process.spawn: Add returnproc parameter

In order to migrate away from unsafe os.fork() usage in threaded
processes (https://github.com/python/cpython/issues/84559), add a
returnproc parameter that is similar to returnpid, which causes
spawn to return a single Process object instead of a list of pids.
The Process API is a subset of asyncio.subprocess.Process. The
returnproc parameter conflicts with the logfile parameter, since
the caller is expected to use the fd_pipes parameter to implement
logging (this was also true for the returnpid parameter).

In the future, spawn will return objects of a different type but
with a compatible interface to Process, in order to encapsulate
implementation-dependent objects like multiprocessing.Process which
are designed to manage the process lifecycle and need to persist
until it exits.

Trigger a UserWarning when the returnpid parameter is used, in
order to encourage migration to returnproc (do not use
DeprecationWarning since it is hidden by default). This warning
will be temporarily suppressed for portage internals, until they
finish migrating to returnproc. There are probably very few if
any external consumers of spawn with the returnpid parameter,
so it seems safe to move quickly with this deprecation.

Bug: https://bugs.gentoo.org/916566
Signed-off-by: Zac Medico <zmedico@gentoo.org>

3 files changed, 124 insertions, 0 deletions

diff --git a/lib/portage/process.py b/lib/portage/process.py
index 71750a715..6ec52efc4 100644
--- a/lib/portage/process.py
+++ b/lib/portage/process.py
@@ -15,6 +15,7 @@ import subprocess
 import sys
 import traceback
 import os as _os
+import warnings
 
 from dataclasses import dataclass
 from functools import lru_cache
@@ -27,6 +28,7 @@ import portage
 
 portage.proxy.lazyimport.lazyimport(
     globals(),
+    "portage.util._eventloop.global_event_loop:global_event_loop",
     "portage.util:dump_traceback,writemsg,writemsg_level",
 )
 
@@ -277,12 +279,78 @@ def calc_env_stats(env) -> EnvStats:
 env_too_large_warnings = 0
 
 
+class Process:
+    """
+    An object that wraps OS processes created by spawn.
+    In the future, spawn will return objects of a different type
+    but with a compatible interface to this class, in order
+    to encapsulate implementation-dependent objects like
+    multiprocessing.Process which are designed to manage
+    the process lifecycle and need to persist until it exits.
+    """
+
+    def __init__(self, pid):
+        self.pid = pid
+        self.returncode = None
+        self._exit_waiters = []
+
+    def __repr__(self):
+        return f"<{self.__class__.__name__} {self.pid}>"
+
+    async def wait(self):
+        """
+        Wait for the child process to terminate.
+
+        Set and return the returncode attribute.
+        """
+        if self.returncode is not None:
+            return self.returncode
+
+        loop = global_event_loop()
+        if not self._exit_waiters:
+            loop._asyncio_child_watcher.add_child_handler(self.pid, self._child_handler)
+        waiter = loop.create_future()
+        self._exit_waiters.append(waiter)
+        return await waiter
+
+    def _child_handler(self, pid, returncode):
+        if pid != self.pid:
+            raise AssertionError(f"expected pid {self.pid}, got {pid}")
+        self.returncode = returncode
+
+        for waiter in self._exit_waiters:
+            if not waiter.cancelled():
+                waiter.set_result(returncode)
+        self._exit_waiters = None
+
+    def send_signal(self, sig):
+        """Send a signal to the process."""
+        if self.returncode is not None:
+            # Skip signalling a process that we know has already died.
+            return
+
+        try:
+            os.kill(self.pid, sig)
+        except ProcessLookupError:
+            # Suppress the race condition error; bpo-40550.
+            pass
+
+    def terminate(self):
+        """Terminate the process with SIGTERM"""
+        self.send_signal(signal.SIGTERM)
+
+    def kill(self):
+        """Kill the process with SIGKILL"""
+        self.send_signal(signal.SIGKILL)
+
+
 def spawn(
     mycommand,
     env=None,
     opt_name=None,
     fd_pipes=None,
     returnpid=False,
+    returnproc=False,
     uid=None,
     gid=None,
     groups=None,
@@ -315,6 +383,9 @@ def spawn(
     @param returnpid: Return the Process IDs for a successful spawn.
     NOTE: This requires the caller clean up all the PIDs, otherwise spawn will clean them.
     @type returnpid: Boolean
+    @param returnproc: Return a Process object for a successful spawn (conflicts with logfile parameter).
+    NOTE: This requires the caller to asynchronously wait for the Process.
+    @type returnproc: Boolean
     @param uid: User ID to spawn as; useful for dropping privilages
     @type uid: Integer
     @param gid: Group ID to spawn as; useful for dropping privilages
@@ -350,6 +421,11 @@ def spawn(
 
     """
 
+    if logfile and returnproc:
+        raise ValueError(
+            "logfile parameter conflicts with returnproc (use fd_pipes instead)"
+        )
+
     # mycommand is either a str or a list
     if isinstance(mycommand, str):
         mycommand = mycommand.split()
@@ -491,7 +567,15 @@ def spawn(
     # If the caller wants to handle cleaning up the processes, we tell
     # it about all processes that were created.
     if returnpid:
+        if not portage._internal_caller:
+            warnings.warn(
+                "The portage.process.spawn returnpid paramenter is deprecated and replaced by returnproc",
+                UserWarning,
+                stacklevel=1,
+            )
         return mypids
+    if returnproc:
+        return Process(mypids[0])
 
     # Otherwise we clean them up.
     while mypids:
diff --git a/lib/portage/tests/process/meson.build b/lib/portage/tests/process/meson.build
index b86fa10fb..e2b3c11d3 100644
--- a/lib/portage/tests/process/meson.build
+++ b/lib/portage/tests/process/meson.build
@@ -8,6 +8,7 @@ py.install_sources(
         'test_pickle.py',
         'test_poll.py',
         'test_spawn_fail_e2big.py',
+        'test_spawn_returnproc.py',
         'test_spawn_warn_large_env.py',
         'test_unshare_net.py',
         '__init__.py',
diff --git a/lib/portage/tests/process/test_spawn_returnproc.py b/lib/portage/tests/process/test_spawn_returnproc.py
new file mode 100644
index 000000000..6d823d9c3
--- /dev/null
+++ b/lib/portage/tests/process/test_spawn_returnproc.py
@@ -0,0 +1,39 @@
+# Copyright 2024 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+import os
+import signal
+
+from portage.process import find_binary, spawn
+from portage.tests import TestCase
+from portage.util._eventloop.global_event_loop import global_event_loop
+
+
+class SpawnReturnProcTestCase(TestCase):
+    def testSpawnReturnProcWait(self):
+        true_binary = find_binary("true")
+        self.assertNotEqual(true_binary, None)
+
+        loop = global_event_loop()
+
+        async def watch_pid():
+            proc = spawn([true_binary], returnproc=True)
+            self.assertEqual(await proc.wait(), os.EX_OK)
+
+            # A second wait should also work.
+            self.assertEqual(await proc.wait(), os.EX_OK)
+
+        loop.run_until_complete(watch_pid())
+
+    def testSpawnReturnProcTerminate(self):
+        sleep_binary = find_binary("sleep")
+        self.assertNotEqual(sleep_binary, None)
+
+        loop = global_event_loop()
+
+        async def watch_pid():
+            proc = spawn([sleep_binary, 9999], returnproc=True)
+            proc.terminate()
+            self.assertEqual(await proc.wait(), -signal.SIGTERM)
+
+        loop.run_until_complete(watch_pid())