Fix failure to cover scalar-vs-rowtype cases in exec_stmt_return().

In commit 9e3ad1aac52454569393a947c06be0d301749362 I modified plpgsql
to use exec_stmt_return's simple-variables fast path in more cases.
However, I overlooked that there are really two different return
conventions in use here, depending on whether estate->retistuple is true,
and the existing fast-path code had only bothered to handle one of them.
So trying to return a scalar in a function returning composite, or vice
versa, could lead to unexpected error messages (typically "cache lookup
failed for type 0") or to a null-pointer-dereference crash.

In the DTYPE_VAR case, we can just throw error if retistuple is true,
corresponding to what happens in the general-expression code path that was
being used previously.  (Perhaps someday both of these code paths should
attempt a coercion, but today is not that day.)

In the REC and ROW cases, just hand the problem to exec_eval_datum()
when not retistuple.  Also clean up the ROW coding slightly so it looks
more like exec_eval_datum().

The previous commit also caused exec_stmt_return_next() to be used in
more cases, but that code seems to be OK as-is.

Per off-list report from Serge Rielau.  This bug is new in 9.5 so no need
to back-patch.

src/pl/plpgsql/src/pl_exec.c

@@ -2508,6 +2508,7 @@ exec_stmt_return(PLpgSQL_execstate *estate, PLpgSQL_stmt_return *stmt)
 	estate->retval = (Datum) 0;
 	estate->rettupdesc = NULL;
 	estate->retisnull = true;
+	estate->rettype = InvalidOid;
 
 	/*
 	 * Special case path when the RETURN expression is a simple variable
@@ -2534,18 +2535,40 @@ exec_stmt_return(PLpgSQL_execstate *estate, PLpgSQL_stmt_return *stmt)
 					estate->retval = var->value;
 					estate->retisnull = var->isnull;
 					estate->rettype = var->datatype->typoid;
+
+					/*
+					 * Cope with retistuple case.  A PLpgSQL_var could not be
+					 * of composite type, so we needn't make any effort to
+					 * convert.  However, for consistency with the expression
+					 * code path, don't throw error if the result is NULL.
+					 */
+					if (estate->retistuple && !estate->retisnull)
+						ereport(ERROR,
+								(errcode(ERRCODE_DATATYPE_MISMATCH),
+								 errmsg("cannot return non-composite value from function returning composite type")));
 				}
 				break;
 
 			case PLPGSQL_DTYPE_REC:
 				{
 					PLpgSQL_rec *rec = (PLpgSQL_rec *) retvar;
+					int32		rettypmod;
 
 					if (HeapTupleIsValid(rec->tup))
 					{
-						estate->retval = PointerGetDatum(rec->tup);
-						estate->rettupdesc = rec->tupdesc;
-						estate->retisnull = false;
+						if (estate->retistuple)
+						{
+							estate->retval = PointerGetDatum(rec->tup);
+							estate->rettupdesc = rec->tupdesc;
+							estate->retisnull = false;
+						}
+						else
+							exec_eval_datum(estate,
+											retvar,
+											&estate->rettype,
+											&rettypmod,
+											&estate->retval,
+											&estate->retisnull);
 					}
 				}
 				break;
@@ -2553,15 +2576,28 @@ exec_stmt_return(PLpgSQL_execstate *estate, PLpgSQL_stmt_return *stmt)
 			case PLPGSQL_DTYPE_ROW:
 				{
 					PLpgSQL_row *row = (PLpgSQL_row *) retvar;
+					int32		rettypmod;
 
-					Assert(row->rowtupdesc);
-					estate->retval =
-						PointerGetDatum(make_tuple_from_row(estate, row,
-															row->rowtupdesc));
-					if (DatumGetPointer(estate->retval) == NULL)		/* should not happen */
-						elog(ERROR, "row not compatible with its own tupdesc");
-					estate->rettupdesc = row->rowtupdesc;
-					estate->retisnull = false;
+					if (estate->retistuple)
+					{
+						HeapTuple	tup;
+
+						if (!row->rowtupdesc)	/* should not happen */
+							elog(ERROR, "row variable has no tupdesc");
+						tup = make_tuple_from_row(estate, row, row->rowtupdesc);
+						if (tup == NULL)		/* should not happen */
+							elog(ERROR, "row not compatible with its own tupdesc");
+						estate->retval = PointerGetDatum(tup);
+						estate->rettupdesc = row->rowtupdesc;
+						estate->retisnull = false;
+					}
+					else
+						exec_eval_datum(estate,
+										retvar,
+										&estate->rettype,
+										&rettypmod,
+										&estate->retval,
+										&estate->retisnull);
 				}
 				break;
 

src/test/regress/expected/plpgsql.out

@@ -4001,6 +4001,38 @@ $$ language plpgsql;
 select compos();
 ERROR:  cannot return non-composite value from function returning composite type
 CONTEXT:  PL/pgSQL function compos() line 3 at RETURN
+-- RETURN variable is a different code path ...
+create or replace function compos() returns compostype as $$
+declare x int := 42;
+begin
+  return x;
+end;
+$$ language plpgsql;
+select * from compos();
+ERROR:  cannot return non-composite value from function returning composite type
+CONTEXT:  PL/pgSQL function compos() line 4 at RETURN
+drop function compos();
+-- test: invalid use of composite variable in scalar-returning function
+create or replace function compos() returns int as $$
+declare
+  v compostype;
+begin
+  v := (1, 'hello');
+  return v;
+end;
+$$ language plpgsql;
+select compos();
+ERROR:  invalid input syntax for integer: "(1,hello)"
+CONTEXT:  PL/pgSQL function compos() while casting return value to function's return type
+-- test: invalid use of composite expression in scalar-returning function
+create or replace function compos() returns int as $$
+begin
+  return (1, 'hello')::compostype;
+end;
+$$ language plpgsql;
+select compos();
+ERROR:  invalid input syntax for integer: "(1,hello)"
+CONTEXT:  PL/pgSQL function compos() while casting return value to function's return type
 drop function compos();
 drop type compostype;
 --

src/test/regress/sql/plpgsql.sql

@@ -3248,6 +3248,39 @@ $$ language plpgsql;
 
 select compos();
 
+-- RETURN variable is a different code path ...
+create or replace function compos() returns compostype as $$
+declare x int := 42;
+begin
+  return x;
+end;
+$$ language plpgsql;
+
+select * from compos();
+
+drop function compos();
+
+-- test: invalid use of composite variable in scalar-returning function
+create or replace function compos() returns int as $$
+declare
+  v compostype;
+begin
+  v := (1, 'hello');
+  return v;
+end;
+$$ language plpgsql;
+
+select compos();
+
+-- test: invalid use of composite expression in scalar-returning function
+create or replace function compos() returns int as $$
+begin
+  return (1, 'hello')::compostype;
+end;
+$$ language plpgsql;
+
+select compos();
+
 drop function compos();
 drop type compostype;