docserver: survive MinIO outages instead of exiting

The worker called sys.exit(1) on any MinIO connection error, so a single
transient 502 from MinIO/its reverse proxy left it dead until a manual restart
or reboot (its scheduled task only runs at system startup). It had been dead
~5 weeks after a 502 on May 9.

- _connect_minio_forever(): retry the initial MinIO connect indefinitely with
  capped exponential backoff (5s..120s) instead of exiting.
- main loop: wrap each poll cycle; on any error, log + rebuild the client and
  keep polling rather than crashing.

Verified on the box: normal DOCX->PDF still works (~11s e2e); a bogus endpoint
now retries forever without ever calling sys.exit (was the exact May-9 failure).

docserver/docserver_worker.py

@@ -195,6 +195,29 @@ def _ensure_bucket(mc) -> None:
         LOG.info("Created bucket: %s", _BUCKET)
 
 
+def _connect_minio_forever():
+    """Build a MinIO client and verify the bucket, retrying forever with capped
+    exponential backoff. Returns a working client once MinIO is reachable.
+
+    The worker used to ``sys.exit(1)`` on a connection error, so a single
+    transient 502 from MinIO (or its reverse proxy) left it dead until a reboot.
+    """
+    delay = 5
+    attempt = 0
+    while True:
+        attempt += 1
+        try:
+            mc = _mc()
+            _ensure_bucket(mc)
+            LOG.info("MinIO connected — bucket '%s' ready", _BUCKET)
+            return mc
+        except Exception as exc:
+            LOG.error("MinIO connection failed (attempt %d): %s; retrying in %ds",
+                      attempt, exc, delay)
+            time.sleep(delay)
+            delay = min(delay * 2, 120)  # 5,10,20,40,80,120,120...
+
+
 def _list_pending(mc) -> list[str]:
     """Return object names under to-convert/ that end in .docx.
 
@@ -334,15 +357,11 @@ def main() -> None:
         LOG.error("Or RDP in to create an interactive session, then the AtLogOn task will fire.")
         sys.exit(1)
 
-    # Verify MinIO connectivity
-    LOG.info("Connecting to MinIO...")
-    try:
-        mc = _mc()
-        _ensure_bucket(mc)
-        LOG.info("MinIO connected — bucket '%s' ready", _BUCKET)
-    except Exception as exc:
-        LOG.error("MinIO connection failed: %s", exc)
-        sys.exit(1)
+    # Connect to MinIO, retrying indefinitely. MinIO (or the nginx vhost in
+    # front of it) can return transient 502s / be briefly unreachable; the
+    # worker must wait it out rather than exit, otherwise it stays dead until a
+    # reboot or a manual restart.
+    mc = _connect_minio_forever()
 
     # Start heartbeat background thread
     hb = threading.Thread(target=_heartbeat_loop, args=(word_version,), daemon=True)
@@ -353,11 +372,17 @@ def main() -> None:
 
     try:
         while True:
-            pending = _list_pending(mc)
-            if pending:
-                LOG.info("Found %d pending job(s)", len(pending))
-                for key in pending:
-                    _process_one(mc, key)
+            try:
+                pending = _list_pending(mc)
+                if pending:
+                    LOG.info("Found %d pending job(s)", len(pending))
+                    for key in pending:
+                        _process_one(mc, key)
+            except Exception as exc:
+                # Never let a transient MinIO/network error kill the loop.
+                # Rebuild the client and keep going after a short pause.
+                LOG.error("Poll cycle failed (%s); reconnecting to MinIO...", exc)
+                mc = _connect_minio_forever()
             time.sleep(_POLL_INTERVAL)
     except KeyboardInterrupt:
         LOG.info("Shutting down...")