[lldb/test] Ensure launched processes are ready to be attached

Linux systems can be configured (and most of them are configured that
way) to disable attaching to unrelated processes, /unless/ those
processes explicitly allow that.

Our test inferiors do that by explicitly calling prctl(PR_SET_PTRACER,
PR_SET_PTRACER_ANY) (a.k.a., lldb_enable_attach). This requires
additional synchronization to ensure that the test does not attempt
attach before that statement is executed.

This is working fine (albeit cumbersome) for most tests but
TestGdbRemoteAttachWait is special in that it wants to start the
inferior _after_ issuing the attach request. This means that the usual
synchronization method does not work.

This patch introduces a different solution -- enable attaching in the
test harness, before the process is launched. Besides fixing this
problem, this is also better because it avoids the need to add special
code to each attach test (which is a common error).

One gotcha here is that it won't work for remote test suites, as we
don't control launching there. However, we could add a similar option to
lldb-platform, or require that lldb-platform itself is started with
attaching enabled. At that point we could delete all lldb_enable_attach
logic.

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

@@ -4,11 +4,12 @@ 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
@@ -198,3 +199,14 @@ 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

@@ -384,7 +384,8 @@ class _LocalProcess(_BaseProcess):
             [executable] + args,
             stdout=open(
                 os.devnull) if not self._trace_on else None,
-            stdin=PIPE)
+            stdin=PIPE,
+            preexec_fn=lldbplatformutil.enable_attach)
 
     def terminate(self):
         if self._proc.poll() is None: