[lldb-dev] Unreliable process attach on Linux

[Florian Weimer via lldb-dev]

Consider this example program:

#include <err.h>
#include <sys/wait.h>
#include <unistd.h>

#include <lldb/API/SBDebugger.h>
#include <lldb/API/SBProcess.h>
#include <lldb/API/SBTarget.h>

int
main(void)
{
  // Target process for the debugger.
  pid_t pid = fork();
  if (pid < 0)
    err(1, "fork");
  if (pid == 0)
    while (true)
      pause();

  lldb::SBDebugger::Initialize();
  {
    auto debugger(lldb::SBDebugger::Create());
    if (!debugger.IsValid())
      errx(1, "SBDebugger::Create failed");

    auto target(debugger.CreateTarget(nullptr));
    if (!target.IsValid())
      errx(1, "SBDebugger::CreateTarget failed");

    lldb::SBAttachInfo attachinfo(pid);
    lldb::SBError error;
    auto process(target.Attach(attachinfo, error));
    if (!process.IsValid())
      errx(1, "SBTarget::Attach failed: %s", error.GetCString());
    error = process.Detach();
    if (error.Fail())
      errx(1, "SBProcess::Detach failed: %s", error.GetCString());
  }
  lldb::SBDebugger::Terminate();

  if (kill(pid, SIGKILL) != 0)
    err(1, "kill");
  if (waitpid(pid, NULL, 0) < 0)
    err(1, "waitpid");

  return 0;
}

Run it in a loop like this:

$ while ./test-attach ; do date; done

On Linux x86-64 (Fedora 29), with LLDB 7 (lldb-7.0.0-1.fc29.x86_64) and
kernel 4.19.12 (kernel-4.19.12-301.fc29.x86_64), after 100 iterations or
so, attaching to the newly created process fails:

test-attach: SBTarget::Attach failed: lost connection

This also reproduces occasionally with LLDB itself (with “lldb -p PID”).

Any suggestions how to get more information about the cause of this
error?

Thanks,
Florian