Fix error handling in PLy_spi_execute_fetch_result().

If an error is thrown out of the datatype I/O functions called by this
function, we need to do subtransaction cleanup, which the previous coding
entirely failed to do.  Fortunately, both existing callers of this function
already have proper cleanup logic, so re-throwing the exception is enough.

Also, postpone creation of the resultset tupdesc until after the I/O
conversions are complete, so that we won't leak memory in TopMemoryContext
when such an error happens.

src/pl/plpython/plpy_spi.c

@@ -407,16 +407,6 @@ PLy_spi_execute_fetch_result(SPITupleTable *tuptable, int rows, int status)
 		{
 			MemoryContext oldcontext2;
 
-			/*
-			 * Save tuple descriptor for later use by result set metadata
-			 * functions.  Save it in TopMemoryContext so that it survives
-			 * outside of an SPI context.  We trust that PLy_result_dealloc()
-			 * will clean it up when the time is right.
-			 */
-			oldcontext2 = MemoryContextSwitchTo(TopMemoryContext);
-			result->tupdesc = CreateTupleDescCopy(tuptable->tupdesc);
-			MemoryContextSwitchTo(oldcontext2);
-
 			if (rows)
 			{
 				Py_DECREF(result->rows);
@@ -425,23 +415,33 @@ PLy_spi_execute_fetch_result(SPITupleTable *tuptable, int rows, int status)
 				PLy_input_tuple_funcs(&args, tuptable->tupdesc);
 				for (i = 0; i < rows; i++)
 				{
-					PyObject   *row = PLyDict_FromTuple(&args, tuptable->vals[i],
+					PyObject   *row = PLyDict_FromTuple(&args,
+														tuptable->vals[i],
 														tuptable->tupdesc);
 
 					PyList_SetItem(result->rows, i, row);
 				}
 			}
+
+			/*
+			 * Save tuple descriptor for later use by result set metadata
+			 * functions.  Save it in TopMemoryContext so that it survives
+			 * outside of an SPI context.  We trust that PLy_result_dealloc()
+			 * will clean it up when the time is right.  (Do this as late as
+			 * possible, to minimize the number of ways the tupdesc could get
+			 * leaked due to errors.)
+			 */
+			oldcontext2 = MemoryContextSwitchTo(TopMemoryContext);
+			result->tupdesc = CreateTupleDescCopy(tuptable->tupdesc);
+			MemoryContextSwitchTo(oldcontext2);
 		}
 		PG_CATCH();
 		{
 			MemoryContextSwitchTo(oldcontext);
-			if (!PyErr_Occurred())
-				PLy_exception_set(PLy_exc_error,
-					   "unrecognized error in PLy_spi_execute_fetch_result");
 			PLy_typeinfo_dealloc(&args);
 			SPI_freetuptable(tuptable);
 			Py_DECREF(result);
-			return NULL;
+			PG_RE_THROW();
 		}
 		PG_END_TRY();