Improve error message if systemd is not running

Summary:
If the systemd user manager is not running (e.g. it crashed or was manually stopped), 'eden stop' fails with an unhelpful error message:

> pystemd.dbusexc.DBusConnectionRefusedError: [err -111]: Could not open a bus to DBus

or

> pystemd.dbusexc.DBusBaseError: [err -2]: Could not open a bus to DBus

Provide a better experience: tell the user the most likely cause, and suggest a solution:

> error: The systemd user manager is not running. Run the following command to start it, then try again:
> sudo systemctl start user@strager.service

Reviewed By: wez

Differential Revision: D13791023

fbshipit-source-id: 5172df0a52d21c311b27b8a527cad934f9882154

eden/cli/daemon.py

@@ -9,6 +9,7 @@
 
 import asyncio
 import errno
+import getpass
 import os
 import pathlib
 import signal
@@ -20,6 +21,8 @@ from .config import EdenInstance
 from .logfile import forward_log_file
 from .systemd import (
     EdenFSSystemdServiceConfig,
+    SystemdConnectionRefusedError,
+    SystemdFileNotFoundError,
     SystemdServiceFailedToStartError,
     SystemdUserBus,
     edenfs_systemd_service_name,
@@ -274,6 +277,15 @@ def start_systemd_service(
             start_task = loop.create_task(start_service_async())
             loop.create_task(log_forwarder.poll_forever_async())
             return loop.run_until_complete(start_task)
+        except (SystemdConnectionRefusedError, SystemdFileNotFoundError):
+            print_stderr(
+                f"error: The systemd user manager is not running. Run the "
+                f"following command to\n"
+                f"start it, then try again:\n"
+                f"\n"
+                f"  sudo systemctl start user@{getpass.getuser()}.service"
+            )
+            return 1
         except SystemdServiceFailedToStartError as e:
             print_stderr(f"error: {e}")
             return 1

eden/cli/systemd.py

@@ -21,6 +21,7 @@ import typing
 pystemd_import_error = None
 try:
     import pystemd
+    import pystemd.dbusexc  # pyre-ignore[21]: T32805591
     import pystemd.dbuslib  # pyre-ignore[21]: T32805591
     import pystemd.systemd1.manager
     import pystemd.systemd1.unit
@@ -749,6 +750,18 @@ def escape_dbus_address(input: bytes) -> bytes:
     return b"".join(result_pieces)
 
 
+if pystemd_import_error is None:
+    SystemdConnectionRefusedError = (
+        pystemd.dbusexc.DBusConnectionRefusedError  # pyre-ignore[16]: T32805591
+    )
+    SystemdFileNotFoundError = (
+        pystemd.dbusexc.DBusFileNotFoundError  # pyre-ignore[16]: T32805591
+    )
+else:
+    SystemdConnectionRefusedError = Exception
+    SystemdFileNotFoundError = Exception
+
+
 class SystemdServiceFailedToStartError(Exception):
     def __init__(
         self,

eden/integration/systemd_test.py

@@ -8,6 +8,7 @@
 # of patent rights can be found in the PATENTS file in the same directory.
 
 import pathlib
+import shutil
 import subprocess
 import sys
 import typing
@@ -147,15 +148,38 @@ class SystemdTest(
             "journalctl doesn't work and should not be mentioned",
         )
 
-    def test_eden_start_reports_error_if_systemd_is_not_running(self) -> None:
+    def test_eden_start_reports_error_if_systemd_is_dead(self) -> None:
         self.set_up_edenfs_systemd_service()
         systemd = self.systemd
         assert systemd is not None
         systemd.exit()
+        self.assertTrue(
+            (systemd.xdg_runtime_dir / "systemd" / "private").exists(),
+            "systemd's socket file should still exist",
+        )
 
+        self.spoof_user_name("testuser")
         start_process = self.spawn_start_with_fake_edenfs()
-        # TODO(strager): Improve this message.
-        start_process.expect_exact("Could not open a bus to DBus")
+        start_process.expect_exact(
+            "error: The systemd user manager is not running. Run the following "
+            "command to\r\nstart it, then try again:"
+        )
+        start_process.expect_exact("sudo systemctl start user@testuser.service")
+
+    def test_eden_start_reports_error_if_systemd_is_dead_and_cleaned_up(self) -> None:
+        self.set_up_edenfs_systemd_service()
+        systemd = self.systemd
+        assert systemd is not None
+        systemd.exit()
+        shutil.rmtree(systemd.xdg_runtime_dir)
+
+        self.spoof_user_name("testuser")
+        start_process = self.spawn_start_with_fake_edenfs()
+        start_process.expect_exact(
+            "error: The systemd user manager is not running. Run the following "
+            "command to\r\nstart it, then try again:"
+        )
+        start_process.expect_exact("sudo systemctl start user@testuser.service")
 
     def test_eden_start_uses_fallback_if_systemd_environment_is_missing(self) -> None:
         self.set_up_edenfs_systemd_service()
@@ -208,3 +232,7 @@ class SystemdTest(
         config_d.mkdir()
         with open(config_d / "systemd.toml", "w") as config_file:
             toml.dump(config, config_file)
+
+    def spoof_user_name(self, user_name: str) -> None:
+        self.set_environment_variable("LOGNAME", user_name)
+        self.set_environment_variable("USER", user_name)