Refactor PopenHandler API for better subclassing for replica popen handling

Summary:
## No functional changes
- refactor Local Scheduler process popen local into a PopenHandler class as a field for easier subclass.

Differential Revision: D56162173

Author: cniii

torchx/schedulers/local_scheduler.py

@@ -333,6 +333,111 @@ def failed(self) -> bool:
             return self.proc.returncode != 0
 
 
+class _PopenHandler:
+    """
+    Handles the creation ``subprocess.Popen`` for each replica process with
+    configuration extracted from ``ReplicaParam``.
+    """
+
+    def __init__(self) -> None:
+        pass
+
+    def _get_file_io(self, file: Optional[str]) -> Optional[io.FileIO]:
+        """
+        Given a file name, opens the file for write and returns the IO.
+        If no file name is given, then returns ``None``
+        Raises a ``FileExistsError`` if the file is already present.
+        """
+
+        if not file:
+            return None
+
+        if os.path.isfile(file):
+            raise FileExistsError(
+                f"log file: {file} already exists,"
+                f" specify a different log_dir, app_name, or remove the file and retry"
+            )
+
+        os.makedirs(os.path.dirname(file), exist_ok=True)
+        return io.open(file, mode="wb", buffering=0)
+
+    def _popen(
+        self,
+        role_name: RoleName,
+        replica_id: int,
+        replica_params: ReplicaParam,
+    ) -> _LocalReplica:
+        """
+        Same as ``subprocess.Popen(**popen_kwargs)`` but is able to take ``stdout`` and ``stderr``
+        as file name ``str`` rather than a file-like obj.
+        """
+
+        stdout_, stderr_, combined_ = self._get_replica_output_handles(replica_params)
+
+        args_pfmt = pprint.pformat(asdict(replica_params), indent=2, width=80)
+        log.debug(f"Running {role_name} (replica {replica_id}):\n {args_pfmt}")
+        env = self._get_replica_env(replica_params)
+
+        proc = subprocess.Popen(
+            args=replica_params.args,
+            env=env,
+            stdout=stdout_,
+            stderr=stderr_,
+            start_new_session=True,
+            cwd=replica_params.cwd,
+        )
+        return _LocalReplica(
+            role_name,
+            replica_id,
+            proc,
+            stdout=stdout_,
+            stderr=stderr_,
+            combined=combined_,
+            error_file=env.get("TORCHELASTIC_ERROR_FILE", "<N/A>"),
+        )
+
+    def _get_replica_env(
+        self,
+        replica_params: ReplicaParam,
+    ) -> Dict[str, str]:
+        """
+        Returns environment variables for the ``_LocalReplica``
+        """
+
+        # inherit parent's env vars since 99.9% of the time we want this behavior
+        # just make sure we override the parent's env vars with the user_defined ones
+        env = os.environ.copy()
+        env.update(replica_params.env)
+        # PATH is a special one, instead of overriding, append
+        env["PATH"] = _join_PATH(replica_params.env.get("PATH"), os.getenv("PATH"))
+
+        # default to unbuffered python for faster responsiveness locally
+        env.setdefault("PYTHONUNBUFFERED", "x")
+
+        return env
+
+    def _get_replica_output_handles(
+        self,
+        replica_params: ReplicaParam,
+    ) -> Tuple[Optional[io.FileIO], Optional[io.FileIO], Optional[Tee]]:
+        """
+        Returns the stdout, stderr, and combined outputs of the replica.
+        If the combined output file is not specified, then the combined output is ``None``.
+        """
+
+        stdout_ = self._get_file_io(replica_params.stdout)
+        stderr_ = self._get_file_io(replica_params.stderr)
+        combined_: Optional[Tee] = None
+        combined_file = self._get_file_io(replica_params.combined)
+        if combined_file:
+            combined_ = Tee(
+                combined_file,
+                none_throws(replica_params.stdout),
+                none_throws(replica_params.stderr),
+            )
+        return stdout_, stderr_, combined_
+
+
 class _LocalAppDef:
     """
     Container object used by ``LocalhostScheduler`` to group the pids that
@@ -600,6 +705,8 @@ def __init__(
         self._cache_size = cache_size
         _register_termination_signals()
 
