update_analytics_count: Use a correct lock mechanism.

Adds a re-usable lockfile_nonblocking helper to context_managers.

Relying on naive `os.mkdir` is not enough especially now that the
successful operation of this command is necessary for push notifications
to work for many servers.

We can't use `lockfile` context manager from
`zerver.lib.context_managers`, because we want the custom behavior of
failing if the lock can't be acquired, instead of waiting.
That's because if an instance of this gets stuck, we don't want to start
queueing up more processes waiting forever whenever the cronjob runs
again and fail->exit is preferrable instead.

analytics/management/commands/update_analytics_counts.py

@@ -1,5 +1,4 @@
 import hashlib
-import os
 import time
 from argparse import ArgumentParser
 from datetime import timezone
@@ -13,6 +12,7 @@ from typing_extensions import override
 
 from analytics.lib.counts import ALL_COUNT_STATS, logger, process_count_stat
 from scripts.lib.zulip_tools import ENDC, WARNING
+from zerver.lib.context_managers import lockfile_nonblocking
 from zerver.lib.remote_server import send_server_data_to_push_bouncer
 from zerver.lib.timestamp import floor_to_hour
 from zerver.models import Realm
@@ -42,19 +42,16 @@ class Command(BaseCommand):
 
     @override
     def handle(self, *args: Any, **options: Any) -> None:
-        try:
-            os.mkdir(settings.ANALYTICS_LOCK_DIR)
-        except OSError:
-            print(
-                f"{WARNING}Analytics lock {settings.ANALYTICS_LOCK_DIR} is unavailable;"
-                f" exiting.{ENDC}"
-            )
-            return
-
-        try:
-            self.run_update_analytics_counts(options)
-        finally:
-            os.rmdir(settings.ANALYTICS_LOCK_DIR)
+        with lockfile_nonblocking(
+            settings.ANALYTICS_LOCK_FILE,
+        ) as lock_acquired:
+            if lock_acquired:
+                self.run_update_analytics_counts(options)
+            else:
+                print(
+                    f"{WARNING}Analytics lock {settings.ANALYTICS_LOCK_FILE} is unavailable;"
+                    f" exiting.{ENDC}"
+                )
 
     def run_update_analytics_counts(self, options: Dict[str, Any]) -> None:
         # installation_epoch relies on there being at least one realm; we

zerver/lib/context_managers.py

@@ -30,3 +30,21 @@ def lockfile(filename: str, shared: bool = False) -> Iterator[None]:
     with open(filename, "w") as lock:
         with flock(lock, shared=shared):
             yield
+
+
+@contextmanager
+def lockfile_nonblocking(filename: str) -> Iterator[bool]:  # nocoverage
+    """Lock a file using flock(2) for the duration of a 'with' statement.
+
+    Doesn't block, yields False immediately if the lock can't be acquired."""
+    with open(filename) as f:
+        lock_acquired = False
+        try:
+            fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
+            lock_acquired = True
+            yield lock_acquired
+        except BlockingIOError:
+            yield False
+        finally:
+            if lock_acquired:
+                fcntl.flock(f, fcntl.LOCK_UN)

zproject/computed_settings.py

@@ -676,7 +676,7 @@ QUEUE_ERROR_DIR = zulip_path("/var/log/zulip/queue_error")
 QUEUE_STATS_DIR = zulip_path("/var/log/zulip/queue_stats")
 DIGEST_LOG_PATH = zulip_path("/var/log/zulip/digest.log")
 ANALYTICS_LOG_PATH = zulip_path("/var/log/zulip/analytics.log")
-ANALYTICS_LOCK_DIR = zulip_path("/home/zulip/deployments/analytics-lock-dir")
+ANALYTICS_LOCK_FILE = zulip_path("/home/zulip/deployments/analytics-lock.lock")
 WEBHOOK_LOG_PATH = zulip_path("/var/log/zulip/webhooks_errors.log")
 WEBHOOK_ANOMALOUS_PAYLOADS_LOG_PATH = zulip_path("/var/log/zulip/webhooks_anomalous_payloads.log")
 WEBHOOK_UNSUPPORTED_EVENTS_LOG_PATH = zulip_path("/var/log/zulip/webhooks_unsupported_events.log")