Fix race condition in drop subscription's handling of tablesync slots.

Amit Kapila · Amit Kapila · commit 6b67d72b604c · 2021-03-17T08:15:12.000+05:30
Commit ce0fdbf made tablesync slots permanent and allow Drop Subscription to drop such slots. However, it is possible that before tablesync worker could get the acknowledgment of slot creation, drop subscription stops it and that can lead to a dangling slot on the publisher. Prevent cancel/die interrupts while creating a slot in the tablesync worker. Reported-by: Thomas Munro as per buildfarm Author: Amit Kapila Reviewed-by: Vignesh C, Takamichi Osumi Discussion: https://postgr.es/m/CA+hUKGJG9dWpw1cOQ2nzWU8PHjm=PTraB+KgE5648K9nTfwvxg@mail.gmail.com
diff --git a/src/backend/replication/logical/tablesync.c b/src/backend/replication/logical/tablesync.c
@@ -1051,9 +1051,16 @@ LogicalRepSyncTableStart(XLogRecPtr *origin_startpos)
 	 * Create a new permanent logical decoding slot. This slot will be used
 	 * for the catchup phase after COPY is done, so tell it to use the
 	 * snapshot to make the final data consistent.
+	 *
+	 * Prevent cancel/die interrupts while creating slot here because it is
+	 * possible that before the server finishes this command, a concurrent
+	 * drop subscription happens which would complete without removing this
+	 * slot leading to a dangling slot on the server.
 	 */
+	HOLD_INTERRUPTS();
 	walrcv_create_slot(wrconn, slotname, false /* permanent */ ,
 					   CRS_USE_SNAPSHOT, origin_startpos);
+	RESUME_INTERRUPTS();
 
 	/*
 	 * Setup replication origin tracking. The purpose of doing this before the