+        self._popen_handler = _PopenHandler()
+
         self._extra_paths: List[str] = extra_paths or []
 
         # sets lazily on submit or dryrun based on log_dir cfg
@@ -660,101 +767,6 @@ def _evict_lru(self) -> bool:
             log.debug(f"no apps evicted, all {len(self._apps)} apps are running")
             return False
 
-    def _get_file_io(self, file: Optional[str]) -> Optional[io.FileIO]:
-        """
-        Given a file name, opens the file for write and returns the IO.
-        If no file name is given, then returns ``None``
-        Raises a ``FileExistsError`` if the file is already present.
-        """
-
-        if not file:
-            return None
-
-        if os.path.isfile(file):
-            raise FileExistsError(
-                f"log file: {file} already exists,"
-                f" specify a different log_dir, app_name, or remove the file and retry"
-            )
-
-        os.makedirs(os.path.dirname(file), exist_ok=True)
-        return io.open(file, mode="wb", buffering=0)
-
-    def _popen(
-        self,
-        role_name: RoleName,
-        replica_id: int,
-        replica_params: ReplicaParam,
-    ) -> _LocalReplica:
-        """
-        Same as ``subprocess.Popen(**popen_kwargs)`` but is able to take ``stdout`` and ``stderr``
-        as file name ``str`` rather than a file-like obj.
-        """
-
-        stdout_, stderr_, combined_ = self._get_replica_output_handles(replica_params)
-
-        args_pfmt = pprint.pformat(asdict(replica_params), indent=2, width=80)
-        log.debug(f"Running {role_name} (replica {replica_id}):\n {args_pfmt}")
-        env = self._get_replica_env(replica_params)
-
-        proc = subprocess.Popen(
-            args=replica_params.args,
-            env=env,
-            stdout=stdout_,
-            stderr=stderr_,
-            start_new_session=True,
-            cwd=replica_params.cwd,
-        )
-        return _LocalReplica(
-            role_name,
-            replica_id,
-            proc,
-            stdout=stdout_,
-            stderr=stderr_,
-            combined=combined_,
-            error_file=env.get("TORCHELASTIC_ERROR_FILE", "<N/A>"),
-        )
-
-    def _get_replica_output_handles(
-        self,
-        replica_params: ReplicaParam,
-    ) -> Tuple[Optional[io.FileIO], Optional[io.FileIO], Optional[Tee]]:
-        """
-        Returns the stdout, stderr, and combined outputs of the replica.
-        If the combined output file is not specified, then the combined output is ``None``.
-        """
-
-        stdout_ = self._get_file_io(replica_params.stdout)
-        stderr_ = self._get_file_io(replica_params.stderr)
-        combined_: Optional[Tee] = None
-        combined_file = self._get_file_io(replica_params.combined)
-        if combined_file:
-            combined_ = Tee(
-                combined_file,
-                none_throws(replica_params.stdout),
-                none_throws(replica_params.stderr),
-            )
-        return stdout_, stderr_, combined_
-
-    def _get_replica_env(
-        self,
-        replica_params: ReplicaParam,
-    ) -> Dict[str, str]:
-        """
-        Returns environment variables for the ``_LocalReplica``
-        """
-
-        # inherit parent's env vars since 99.9% of the time we want this behavior
-        # just make sure we override the parent's env vars with the user_defined ones
-        env = os.environ.copy()
-        env.update(replica_params.env)
-        # PATH is a special one, instead of overriding, append
-        env["PATH"] = _join_PATH(replica_params.env.get("PATH"), os.getenv("PATH"))
-
-        # default to unbuffered python for faster responsiveness locally
-        env.setdefault("PYTHONUNBUFFERED", "x")
-
-        return env
-
     def _get_app_log_dir(self, app_id: str, cfg: LocalOpts) -> str:
         """
         Returns the log dir. We redirect stdout/err
@@ -802,7 +814,7 @@ def schedule(self, dryrun_info: AppDryRunInfo[PopenRequest]) -> str:
                 replica_log_dir = role_log_dirs[replica_id]
 
                 os.makedirs(replica_log_dir)
-                replica = self._popen(
+                replica = self._popen_handler._popen(
                     role_name,
                     replica_id,
                     replica_params,