update the systemd-related integration tests to work on Ubuntu

Summary:
The systemd binary is installed in a different location on Debian-based
distributions (e.g., Ubuntu) from RedHat-based distributions.  Update the
integration tests to look in both places.

Reviewed By: wez

Differential Revision: D21297591

fbshipit-source-id: 67e5a698080205b8624c1f38aa49e75c20b6a28c

eden/integration/lib/find_executables.py

@@ -116,6 +116,17 @@ class FindExeClass(object):
     def SYSTEMD_FB_EDENFS_SERVICE(self) -> str:
         return os.path.join(self.EDEN_SRC_ROOT, "eden/fs/service/fb-edenfs@.service")
 
+    @cached_property
+    def SYSTEMD(self) -> str:
+        # systemd is installed at /usr/lib/systemd/systemd on RedHat-like distributions,
+        # and /bin/systemd on Debian-like distributions.
+        candidates = ["/usr/lib/systemd/systemd", "/bin/systemd"]
+        for path in candidates:
+            if os.access(path, os.X_OK):
+                return path
+
+        raise Exception(f"unable to find systemd: candidates checked={candidates}")
+
     @cached_property
     def TAKEOVER_TOOL(self) -> str:
         return self._find_exe(

eden/integration/lib/systemd.py

@@ -413,7 +413,8 @@ def _transient_managed_systemd_user_service_manager(
     """Create an isolated systemd instance using 'systemd-run systemd'."""
 
     child_systemd_service = parent_systemd.systemd_run(
-        command=["/usr/lib/systemd/systemd", "--user", "--unit=basic.target"],
+        # pyre-ignore[6]: T38947910
+        command=[FindExe.SYSTEMD, "--user", "--unit=basic.target"],
         properties={
             "Description": f"Eden test systemd user service manager "
             f"({xdg_runtime_dir})",
@@ -486,10 +487,11 @@ class _TransientUnmanagedSystemdUserServiceManager:
             pathlib.Path(FindExe.FORCE_SD_BOOTED).resolve(strict=True)
         )
         process = subprocess.Popen(
+            # pyre-ignore[6]: T38947910
             [
                 "timeout",
                 f"{self.__lifetime_duration}s",
-                "/usr/lib/systemd/systemd",
+                FindExe.SYSTEMD,
                 "--user",
                 "--unit=basic.target",
                 "--log-target=console",