From a75ebb7c6b558feb9a3b93ace6cbca54bccb067a Mon Sep 17 00:00:00 2001
From: Helmut Grohne <helmut@subdivi.de>
Date: Fri, 19 Apr 2024 05:29:50 +0200
Subject: examples/cgroup.py: do not fail when ravel is unavailable

---
 examples/cgroup.py | 145 +++++++++++++++++++++++++++--------------------------
 1 file changed, 73 insertions(+), 72 deletions(-)

diff --git a/examples/cgroup.py b/examples/cgroup.py
index 8f81a85..da222f9 100755
--- a/examples/cgroup.py
+++ b/examples/cgroup.py
@@ -34,80 +34,81 @@ def get_cgroup(pid: int = -1) -> pathlib.PurePath:
     )
 
 
-class SystemdJobWaiter:
-    """Context manager for waiting for a systemd job to complete.
-    Typical usage:
+if ravel is not None:
+    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)
+        """
+
+        systemd_path = "/org/freedesktop/systemd1"
+        systemd_iface = "org.freedesktop.systemd1.Manager"
+
+        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 _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,
+            )
+            return self
+
+        async def __call__(self, job: str, timeout: int | float = 60) -> str:
+            assert self.target_job is None
+            self.target_job = job
+            try:
+                return self.jobs_removed[job]
+            except KeyError:
+                return await asyncio.wait_for(self.job_done, timeout)
+
+        def __exit__(self, *exc_info: typing.Any) -> None:
+            self.bus.unlisten_signal(
+                self.systemd_path,
+                False,
+                self.systemd_iface,
+                "JobRemoved",
+                self._on_job_removed,
+            )
 
-        with SystemdJobWaiter(bus) as wait:
-            job = create_a_job_on(bus)
-            result = await wait(job)
-    """
 
-    systemd_path = "/org/freedesktop/systemd1"
-    systemd_iface = "org.freedesktop.systemd1.Manager"
-
-    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 _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,
-        )
-        return self
-
-    async def __call__(self, job: str, timeout: int | float = 60) -> str:
-        assert self.target_job is None
-        self.target_job = job
-        try:
-            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])), ("Delegate", ("b", True))],
-                [],
-            )
-        )[0]
-        result = await wait(scope_job)
-    if result != "done":
-        raise OSError("StartTransientUnit failed: " + result)
+    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])), ("Delegate", ("b", True))],
+                    [],
+                )
+            )[0]
+            result = await wait(scope_job)
+        if result != "done":
+            raise OSError("StartTransientUnit failed: " + result)
 
 
 def main() -> None:
-- 
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