[lldb][test] Wait for a file before attempting to attach to lldb-server test inferior

Fixes #161510

Addresses the Linux parts of #138085

The situation we have to handle here is systems where
Yama ptrace_scope set to 1.

> 1 - restricted ptrace: a process must have a predefined relationship
>     with the inferior it wants to call PTRACE_ATTACH on. By default,
>     this relationship is that of only its descendants when the above
>     classic criteria is also met. To change the relationship, an
>     inferior can call prctl(PR_SET_PTRACER, debugger, ...) to declare
>     an allowed debugger PID to call PTRACE_ATTACH on the inferior.
>     Using PTRACE_TRACEME is unchanged.

(https://www.kernel.org/doc/Documentation/security/Yama.txt)

The inferior was addressing this by calling this at the start
of main():
prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, 0, 0, 0);

Which is ok if lldb-server tries to attach after that call has happened,
but there was nothing to synchronise this. So if the system was heavily
loaded, the inferior may be stalled, delaying the call, causing lldb-server
to fail to attach with EPERM (permission denied).

We were not using any mechanism to retry the attach or wait for some
signal from the inferior.

Except we do do this in other tests, even other lldb-server tests.

So I have adopted that mechanism to these tests:
* The inferior is launched with `syncfile:<path>` as its first
  argument.
* It creates this file at `<path>`, at a point where we know attaching has
  been allowed.
* The test framework launches the inferior then waits for the file
  to appear.
* This check is retried a few times, increasing the delay each time
  and eventually giving up.
* Only once it has seen the file does it start lldb-server and
  tell it to attach to the inferior.

I have tested this by insterting a `sleep()` call before the
attach enable call and running the test on a machine with
ptrace_scope set to 1. I was able to increase the sleep to 6 seconds
before tests failed (when running just these tests, single threaded).

With OS scheduling, you could be stalled indefinitely, so we may
have to increase this timeout but this is easy to do with
wait_for_file_on_target.

The alternative is to have the test runner check ptrace_scope
and only enable these on systems where it's 0. Would be good
to keep them running if we can though.

Author: DavidSpickett

lldb/packages/Python/lldbsuite/test/tools/lldb-server/gdbremote_testcase.py

@@ -444,13 +444,20 @@ def launch_process_for_attach(
         if not exe_path:
             exe_path = self.getBuildArtifact("a.out")
 
-        args = []
+        # This file will be created once the inferior has enabled attaching.
+        sync_file_path = lldbutil.append_to_process_working_directory(
+            self, "process_ready"
+        )
+        args = [f"syncfile:{sync_file_path}"]
         if inferior_args:
             args.extend(inferior_args)
         if sleep_seconds:
             args.append("sleep:%d" % sleep_seconds)
 
-        return self.spawnSubprocess(exe_path, args)
+        inferior = self.spawnSubprocess(exe_path, args)
+        lldbutil.wait_for_file_on_target(self, sync_file_path)
+
+        return inferior
 
     def prep_debug_monitor_and_inferior(
         self,

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

@@ -6,6 +6,7 @@
 #include <cstring>
 #include <errno.h>
 #include <future>
+#include <fstream>
 #include <inttypes.h>
 #include <memory>
 #include <mutex>
@@ -265,7 +266,10 @@ int main(int argc, char **argv) {
   // Process command line args.
   for (int i = 1; i < argc; ++i) {
     std::string arg = argv[i];
-    if (consume_front(arg, "stderr:")) {
+    if (consume_front(arg, "syncfile:")) {
+      // Write to this file to tell test framework that attaching is now possible.
+      std::ofstream(arg).close();
+    } else if (consume_front(arg, "stderr:")) {
       // Treat remainder as text to go to stderr.
       fprintf(stderr, "%s\n", arg.c_str());
     } else if (consume_front(arg, "retval:")) {