Hang during attach

I have some code which worked under LLVM+LLDB 3.6.0 which runs as follows: its purpose is to run some code, and print a backtrace if the code segfaults. My approach was:

  • fork

  • the child runs the main part of the program.

  • the parent creates a debugger and attaches to the child. The child is continued and allowed to run until either it terminates or faults.

This code worked in 3.6.0:

switch (pid = fork()) {
case -1:
return; // no trace

case 0:
// child
fprintf(stderr, “child 1\n”);
fprintf(stderr, “child 2\n”);
signal(SIGUSR1, SIG_DFL);
fprintf(stderr, “child 3\n”);

// parent: create debugger
StateType state;
//SBAttachInfo ai(pid);

m_Debugger = SBDebugger::Create(false);
if (!m_Debugger.IsValid())
fprintf (stderr, “error: failed to create a debugger object\n”);
m_Listener = m_Debugger.GetListener();
m_Target = m_Debugger.CreateTarget(exec_name);
fprintf(stderr, “parent 1\n”);
//m_Target.Attach(ai, m_Error);
m_Target.AttachToProcessWithID(m_Listener, pid, m_Error);
fprintf(stderr, “parent 2\n”);

Under LLVM+LLDB 3.7.1, I see the following instead:

  • the child runs to completion (child 1/2/3 messages print out).

  • the parent hangs in attach (parent 1 prints out but parent 2 does not)

Debugging the whole thing under GDB (unsure how reliable this is, but the hang happens w/o GDB as well) shows:

child 1
child 2
child 3

[ child prints out other stuff and runs happily ]

parent 1

[ child terminates ]

[New Thread 0x7fffed043700 (LWP 470)]
[New Thread 0x7fffec842700 (LWP 472)]
[Thread 0x7fffec842700 (LWP 472) exited]
Program received signal SIGINT, Interrupt.
0x00007ffff2bd566b in pthread_join (threadid=140737169864448, thread_return=0x0)
at pthread_join.c:92
92 pthread_join.c: No such file or directory.
(gdb) bt
#0 0x00007ffff2bd566b in pthread_join (threadid=140737169864448,
thread_return=0x0) at pthread_join.c:92
#1 0x00007ffff5f8247c in lldb_private::HostThreadPosix::Join(void**) ()
from /tools/llvm/3.7.1dbg/lib/liblldb.so
#2 0x00007ffff5f7034d in lldb_private::HostThread::Join(void**) ()
from /tools/llvm/3.7.1dbg/lib/liblldb.so
#3 0x00007ffff610802e in lldb_private::process_gdb_remote::GDBRemoteCommunication::StartDebugserverProcess(char const*, unsigned short, lldb_private::ProcessLaunchInfo&, unsigned short&) () from /tools/llvm/3.7.1dbg/lib/liblldb.so
#4 0x00007ffff612f5c0 in lldb_private::process_gdb_remote::ProcessGDBRemote::LaunchAndConnectToDebugserver(lldb_private::ProcessInfo const&) ()
from /tools/llvm/3.7.1dbg/lib/liblldb.so
#5 0x00007ffff612fbde in lldb_private::process_gdb_remote::ProcessGDBRemote::DoAttachToProcessWithID(unsigned long, lldb_private::ProcessAttachInfo const&) ()
from /tools/llvm/3.7.1dbg/lib/liblldb.so
#6 0x00007ffff623efa5 in lldb_private::Process::Attach(lldb_private::ProcessAttachInfo&) () from /tools/llvm/3.7.1dbg/lib/liblldb.so
#7 0x00007ffff7025cc4 in PlatformPOSIX::Attach(lldb_private::ProcessAttachInfo&, lldb_private::Debugger&, lldb_private::Target*, lldb_private::Error&) ()
from /tools/llvm/3.7.1dbg/lib/liblldb.so
#8 0x00007ffff626ef52 in lldb_private::Target::Attach(lldb_private::ProcessAttachInfo&, lldb_private::Stream*) () from /tools/llvm/3.7.1dbg/lib/liblldb.so
#9 0x00007ffff5d4a4d7 in (anonymous namespace)::AttachToProcess(lldb_private::ProcessAttachInfo&, lldb_private::Target&) ()
from /tools/llvm/3.7.1dbg/lib/liblldb.so
#10 0x00007ffff5d4a149 in lldb::SBTarget::Attach(lldb::SBAttachInfo&, lldb::SBError&) () from /tools/llvm/3.7.1dbg/lib/liblldb.so

Target is Ubuntu LTS 14.04 on x86_64.

Has anything changed in the API over the past year? It looks like LLDB is creating a server internally; it is terminating quickly for some reason, and the LLDB parent is waiting forever for its server.

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