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

Commit ce0fdbfe97 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

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