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

This recommits b15b1421, which reverted in was reverted in f51c47d98 due to
failures on apple systems. The problem was that the patch introduced a race
where the debug server could start the attach process before the first process
(which isn't supposed to be attached to) was set up. This caused us to attach
to the wrong process.

The new version introduces additional synchronization to ensure that does not
happen.

Original commit message was:
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 +14,13 @@ 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
+        sync_file_path = lldbutil.append_to_process_working_directory(self, "process_ready")
+        args = [sync_file_path]
 
         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 +29,14 @@ 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)] + args
+            exe = "shim"
+            self.build(dictionary={'EXE': exe, 'CXX_SOURCES': 'shim.cpp'})
+
         self.set_inferior_startup_attach_manually()
 
         server = self.connect_to_debug_monitor()
@@ -34,11 +44,14 @@ class TestGdbRemoteAttachWait(gdbremote_testcase.GdbRemoteTestCaseBase):
 
         # Launch the first inferior (we shouldn't attach to this one).
         launch_inferior()
-        
+
+        lldbutil.wait_for_file_on_target(self, sync_file_path)
+
         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 +1,12 @@
+#include <thread>
+#include <fstream>
+
+int main(int argc, char *argv[])
+{
+  if (argc >= 2) {
+    std::ofstream(argv[1]).close();
+  }
+  // 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();
+  execvp(argv[1], argv+1);
+  perror("execlp");
+  return 1;
+}