[lldb/test] Don't use preexec_fn for launching inferiors

As the documentation states, using this is not safe in multithreaded
programs, and I have traced it to a rare deadlock in some of the tests.

The reason this was introduced was to be able to attach to a program
from the very first instruction, where our usual mechanism of
synchronization -- waiting for a file to appear -- does not work.

However, this is only needed for a single test
(TestGdbRemoteAttachWait) so instead of doing this everywhere, I create
a bespoke solution for that single test. The solution basically
consists of outsourcing the preexec_fn code to a separate (and
single-threaded) shim process, which enables attaching and then executes
the real program.

This pattern could be generalized in case we needed to use it for other
tests, but I suspect that we will not be having many tests like this.

This effectively reverts commit
a997a1d7fb.

lldb/packages/Python/lldbsuite/test/lldbplatformutil.py

@@ -4,12 +4,11 @@ architecture and/or the platform dependent nature of the tests. """
 from __future__ import absolute_import
 
 # System modules
-import ctypes
 import itertools
-import os
 import re
 import subprocess
 import sys
+import os
 
 # Third-party modules
 import six
@@ -192,14 +191,3 @@ def hasChattyStderr(test_case):
     if match_android_device(test_case.getArchitecture(), ['aarch64'], range(22, 25+1)):
         return True  # The dynamic linker on the device will complain about unknown DT entries
     return False
-
-if getHostPlatform() == "linux":
-    def enable_attach():
-        """Enable attaching to _this_ process, if host requires such an action.
-        Suitable for use as a preexec_fn in subprocess.Popen and similar."""
-        c = ctypes.CDLL(None)
-        PR_SET_PTRACER = ctypes.c_int(0x59616d61)
-        PR_SET_PTRACER_ANY = ctypes.c_ulong(-1)
-        c.prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY)
-else:
-    enable_attach = None

lldb/packages/Python/lldbsuite/test/lldbtest.py

@@ -393,7 +393,6 @@ class _LocalProcess(_BaseProcess):
             stdout=open(
                 os.devnull) if not self._trace_on else None,
             stdin=PIPE,
-            preexec_fn=lldbplatformutil.enable_attach,
             env=env)
 
     def terminate(self):

lldb/test/API/tools/lldb-server/attach-wait/Makefile

@@ -0,0 +1 @@
+include Makefile.rules

lldb/test/API/tools/lldb-server/attach-wait/TestGdbRemoteAttachWait.py

@@ -14,10 +14,12 @@ class TestGdbRemoteAttachWait(gdbremote_testcase.GdbRemoteTestCaseBase):
     @skipIfWindows # This test is flaky on Windows
     def test_attach_with_vAttachWait(self):
         exe = '%s_%d' % (self.testMethodName, os.getpid())
+        exe_to_attach = exe
+        args = []
 
         def launch_inferior():
             inferior = self.launch_process_for_attach(
-                inferior_args=["sleep:60"],
+                inferior_args=args,
                 exe_path=self.getBuildArtifact(exe))
             self.assertIsNotNone(inferior)
             self.assertTrue(inferior.pid > 0)
@@ -26,7 +28,14 @@ class TestGdbRemoteAttachWait(gdbremote_testcase.GdbRemoteTestCaseBase):
                     inferior.pid, True))
             return inferior
 
-        self.build(dictionary={'EXE': exe})
+        self.build(dictionary={'EXE': exe, 'CXX_SOURCES': 'main.cpp'})
+        if self.getPlatform() != "windows":
+            # Use a shim to ensure that the process is ready to be attached from
+            # the get-go.
+            args = [self.getBuildArtifact(exe)]
+            exe = "shim"
+            self.build(dictionary={'EXE': exe, 'CXX_SOURCES': 'shim.cpp'})
+
         self.set_inferior_startup_attach_manually()
 
         server = self.connect_to_debug_monitor()
@@ -38,7 +47,8 @@ class TestGdbRemoteAttachWait(gdbremote_testcase.GdbRemoteTestCaseBase):
         self.do_handshake()
         self.test_sequence.add_log_lines([
             # Do the attach.
-            "read packet: $vAttachWait;{}#00".format(lldbgdbserverutils.gdbremote_hex_encode_string(exe)),
+            "read packet: $vAttachWait;{}#00".format(
+                lldbgdbserverutils.gdbremote_hex_encode_string(exe_to_attach)),
         ], True)
         # Run the stream until attachWait.
         context = self.expect_gdbremote_sequence()

lldb/test/API/tools/lldb-server/attach-wait/main.cpp

@@ -0,0 +1,8 @@
+#include <thread>
+
+int main()
+{
+  // Wait to be attached.
+  std::this_thread::sleep_for(std::chrono::minutes(1));
+  return 0;
+}

lldb/test/API/tools/lldb-server/attach-wait/shim.cpp

@@ -0,0 +1,9 @@
+#include <unistd.h>
+#include <cstdio>
+
+int main(int argc, char *argv[]) {
+  lldb_enable_attach();
+  execlp(argv[1], argv[1], nullptr);
+  perror("execlp");
+  return 1;
+}