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.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/include/funcapi.h b/src/include/funcapi.h
index 6590a088c92..37f9b3b56c7 100644
--- a/src/include/funcapi.h
+++ b/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.
@@ -314,4 +315,26 @@ 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 */