (ptid=...) at .../src/gdb/thread.c:1010
  #10 0x00000000004c23bb in get_pending_status (lp=0x12c5cc0, status=0x7fffffffdc0c) at .../src/gdb/linux-nat.c:1235
  #11 0x00000000004c2738 in detach_callback (lp=0x12c5cc0, data=0x0) at .../src/gdb/linux-nat.c:1317
  #12 0x00000000004c1a2a in iterate_over_lwps (filter=..., callback=0x4c2599 <detach_callback>, data=0x0) at .../src/gdb/linux-nat.c:899
  #13 0x00000000004c295c in linux_nat_detach (ops=0xe7bd30, args=0x0, from_tty=1) at .../src/gdb/linux-nat.c:1358
  #14 0x000000000068284d in delegate_detach (self=0xe7bd30, arg1=0x0, arg2=1) at .../src/gdb/target-delegates.c:34
  #15 0x0000000000694141 in target_detach (args=0x0, from_tty=1) at .../src/gdb/target.c:2241
  #16 0x0000000000630582 in detach_command (args=0x0, from_tty=1) at .../src/gdb/infcmd.c:2975
  ...

Tested on x86-64 Fedora 23.  Also confirmed the test passes against
gdbserver with "maint set target-non-stop".

gdb/ChangeLog:
2016-05-24  Pedro Alves  <palves@redhat.com>

	PR gdb/19828
	* linux-nat.c (attach_proc_task_lwp_callback): Mark the lwp
	resumed, and add the thread to GDB's thread list.

testsuite/ChangeLog:
2016-05-24  Pedro Alves  <palves@redhat.com>

	PR gdb/19828
	* gdb.threads/clone-attach-detach.c: New file.
	* gdb.threads/clone-attach-detach.exp: New file.

3 files changed, 170 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index c50edad..16b756c 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2016-05-24  Pedro Alves  <palves@redhat.com>
+
+	PR gdb/19828
+	* gdb.threads/clone-attach-detach.c: New file.
+	* gdb.threads/clone-attach-detach.exp: New file.
+
 2016-05-24  Francis Ricci  <francisjricci@gmail.com>  (tiny patch)
 
 	* gdb.base/annota-input-while-running.exp: Fix syntax error.
diff --git a/gdb/testsuite/gdb.threads/clone-attach-detach.c b/gdb/testsuite/gdb.threads/clone-attach-detach.c
new file mode 100644
index 0000000..daa8f1f
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/clone-attach-detach.c
@@ -0,0 +1,66 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2016 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 <sched.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#define STACK_SIZE 0x1000
+
+int clone_pid;
+
+static int
+clone_fn (void *unused)
+{
+  /* Wait for alarm.  */
+  while (1)
+    sleep (1);
+  return 0;
+}
+
+int
+main (int argc, char **argv)
+{
+  unsigned char *stack;
+  int res;
+
+  alarm (300);
+
+  stack = malloc (STACK_SIZE);
+  assert (stack != NULL);
+
+#define CLONE_FLAGS (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)
+
+#ifdef __ia64__
+  clone_pid = __clone2 (clone_fn, stack, STACK_SIZE, CLONE_FLAGS, NULL);
+#else
+  clone_pid = clone (clone_fn, stack + STACK_SIZE, CLONE_FLAGS, NULL);
+#endif
+
+  assert (clone_pid > 0);
+
+  /* Wait for alarm.  */
+  while (1)
+    sleep (1);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/clone-attach-detach.exp b/gdb/testsuite/gdb.threads/clone-attach-detach.exp
new file mode 100644
index 0000000..42e9914
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/clone-attach-detach.exp
@@ -0,0 +1,98 @@
+# This testcase is part of GDB, the GNU debugger.
+
+# Copyright 2016 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 attach / detach from a process that uses raw clone.  We use raw
+# clone as proxy for when libthread_db is not available.
+
+# This only works on targets with the Linux kernel.
+if ![istarget *-*-linux*] {
+    return
+}
+
+if {![can_spawn_for_attach]} {
+    return 0
+}
+
+standard_testfile
+
+if [prepare_for_testing "failed to prepare" $testfile $srcfile {debug}] {
+    return -1
+}
+
+clean_restart ${binfile}
+
+set test_spawn_id [spawn_wait_for_attach $binfile]
+set testpid [spawn_id_get_pid $test_spawn_id]
+
+# Native/gdbserver.
+set thread_re "(LWP $decimal|Thread )"
+
+# Try attach / detach a few times, in case GDB ends up with stale
+# state after detaching.
+
+set attempts 3
+for {set attempt 1} {$attempt <= $attempts} {incr attempt} {
+    with_test_prefix "fg attach $attempt" {
+
+	gdb_test "attach $testpid" \
+	    "Attaching to program.*process $testpid.*" \
+	    "attach"
+
+	gdb_test "info threads" \
+	    "1.*${thread_re}.*\r\n.*2.*${thread_re}.*" \
+	    "info threads shows two LWPs"
+
+	gdb_test "detach" "Detaching from .*, process $testpid"
+    }
+}
+
+# Same, but use async/background attach.
+
+# If debugging with target remote, check whether the all-stop variant
+# of the RSP is being used.  If so, we can't run the background tests.
+if {[target_info exists gdb_protocol]
+    && ([target_info gdb_protocol] == "remote"
+	|| [target_info gdb_protocol] == "extended-remote")} {
+
+    set test "maint show target-non-stop"
+    gdb_test_multiple "maint show target-non-stop" $test {
+	-re "(is|currently) on.*$gdb_prompt $" {
+	}
+	-re "(is|currently) off.*$gdb_prompt $" {
+	    unsupported "bg attach: can't issue info threads while target is running"
+	    return 0
+	}
+    }
+}
+
+set attempts 3
+for {set attempt 1} {$attempt <= $attempts} {incr attempt} {
+    with_test_prefix "bg attach $attempt" {
+
+	gdb_test "attach $testpid &" \
+	    "Attaching to program.*process $testpid.*" \
+	    "attach"
+
+	gdb_test "info threads" \
+	    "1.*${thread_re}.*\\(running\\)\r\n.*2.*${thread_re}.*\\(running\\)" \
+	    "info threads shows two LWPs"
+
+	gdb_test "detach" "Detaching from .*, process $testpid"
+    }
+}
+
+kill_wait_spawned_process $test_spawn_id