Add a utility function to extract variadic function arguments

This is epecially useful in the case or "VARIADIC ANY" functions. The
caller can get the artguments and types regardless of whether or not and
explicit VARIADIC array argument has been used. The function also
provides an option to convert arguments on type "unknown" to to "text".

Michael Paquier and me, reviewed by Tom Lane.

Backpatch to 9.4 in order to support the following json bug fix.

src/backend/utils/fmgr/funcapi.c

@@ -2,7 +2,7 @@
  *
  * funcapi.c
  *	  Utility and convenience functions for fmgr functions that return
- *	  sets and/or composite types.
+ *	  sets and/or composite types, or deal with VARIADIC inputs.
  *
  * Copyright (c) 2002-2017, PostgreSQL Global Development Group
  *
@@ -1400,3 +1400,116 @@ TypeGetTupleDesc(Oid typeoid, List *colaliases)
 
 	return tupdesc;
 }
+
+/*
+ * extract_variadic_args
+ *
+ * Extract a set of argument values, types and NULL markers for a given
+ * input function which makes use of a VARIADIC input whose argument list
+ * depends on the caller context. When doing a VARIADIC call, the caller
+ * has provided one argument made of an array of values, so deconstruct the
+ * array data before using it for the next processing. If no VARIADIC call
+ * is used, just fill in the status data based on all the arguments given
+ * by the caller.
+ *
+ * This function returns the number of arguments generated, or -1 in the
+ * case of "VARIADIC NULL".
+ */
+int
+extract_variadic_args(FunctionCallInfo fcinfo, int variadic_start,
+					  bool convert_unknown, Datum **args, Oid **types,
+					  bool **nulls)
+{
+	bool		variadic = get_fn_expr_variadic(fcinfo->flinfo);
+	Datum	   *args_res;
+	bool	   *nulls_res;
+	Oid		   *types_res;
+	int			nargs, i;
+
+	*args = NULL;
+	*types = NULL;
+	*nulls = NULL;
+
+	if (variadic)
+	{
+		ArrayType  *array_in;
+		Oid			element_type;
+		bool		typbyval;
+		char		typalign;
+		int16		typlen;
+
+		Assert(PG_NARGS() == variadic_start + 1);
+
+		if (PG_ARGISNULL(variadic_start))
+			return -1;
+
+		array_in = PG_GETARG_ARRAYTYPE_P(variadic_start);
+		element_type = ARR_ELEMTYPE(array_in);
+
+		get_typlenbyvalalign(element_type,
+							 &typlen, &typbyval, &typalign);
+		deconstruct_array(array_in, element_type, typlen, typbyval,
+						  typalign, &args_res, &nulls_res,
+						  &nargs);
+
+		/* All the elements of the array have the same type */
+		types_res = (Oid *) palloc0(nargs * sizeof(Oid));
+		for (i = 0; i < nargs; i++)
+			types_res[i] = element_type;
+	}
+	else
+	{
+		nargs = PG_NARGS() - variadic_start;
+		Assert (nargs > 0);
+		nulls_res = (bool *) palloc0(nargs * sizeof(bool));
+		args_res = (Datum *) palloc0(nargs * sizeof(Datum));
+		types_res = (Oid *) palloc0(nargs * sizeof(Oid));
+
+		for (i = 0; i < nargs; i++)
+		{
+			nulls_res[i] = PG_ARGISNULL(i + variadic_start);
+			types_res[i] = get_fn_expr_argtype(fcinfo->flinfo,
+											   i + variadic_start);
+
+			/*
+			 * Turn a constant (more or less literal) value that's of unknown
+			 * type into text if required . Unknowns come in as a cstring
+			 * pointer.
+			 * Note: for functions declared as taking type "any", the parser
+			 * will not do any type conversion on unknown-type literals (that
+			 * is, undecorated strings or NULLs).
+			 */
+			if (convert_unknown &&
+				types_res[i] == UNKNOWNOID &&
+				get_fn_expr_arg_stable(fcinfo->flinfo, i + variadic_start))
+			{
+				types_res[i] = TEXTOID;
+
+				if (PG_ARGISNULL(i + variadic_start))
+					args_res[i] = (Datum) 0;
+				else
+					args_res[i] =
+						CStringGetTextDatum(PG_GETARG_POINTER(i + variadic_start));
+			}
+			else
+			{
+				/* no conversion needed, just take the datum as given */
+				args_res[i] = PG_GETARG_DATUM(i + variadic_start);
+			}
+
+			if (!OidIsValid(types_res[i]) ||
+				(convert_unknown && types_res[i] == UNKNOWNOID))
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+						 errmsg("could not determine data type for argument %d",
+								i + 1)));
+		}
+	}
+
+	/* Fill in results */
+	*args = args_res;
+	*nulls = nulls_res;
+	*types = types_res;
+
+	return nargs;
+}

src/include/funcapi.h

@@ -2,6 +2,7 @@
  *
  * funcapi.h
  *	  Definitions for functions which return composite type and/or sets
+ *	  or work on VARIADIC inputs.
  *
  * This file must be included by all Postgres modules that either define
  * or call FUNCAPI-callable functions or macros.
@@ -315,4 +316,26 @@ extern void end_MultiFuncCall(PG_FUNCTION_ARGS, FuncCallContext *funcctx);
 		PG_RETURN_NULL(); \
 	} while (0)
 
+/*----------
+ *	Support to ease writing of functions dealing with VARIADIC inputs
+ *----------
+ *
+ * This function extracts a set of argument values, types and NULL markers
+ * for a given input function. This returns a set of data:
+ * - **values includes the set of Datum values extracted.
+ * - **types the data type OID for each element.
+ * - **nulls tracks if an element is NULL.
+ *
+ * variadic_start indicates the argument number where the VARIADIC argument
+ * starts.
+ * convert_unknown set to true will enforce the conversion of arguments
+ * with unknown data type to text.
+ *
+ * The return result is the number of elements stored, or -1 in the case of
+ * "VARIADIC NULL".
+ */
+extern int extract_variadic_args(FunctionCallInfo fcinfo, int variadic_start,
+								 bool convert_unknown, Datum **values,
+								 Oid **types, bool **nulls);
+
 #endif							/* FUNCAPI_H */