Split client-related logic out of the PantsDaemon class. (#9949)

### Problem

The `PantsDaemon` class was playing double duty as both the entrypoint for the pantsd server, and as a launcher for the client. We will soon need to make significant changes there in order to support #8200 and #7654.

### Solution

Extract a `PantsDaemonProcessManager` base class for process metadata reads/writes and a `PantsDaemonClient` subclass to consume the metadata to decide whether to launch the server. This is 100% code moves... no logic changed.

### Result

The `PantsDaemon` class is now exclusively a server, and `PantsDaemonClient` is now exclusively a client.

[ci skip-rust-tests]
[ci skip-jvm-tests]

src/python/pants/bin/BUILD

@@ -33,7 +33,7 @@ python_library(
     'src/python/pants/help',
     'src/python/pants/init',
     'src/python/pants/option',
-    'src/python/pants/pantsd:pants_daemon',
+    'src/python/pants/pantsd:pants_daemon_client',
     'src/python/pants/reporting',
     'src/python/pants/scm/subsystems:changed',
     'src/python/pants/subsystem',

src/python/pants/bin/remote_pants_runner.py

@@ -13,7 +13,7 @@
 from pants.java.nailgun_client import NailgunClient
 from pants.java.nailgun_protocol import NailgunProtocol
 from pants.option.options_bootstrapper import OptionsBootstrapper
-from pants.pantsd.pants_daemon import PantsDaemon
+from pants.pantsd.pants_daemon_client import PantsDaemonClient
 from pants.util.dirutil import maybe_read_file
 
 logger = logging.getLogger(__name__)
@@ -86,6 +86,7 @@ def __init__(
         self._env = env
         self._options_bootstrapper = options_bootstrapper
         self._bootstrap_options = options_bootstrapper.bootstrap_options
+        self._client = PantsDaemonClient(self._bootstrap_options)
         self._stdin = stdin or sys.stdin
         self._stdout = stdout or sys.stdout.buffer
         self._stderr = stderr or sys.stderr.buffer
@@ -107,7 +108,7 @@ def _backoff(attempt):
         time.sleep(attempt + (attempt - 1))
 
     def _run_pants_with_retry(
-        self, pantsd_handle: PantsDaemon.Handle, retries: int = 3
+        self, pantsd_handle: PantsDaemonClient.Handle, retries: int = 3
     ) -> ExitCode:
         """Runs pants remotely with retry and recovery for nascent executions.
 
@@ -137,7 +138,7 @@ def _run_pants_with_retry(
                 # another lifecycle operation is happening concurrently (incl teardown). To account for
                 # this, we won't begin attempting restarts until at least 1 second has passed (1 attempt).
                 if attempt > 1:
-                    pantsd_handle = self._restart_pantsd()
+                    pantsd_handle = self._client.restart()
                 attempt += 1
             except NailgunClient.NailgunError as e:
                 # Ensure a newline.
@@ -146,7 +147,7 @@ def _run_pants_with_retry(
                 traceback = sys.exc_info()[2]
                 raise self._extract_remote_exception(pantsd_handle.pid, e).with_traceback(traceback)
 
-    def _connect_and_execute(self, pantsd_handle: PantsDaemon.Handle) -> ExitCode:
+    def _connect_and_execute(self, pantsd_handle: PantsDaemonClient.Handle) -> ExitCode:
         port = pantsd_handle.port
         pid = pantsd_handle.pid
         # Merge the nailgun TTY capability environment variables with the passed environment dict.
@@ -204,9 +205,5 @@ def _extract_remote_exception(self, pantsd_pid, nailgun_error):
             )
         )
 
-    def _restart_pantsd(self):
-        return PantsDaemon.Factory.restart(options_bootstrapper=self._options_bootstrapper)
-
     def run(self, start_time: float) -> ExitCode:
-        handle = PantsDaemon.Factory.maybe_launch(options_bootstrapper=self._options_bootstrapper)
-        return self._run_pants_with_retry(handle)
+        return self._run_pants_with_retry(self._client.maybe_launch())

src/python/pants/core_tasks/pantsd_kill.py

@@ -3,7 +3,7 @@
 
 from pants.base.exceptions import TaskError
 from pants.option.options_bootstrapper import OptionsBootstrapper
-from pants.pantsd.pants_daemon import PantsDaemon
+from pants.pantsd.pants_daemon_client import PantsDaemonClient
 from pants.pantsd.process_manager import ProcessManager
 from pants.task.task import Task
 
@@ -13,8 +13,8 @@ class PantsDaemonKill(Task):
 
     def execute(self):
         try:
-            pantsd = PantsDaemon.Factory.create(OptionsBootstrapper.create(), full_init=False)
-            with pantsd.lifecycle_lock:
-                pantsd.terminate()
+            pantsd_client = PantsDaemonClient(OptionsBootstrapper.create().bootstrap_options)
+            with pantsd_client.lifecycle_lock:
+                pantsd_client.terminate()
         except ProcessManager.NonResponsiveProcess as e:
             raise TaskError("failure while terminating pantsd: {}".format(e))

src/python/pants/pantsd/BUILD

@@ -1,11 +1,22 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+python_library(
+  name = 'pants_daemon_client',
+  sources = ['pants_daemon_client.py'],
+  dependencies = [
+    '3rdparty/python:dataclasses',
+    ':pants_daemon',
+    ':process_manager',
+    'src/python/pants/option',
+  ],
+  tags = {'partially_type_checked'},
+)
+
 python_library(
   name = 'pants_daemon',
   sources = ['pants_daemon.py'],
   dependencies = [
-    '3rdparty/python:dataclasses',
     '3rdparty/python:setproctitle',
     ':process_manager',
     ':watchman_launcher',
@@ -24,7 +35,6 @@ python_library(
     'src/python/pants/pantsd/service:store_gc_service',
     'src/python/pants/util:contextutil',
     'src/python/pants/util:logging',
-    'src/python/pants/util:memo',
   ],
   tags = {'partially_type_checked'},
 )