From f3f5068e99dd3a915defd57d9fc276a26164b1ff Mon Sep 17 00:00:00 2001
From: Helmut Grohne <helmut@subdivi.de>
Date: Thu, 18 Apr 2024 17:01:43 +0200
Subject: examples/cgroup.py: extract a context manager waiting for systemd
 jobs

---
 examples/cgroup.py | 103 +++++++++++++++++++++++++++++++++--------------------
 1 file changed, 65 insertions(+), 38 deletions(-)

(limited to 'examples')

diff --git a/examples/cgroup.py b/examples/cgroup.py
index ff1a087..1c94d32 100755
--- a/examples/cgroup.py
+++ b/examples/cgroup.py
@@ -10,6 +10,7 @@ import asyncio
 import os
 import pathlib
 import sys
+import typing
 
 try:
     import ravel
@@ -33,51 +34,77 @@ def get_cgroup(pid: int = -1) -> pathlib.PurePath:
     )
 
 
-async def start_transient_unit_with_ravel(pid: int) -> None:
-    """Call the StartTransientUnit dbus method on the user manager for the
-    given pid.
+class SystemdJobWaiter:
+    """Context manager for waiting for a systemd job to complete.
+    Typical usage:
+
+        with SystemdJobWaiter(bus) as wait:
+            job = create_a_job_on(bus)
+            result = await wait(job)
     """
-    bus = await ravel.session_bus_async()
-    jobs_removed = {}
-    scope_job = None
+
     systemd_path = "/org/freedesktop/systemd1"
     systemd_iface = "org.freedesktop.systemd1.Manager"
-    scope_created = asyncio.get_running_loop().create_future()
+
+    def __init__(self, bus: ravel.Connection):
+        self.bus = bus
+        self.jobs_removed: dict[str, str] = {}
+        self.target_job: str | None = None
+        self.job_done = asyncio.get_running_loop().create_future()
 
     @ravel.signal(name="JobRemoved", in_signature="uoss")
-    def handle_job_removed(_1, path, _2, result):
-        nonlocal jobs_removed
-        nonlocal scope_job
-        nonlocal scope_created
-        if scope_job is None:
-            jobs_removed[path] = result
-        elif path == scope_job:
-            scope_created.set_result(result)
-
-    bus.listen_signal(
-        systemd_path, False, systemd_iface, "JobRemoved", handle_job_removed
-    )
-    scope_job = (
-        bus["org.freedesktop.systemd1"][systemd_path]
-        .get_interface(systemd_iface)
-        .StartTransientUnit(
-            f"cgroup-{pid}.scope", "fail", [("PIDs", ("au", [pid]))], []
+    def _on_job_removed(
+        self, _id: int, path: str, _unit: str, result: str
+    ) -> None:
+        if self.target_job is None:
+            self.jobs_removed[path] = result
+        elif self.target_job == path:
+            self.job_done.set_result(result)
+
+    def __enter__(self) -> "SystemdJobWaiter":
+        self.bus.listen_signal(
+            self.systemd_path,
+            False,
+            self.systemd_iface,
+            "JobRemoved",
+            self._on_job_removed,
         )
-    )[0]
-    if scope_job in jobs_removed:
-        scope_created.set_result(jobs_removed[scope_job])
-    else:
+        return self
+
+    async def __call__(self, job: str, timeout: int | float = 60) -> str:
+        assert self.target_job is None
+        self.target_job = job
         try:
-            await asyncio.wait_for(scope_created, 60)
-        except TimeoutError:
-            print("Error: timed out waiting for StartTransientUnit")
-            sys.exit(1)
-    bus.unlisten_signal(
-        systemd_path, False, systemd_iface, "JobRemoved", handle_job_removed
-    )
-    if scope_created.result() != "done":
-        print("Error: StartTransientUnit failed: " + scope_created.result())
-        sys.exit(1)
+            return self.jobs_removed[job]
+        except KeyError:
+            return await asyncio.wait_for(self.job_done, timeout)
+
+    def __exit__(self, _1: typing.Any, _2: typing.Any, _3: typing.Any) -> None:
+        self.bus.unlisten_signal(
+            self.systemd_path,
+            False,
+            self.systemd_iface,
+            "JobRemoved",
+            self._on_job_removed,
+        )
+
+
+async def start_transient_unit_with_ravel(pid: int) -> None:
+    """Call the StartTransientUnit dbus method on the user manager for the
+    given pid.
+    """
+    bus = await ravel.session_bus_async()
+    with SystemdJobWaiter(bus) as wait:
+        scope_job = (
+            bus["org.freedesktop.systemd1"]["/org/freedesktop/systemd1"]
+            .get_interface("org.freedesktop.systemd1.Manager")
+            .StartTransientUnit(
+                f"cgroup-{pid}.scope", "fail", [("PIDs", ("au", [pid]))], []
+            )
+        )[0]
+        result = await wait(scope_job)
+    if result != "done":
+        raise OSError("StartTransientUnit failed: " + result)
 
 
 def main() -> None:
-- 
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