Give a hint, when [] is incorrectly used for a composite type in array.

That used to be accepted, so let's try to give a hint to users on why
their PL/python functions no longer work.

Reviewed by Pavel Stehule.

Discussion: <CAH38_tmbqwaUyKs9yagyRra=SMaT45FPBxk1pmTYcM0TyXGG7Q@mail.gmail.com>

src/pl/plpython/expected/plpython_composite.out

@@ -579,3 +579,16 @@ SELECT * FROM composite_type_as_list();
  {{"(first,1)","(second,1)"},{"(first,2)","(second,2)"},{"(first,3)","(second,3)"}}
 (1 row)
 
+-- Starting with PostgreSQL 10, a composite type in an array cannot be
+-- represented as a Python list, because it's ambiguous with multi-dimensional
+-- arrays. So this throws an error now. The error should contain a useful hint
+-- on the issue.
+CREATE FUNCTION composite_type_as_list_broken()  RETURNS type_record[] AS $$
+  return [['first', 1]];
+$$ LANGUAGE plpythonu;
+SELECT * FROM composite_type_as_list_broken();
+ERROR:  malformed record literal: "first"
+DETAIL:  Missing left parenthesis.
+HINT:  To return a composite type in an array, return the composite type as a Python tuple, e.g. "[('foo')]"
+CONTEXT:  while creating return value
+PL/Python function "composite_type_as_list_broken"

src/pl/plpython/plpy_cursorobject.c

@@ -240,7 +240,8 @@ PLy_cursor_plan(PyObject *ob, PyObject *args)
 					plan->values[j] =
 						plan->args[j].out.d.func(&(plan->args[j].out.d),
 												 -1,
-												 elem);
+												 elem,
+												 false);
 				}
 				PG_CATCH();
 				{

src/pl/plpython/plpy_exec.c

@@ -245,7 +245,7 @@ PLy_exec_function(FunctionCallInfo fcinfo, PLyProcedure *proc)
 			desc = lookup_rowtype_tupdesc(proc->result.out.d.typoid,
 										  proc->result.out.d.typmod);
 
-			rv = PLyObject_ToCompositeDatum(&proc->result, desc, plrv);
+			rv = PLyObject_ToCompositeDatum(&proc->result, desc, plrv, false);
 			fcinfo->isnull = (rv == (Datum) NULL);
 
 			ReleaseTupleDesc(desc);
@@ -253,7 +253,7 @@ PLy_exec_function(FunctionCallInfo fcinfo, PLyProcedure *proc)
 		else
 		{
 			fcinfo->isnull = false;
-			rv = (proc->result.out.d.func) (&proc->result.out.d, -1, plrv);
+			rv = (proc->result.out.d.func) (&proc->result.out.d, -1, plrv, false);
 		}
 	}
 	PG_CATCH();
@@ -984,7 +984,8 @@ PLy_modify_tuple(PLyProcedure *proc, PyObject *pltd, TriggerData *tdata,
 
 				modvalues[i] = (att->func) (att,
 											tupdesc->attrs[atti]->atttypmod,
-											plval);
+											plval,
+											false);
 				modnulls[i] = ' ';
 			}
 			else

src/pl/plpython/plpy_spi.c

@@ -264,7 +264,8 @@ PLy_spi_execute_plan(PyObject *ob, PyObject *list, long limit)
 					plan->values[j] =
 						plan->args[j].out.d.func(&(plan->args[j].out.d),
 												 -1,
-												 elem);
+												 elem,
+												 false);
 				}
 				PG_CATCH();
 				{

src/pl/plpython/plpy_typeio.h

@@ -10,8 +10,11 @@
 #include "fmgr.h"
 #include "storage/itemptr.h"
 
+/*
+ * Conversion from PostgreSQL Datum to a Python object.
+ */
 struct PLyDatumToOb;
-typedef PyObject *(*PLyDatumToObFunc) (struct PLyDatumToOb *, Datum);
+typedef PyObject *(*PLyDatumToObFunc) (struct PLyDatumToOb *arg, Datum val);
 
 typedef struct PLyDatumToOb
 {
@@ -39,11 +42,15 @@ typedef union PLyTypeInput
 	PLyTupleToOb r;
 } PLyTypeInput;
 
-/* convert PyObject to a Postgresql Datum or tuple.
- * output from Python
+/*
+ * Conversion from Python object to a Postgresql Datum.
+ *
+ * The 'inarray' argument to the conversion function is true, if the
+ * converted value was in an array (Python list). It is used to give a
+ * better error message in some cases.
  */
 struct PLyObToDatum;
-typedef Datum (*PLyObToDatumFunc) (struct PLyObToDatum *, int32, PyObject *);
+typedef Datum (*PLyObToDatumFunc) (struct PLyObToDatum *arg, int32 typmod, PyObject *val, bool inarray);
 
 typedef struct PLyObToDatum
 {
@@ -104,7 +111,7 @@ extern void PLy_output_tuple_funcs(PLyTypeInfo *arg, TupleDesc desc);
 extern void PLy_output_record_funcs(PLyTypeInfo *arg, TupleDesc desc);
 
 /* conversion from Python objects to composite Datums */
-extern Datum PLyObject_ToCompositeDatum(PLyTypeInfo *info, TupleDesc desc, PyObject *plrv);
+extern Datum PLyObject_ToCompositeDatum(PLyTypeInfo *info, TupleDesc desc, PyObject *plrv, bool isarray);
 
 /* conversion from heap tuples to Python dictionaries */
 extern PyObject *PLyDict_FromTuple(PLyTypeInfo *info, HeapTuple tuple, TupleDesc desc);

src/pl/plpython/sql/plpython_composite.sql

@@ -213,3 +213,12 @@ CREATE FUNCTION composite_type_as_list()  RETURNS type_record[] AS $$
   return [[('first', 1), ('second', 1)], [('first', 2), ('second', 2)], [('first', 3), ('second', 3)]];
 $$ LANGUAGE plpythonu;
 SELECT * FROM composite_type_as_list();
+
+-- Starting with PostgreSQL 10, a composite type in an array cannot be
+-- represented as a Python list, because it's ambiguous with multi-dimensional
+-- arrays. So this throws an error now. The error should contain a useful hint
+-- on the issue.
+CREATE FUNCTION composite_type_as_list_broken()  RETURNS type_record[] AS $$
+  return [['first', 1]];
+$$ LANGUAGE plpythonu;
+SELECT * FROM composite_type_as_list_broken();