Add support for coordinating record typmods among parallel workers.

Tuples can have type RECORDOID and a typmod number that identifies a blessed
TupleDesc in a backend-private cache.  To support the sharing of such tuples
through shared memory and temporary files, provide a typmod registry in
shared memory.

To achieve that, introduce per-session DSM segments, created on demand when a
backend first runs a parallel query.  The per-session DSM segment has a
table-of-contents just like the per-query DSM segment, and initially the
contents are a shared record typmod registry and a DSA area to provide the
space it needs to grow.

State relating to the current session is accessed via a Session object
reached through global variable CurrentSession that may require significant
redesign further down the road as we figure out what else needs to be shared
or remodelled.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.com

src/include/access/tupdesc.h

@@ -92,6 +92,12 @@ extern TupleDesc CreateTupleDescCopy(TupleDesc tupdesc);
 
 extern TupleDesc CreateTupleDescCopyConstr(TupleDesc tupdesc);
 
+#define TupleDescSize(src) \
+	(offsetof(struct tupleDesc, attrs) + \
+	 (src)->natts * sizeof(FormData_pg_attribute))
+
+extern void TupleDescCopy(TupleDesc dst, TupleDesc src);
+
 extern void TupleDescCopyEntry(TupleDesc dst, AttrNumber dstAttno,
 				   TupleDesc src, AttrNumber srcAttno);