Fix python process name extraction. (pantsbuild#11966)

Previously we grabbed the 1st 15 characters on Linux and, at least on
some versions of macOS, we didn't grab the process name at all, but the
actual name of the process executable instead. Since pantsd uses process
re-naming to leave a fingerprint it can use to detect its last recorded
pid has not been recycled to another process, the macOS case is 
problematic. In our case, the process executable for pantsd is a Python
binary and processes with that as their executable are ubiquitous. This 
defeats the pid recycling detection scheme.

Fix all this by grabbing the process name from the right place, only
falling back to the prior if there is a problem reading the process
command line (can happen for zombie processes at least). Add an
integration test that verifies we now read the full custom command line
and not a truncated version of it or the name of the process executable.

src/python/pants/pantsd/process_manager.py

@@ -1,6 +1,8 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+from __future__ import annotations
+
 import logging
 import os
 import signal
@@ -331,9 +333,14 @@ def write_pid(self, pid: Optional[int] = None):
         pid = os.getpid() if pid is None else pid
         self.write_metadata_by_name(self.PID_KEY, str(pid))
 
+    def _get_process_name(self, process: psutil.Process | None = None) -> str:
+        proc = process or self._as_process()
+        cmdline = proc.cmdline()
+        return cast(str, cmdline[0] if cmdline else proc.name())
+
     def write_process_name(self, process_name: Optional[str] = None):
         """Write the current process's name."""
-        process_name = process_name or self._as_process().name()
+        process_name = process_name or self._get_process_name()
         self.write_metadata_by_name(self.PROCESS_NAME_KEY, process_name)
 
     def write_socket(self, socket_info: int):
@@ -382,7 +389,7 @@ def is_alive(self, extended_check=None):
                 (process.status() == psutil.STATUS_ZOMBIE)
                 or
                 # Check for stale pids.
-                (self.process_name and self.process_name != process.name())
+                (self.process_name and self.process_name != self._get_process_name(process))
                 or
                 # Extended checking.
                 (extended_check and not extended_check(process))

tests/python/pants_test/pantsd/test_process_manager.py

@@ -1,15 +1,18 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
-
 import errno
 import logging
 import os
+import subprocess
+import sys
 import unittest
 import unittest.mock
 from contextlib import contextmanager
+from textwrap import dedent
 
 import psutil
 import pytest
+from _pytest.tmpdir import TempdirFactory
 
 from pants.pantsd.process_manager import ProcessManager
 from pants.util.contextutil import temporary_dir
@@ -334,3 +337,34 @@ def test_callbacks(self):
         self.pm.pre_fork()
         self.pm.post_fork_child()
         self.pm.post_fork_parent()
+
+
+def test_process_name_setproctitle_integration(tmpdir_factory: TempdirFactory) -> None:
+    buildroot = tmpdir_factory.mktemp("buildroot")
+    manager_name = "Bob"
+    process_name = f"{manager_name} [{buildroot}]"
+    metadata_base_dir = tmpdir_factory.mktemp(".pids")
+
+    subprocess.check_call(
+        args=[
+            sys.executable,
+            "-c",
+            dedent(
+                f"""\
+                from setproctitle import setproctitle as set_process_title
+
+                from pants.pantsd.process_manager import ProcessManager
+
+
+                set_process_title({process_name!r})
+                pm = ProcessManager({manager_name!r}, metadata_base_dir="{metadata_base_dir}")
+                pm.write_pid()
+                pm.write_process_name()
+                """
+            ),
+        ],
+        env=dict(PYTHONPATH=os.pathsep.join(sys.path)),
+    )
+
+    pm = ProcessManager(manager_name, metadata_base_dir=str(metadata_base_dir))
+    assert process_name == pm.process_name