Get rid of shared_record_typmod_registry_worker_detach; it doesn't work.

This code is unsafe, as proven by buildfarm failures, because it tries
to access shared memory that might already be gone.  It's also unnecessary,
because we're about to exit the process anyway and so the record type cache
should never be accessed again.  The idea was to lay some foundations for
someday recycling workers --- which would require attaching to a different
shared tupdesc registry --- but that will require considerably more
thought.  In the meantime let's save some bytes by just removing the
nonfunctional code.

Problem identification, and proposal to fix by removing functionality
from the detach function, by Thomas Munro.  I went a bit further by
removing the function altogether.

Discussion: https://postgr.es/m/E1dsguX-00056N-9x@gemulon.postgresql.org

src/backend/utils/cache/typcache.c

@@ -285,8 +285,6 @@ static EnumItem *find_enumitem(TypeCacheEnumData *enumdata, Oid arg);
 static int	enum_oid_cmp(const void *left, const void *right);
 static void shared_record_typmod_registry_detach(dsm_segment *segment,
 									 Datum datum);
-static void shared_record_typmod_registry_worker_detach(dsm_segment *segment,
-											Datum datum);
 static TupleDesc find_or_make_matching_shared_tupledesc(TupleDesc tupdesc);
 static dsa_pointer share_tupledesc(dsa_area *area, TupleDesc tupdesc,
 				uint32 typmod);
@@ -1768,8 +1766,9 @@ SharedRecordTypmodRegistryAttach(SharedRecordTypmodRegistry *registry)
 	 * a freshly started parallel worker.  If we ever support worker
 	 * recycling, a worker would need to zap its local cache in between
 	 * servicing different queries, in order to be able to call this and
-	 * synchronize typmods with a new leader; see
-	 * shared_record_typmod_registry_detach().
+	 * synchronize typmods with a new leader; but that's problematic because
+	 * we can't be very sure that record-typmod-related state hasn't escaped
+	 * to anywhere else in the process.
 	 */
 	Assert(NextRecordTypmod == 0);
 
@@ -1788,11 +1787,12 @@ SharedRecordTypmodRegistryAttach(SharedRecordTypmodRegistry *registry)
 	MemoryContextSwitchTo(old_context);
 
 	/*
-	 * We install a different detach callback that performs a more complete
-	 * reset of backend local state.
+	 * Set up detach hook to run at worker exit.  Currently this is the same
+	 * as the leader's detach hook, but in future they might need to be
+	 * different.
 	 */
 	on_dsm_detach(CurrentSession->segment,
-				  shared_record_typmod_registry_worker_detach,
+				  shared_record_typmod_registry_detach,
 				  PointerGetDatum(registry));
 
 	/*
@@ -2353,10 +2353,8 @@ find_or_make_matching_shared_tupledesc(TupleDesc tupdesc)
 }
 
 /*
- * Detach hook to forget about the current shared record typmod
- * infrastructure.  This is registered directly in leader backends, and
- * reached only in case of error or shutdown.  It's also reached indirectly
- * via the worker detach callback below.
+ * On-DSM-detach hook to forget about the current shared record typmod
+ * infrastructure.  This is currently used by both leader and workers.
  */
 static void
 shared_record_typmod_registry_detach(dsm_segment *segment, Datum datum)
@@ -2374,57 +2372,3 @@ shared_record_typmod_registry_detach(dsm_segment *segment, Datum datum)
 	}
 	CurrentSession->shared_typmod_registry = NULL;
 }
-
-/*
- * Deatch hook allowing workers to disconnect from shared record typmod
- * registry.  The resulting state should allow a worker to attach to a
- * different leader, if worker reuse pools are invented.
- */
-static void
-shared_record_typmod_registry_worker_detach(dsm_segment *segment, Datum datum)
-{
-	/*
-	 * Forget everything we learned about record typmods as part of the
-	 * session we are disconnecting from, and return to the initial state.
-	 */
-	if (RecordCacheArray != NULL)
-	{
-		int32		i;
-
-		for (i = 0; i < RecordCacheArrayLen; ++i)
-		{
-			if (RecordCacheArray[i] != NULL)
-			{
-				TupleDesc	tupdesc = RecordCacheArray[i];
-
-				/*
-				 * Pointers to tuple descriptors in shared memory are not
-				 * reference counted, so we are not responsible for freeing
-				 * them.  They'll survive as long as the shared session
-				 * exists, which should be as long as the owning leader
-				 * backend exists.  In theory we do need to free local
-				 * reference counted tuple descriptors however, and we can't
-				 * do that with DescTupleDescRefCount() because we aren't
-				 * using a resource owner.  In practice we don't expect to
-				 * find any non-shared TupleDesc object in a worker.
-				 */
-				if (tupdesc->tdrefcount != -1)
-				{
-					Assert(tupdesc->tdrefcount > 0);
-					if (--tupdesc->tdrefcount == 0)
-						FreeTupleDesc(tupdesc);
-				}
-			}
-		}
-		pfree(RecordCacheArray);
-		RecordCacheArray = NULL;
-	}
-	if (RecordCacheHash != NULL)
-	{
-		hash_destroy(RecordCacheHash);
-		RecordCacheHash = NULL;
-	}
-	NextRecordTypmod = 0;
-	/* Call the code common to leader and worker detach. */
-	shared_record_typmod_registry_detach(segment, datum);
-}