[lldb-dev] Why is storing SBTarget in a private field causing random crash?

[Jeffrey Tan via lldb-dev]

Hi,

I have been spending long time troubleshooting a race condition in our lldb
python test. I finally narrowed down to one code change that caused the
race: basically, whenever I store SBTarget in DebuggerTestCase's
self.target field lldb will randomly crash during destruction(see below).

In the code, if I modify the two "self.target" to local variable "target"
the random crash will disappear.

I am not a python expert. Why is holding SBTarget will cause the test to
random crash? Do I have to set every SBXXX fields to None before
calling SBDebugger.Destroy()?


==========================Crash Stack==========================
Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       KERN_INVALID_ADDRESS at 0x0000000000000010

VM Regions Near 0x10:
-->
    __TEXT                 000000010d145000-000000010d146000 [    4K]
r-x/rwx SM=COW
 /System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   com.apple.LLDB.framework       0x00000001101c037d
lldb_private::Listener::BroadcasterWillDestruct(lldb_private::Broadcaster*)
+ 95
1   com.apple.LLDB.framework       0x00000001101a0da2
lldb_private::Broadcaster::Clear() + 50
2   com.apple.LLDB.framework       0x00000001101a0ced
lldb_private::Broadcaster::~Broadcaster() + 75
3   com.apple.LLDB.framework       0x00000001103d6879
lldb_private::Target::~Target() + 741
4   com.apple.LLDB.framework       0x00000001103d6c20
lldb_private::Target::~Target() + 14
5   libc++.1.dylib                 0x00007fff896448a6
std::__1::__shared_weak_count::__release_shared() + 44
6   com.apple.LLDB.framework       0x000000010e560664
_wrap_delete_SBTarget(_object*, _object*) + 123
7   org.python.python             0x000000010d15a50a PyObject_Call + 99


==========================Code==========================
from find_lldb import lldb
from simplest_event_thread import LLDBListenerThread
import unittest
import threading

running_signal = threading.Event()
stopped_signal = threading.Event()

def launch_debugging(debugger, stop_at_entry):
    error = lldb.SBError()
    listener = lldb.SBListener('Chrome Dev Tools Listener')
    target = debugger.GetSelectedTarget()
    process = target.Launch (listener,
                    None,      # argv
                    None,      # envp
                    None,      # stdin_path
                    None,      # stdout_path
                    None,      # stderr_path
                    None,      # working directory
                    0,         # launch flags
                    stop_at_entry,      # Stop at entry
                    error)     # error
    print 'Launch result: %s' % str(error)

    event_thread = LLDBListenerThread(debugger, running_signal,
stopped_signal)
    event_thread.start()

    running_signal.set()
    return event_thread

class DebuggerTestCase:

    def wait_for_process_stop(self):
        running_signal.wait()
        running_signal.clear()
        stopped_signal.wait()
        stopped_signal.clear()

    def test_breakpoint_at_line(self):
        debugger = lldb.SBDebugger.Create()
        debugger.SetAsync(True)
        executable_path =
'~/Personal/compiler/CompilerConstruction/code/compiler'
        *self.target* =
debugger.CreateTargetWithFileAndArch(executable_path,
lldb.LLDB_ARCH_DEFAULT)

        event_thread = launch_debugging(debugger, stop_at_entry=True)

        process = debugger.GetSelectedTarget().process
        self.wait_for_process_stop() # wait for entry breakpoint.
        *self.target*.BreakpointCreateByName('main')
        process.Continue()
        self.wait_for_process_stop() # wait for main breakpoint.

        event_thread.should_quit = True
        event_thread.join()
        lldb.SBDebugger.Destroy(debugger)

if __name__ == '__main__':
    test = DebuggerTestCase()
    for i in range(20):
        test.test_breakpoint_at_line()
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