fix: Atomic job reservation to prevent race condition

src/datajoint/jobs.py

@@ -13,7 +13,7 @@
 import platform
 import subprocess
 
-from .condition import AndList, Not
+from .condition import AndList, Not, make_condition
 from .errors import DataJointError, DuplicateError
 from .heading import Heading
 from .table import Table
@@ -431,8 +431,10 @@ def reserve(self, key: dict) -> bool:
         """
         Attempt to reserve a pending job for processing.
 
-        Updates status to ``'reserved'`` if currently ``'pending'`` and
-        ``scheduled_time <= now``.
+        Atomically updates status to ``'reserved'`` if currently ``'pending'``
+        and ``scheduled_time <= now``, using a single UPDATE with a WHERE clause
+        that includes the status check. This prevents race conditions where
+        multiple workers could reserve the same job simultaneously.
 
         Parameters
         ----------
@@ -444,33 +446,26 @@ def reserve(self, key: dict) -> bool:
         bool
             True if reservation successful, False if job not available.
         """
-        # Check if job is pending and scheduled (use CURRENT_TIMESTAMP(3) for datetime(3) precision)
-        job = (self & key & "status='pending'" & "scheduled_time <= CURRENT_TIMESTAMP(3)").to_dicts()
-
-        if not job:
-            return False
-
-        # Get server time for reserved_time
-        server_now = self.connection.query("SELECT CURRENT_TIMESTAMP").fetchone()[0]
-
-        # Build update row with primary key and new values
         pk = self._get_pk(key)
-        update_row = {
-            **pk,
-            "status": "reserved",
-            "reserved_time": server_now,
-            "host": platform.node(),
-            "pid": os.getpid(),
-            "connection_id": self.connection.connection_id,
-            "user": self.connection.get_user(),
-            "version": _get_job_version(),
-        }
-
-        try:
-            self.update1(update_row)
-            return True
-        except Exception:
-            return False
+        where = make_condition(self, pk, set())
+        qi = self.adapter.quote_identifier
+        assignments = ", ".join(f"{qi(k)}=%s" for k in ("status", "host", "pid", "connection_id", "user", "version"))
+        query = (
+            f"UPDATE {self.full_table_name} "
+            f"SET {assignments}, {qi('reserved_time')}=CURRENT_TIMESTAMP(3) "
+            f"WHERE {where} AND {qi('status')}='pending' "
+            f"AND {qi('scheduled_time')} <= CURRENT_TIMESTAMP(3)"
+        )
+        args = [
+            "reserved",
+            platform.node(),
+            os.getpid(),
+            self.connection.connection_id,
+            self.connection.get_user(),
+            _get_job_version(),
+        ]
+        cursor = self.connection.query(query, args=args)
+        return cursor.rowcount == 1
 
     def complete(self, key: dict, duration: float | None = None) -> None:
         """