Support domains over composite types.

This is the last major omission in our domains feature: you can now
make a domain over anything that's not a pseudotype.

The major complication from an implementation standpoint is that places
that might be creating tuples of a domain type now need to be prepared
to apply domain_check().  It seems better that unprepared code fail
with an error like "<type> is not composite" than that it silently fail
to apply domain constraints.  Therefore, relevant infrastructure like
get_func_result_type() and lookup_rowtype_tupdesc() has been adjusted
to treat domain-over-composite as a distinct case that unprepared code
won't recognize, rather than just transparently treating it the same
as plain composite.  This isn't a 100% solution to the possibility of
overlooked domain checks, but it catches most places.

In passing, improve typcache.c's support for domains (it can now cache
the identity of a domain's base type), and rewrite the argument handling
logic in jsonfuncs.c's populate_record[set]_worker to reduce duplicative
per-call lookups.

I believe this is code-complete so far as the core and contrib code go.
The PLs need varying amounts of work, which will be tackled in followup
patches.

Discussion: https://postgr.es/m/4206.1499798337@sss.pgh.pa.us

src/backend/executor/execExprInterp.c

@@ -3469,8 +3469,12 @@ ExecEvalWholeRowVar(ExprState *state, ExprEvalStep *op, ExprContext *econtext)
 			 * generates an INT4 NULL regardless of the dropped column type).
 			 * If we find a dropped column and cannot verify that case (1)
 			 * holds, we have to use the slow path to check (2) for each row.
+			 *
+			 * If vartype is a domain over composite, just look through that
+			 * to the base composite type.
 			 */
-			var_tupdesc = lookup_rowtype_tupdesc(variable->vartype, -1);
+			var_tupdesc = lookup_rowtype_tupdesc_domain(variable->vartype,
+														-1, false);
 
 			slot_tupdesc = slot->tts_tupleDescriptor;
 

src/backend/executor/execSRF.c

@@ -502,7 +502,7 @@ restart:
 	{
 		TupleTableSlot *slot = fcache->funcResultSlot;
 		MemoryContext oldContext;
-		bool foundTup;
+		bool		foundTup;
 
 		/*
 		 * Have to make sure tuple in slot lives long enough, otherwise
@@ -734,7 +734,8 @@ init_sexpr(Oid foid, Oid input_collation, Expr *node,
 		/* Must save tupdesc in sexpr's context */
 		oldcontext = MemoryContextSwitchTo(sexprCxt);
 
-		if (functypclass == TYPEFUNC_COMPOSITE)
+		if (functypclass == TYPEFUNC_COMPOSITE ||
+			functypclass == TYPEFUNC_COMPOSITE_DOMAIN)
 		{
 			/* Composite data type, e.g. a table's row type */
 			Assert(tupdesc);

src/backend/executor/functions.c

@@ -1665,7 +1665,15 @@ check_sql_fn_retval(Oid func_id, Oid rettype, List *queryTreeList,
 	}
 	else if (fn_typtype == TYPTYPE_COMPOSITE || rettype == RECORDOID)
 	{
-		/* Returns a rowtype */
+		/*
+		 * Returns a rowtype.
+		 *
+		 * Note that we will not consider a domain over composite to be a
+		 * "rowtype" return type; it goes through the scalar case above.  This
+		 * is because SQL functions don't provide any implicit casting to the
+		 * result type, so there is no way to produce a domain-over-composite
+		 * result except by computing it as an explicit single-column result.
+		 */
 		TupleDesc	tupdesc;
 		int			tupnatts;	/* physical number of columns in tuple */
 		int			tuplogcols; /* # of nondeleted columns in tuple */
@@ -1711,7 +1719,10 @@ check_sql_fn_retval(Oid func_id, Oid rettype, List *queryTreeList,
 			}
 		}
 
-		/* Is the rowtype fixed, or determined only at runtime? */
+		/*
+		 * Is the rowtype fixed, or determined only at runtime?  (Note we
+		 * cannot see TYPEFUNC_COMPOSITE_DOMAIN here.)
+		 */
 		if (get_func_result_type(func_id, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
 		{
 			/*

src/backend/executor/nodeFunctionscan.c

@@ -383,7 +383,8 @@ ExecInitFunctionScan(FunctionScan *node, EState *estate, int eflags)
 											&funcrettype,
 											&tupdesc);
 
-		if (functypclass == TYPEFUNC_COMPOSITE)
+		if (functypclass == TYPEFUNC_COMPOSITE ||
+			functypclass == TYPEFUNC_COMPOSITE_DOMAIN)
 		{
 			/* Composite data type, e.g. a table's row type */
 			Assert(tupdesc);