Mark function arguments of type "Datum *" as "const Datum *" where possible

Several functions in the codebase accept "Datum *" parameters but do
not modify the pointed-to data.  These have been updated to take
"const Datum *" instead, improving type safety and making the
interfaces clearer about their intent.  This change helps the compiler
catch accidental modifications and better documents immutability of
arguments.

Most of "Datum *" parameters have a pairing "bool *isnull" parameter,
they are constified as well.

No functional behavior is changed by this patch.

Author: Chao Li <lic@highgo.com>
Discussion: https://www.postgresql.org/message-id/flat/CAEoWx2msfT0knvzUa72ZBwu9LR_RLY4on85w2a9YpE-o2By5HQ@mail.gmail.com

src/backend/access/brin/brin_minmax_multi.c

@@ -276,7 +276,7 @@ static int	compare_values(const void *a, const void *b, void *arg);
  * function (which should be BTLessStrategyNumber).
  */
 static void
-AssertArrayOrder(FmgrInfo *cmp, Oid colloid, Datum *values, int nvalues)
+AssertArrayOrder(FmgrInfo *cmp, Oid colloid, const Datum *values, int nvalues)
 {
 	int			i;
 	Datum		lt;

src/backend/access/hash/hashutil.c

@@ -316,7 +316,7 @@ _hash_get_indextuple_hashkey(IndexTuple itup)
  */
 bool
 _hash_convert_tuple(Relation index,
-					Datum *user_values, bool *user_isnull,
+					const Datum *user_values, const bool *user_isnull,
 					Datum *index_values, bool *index_isnull)
 {
 	uint32		hashkey;

src/backend/access/heap/heaptoast.c

@@ -561,8 +561,8 @@ toast_flatten_tuple_to_datum(HeapTupleHeader tup,
  */
 HeapTuple
 toast_build_flattened_tuple(TupleDesc tupleDesc,
-							Datum *values,
-							bool *isnull)
+							const Datum *values,
+							const bool *isnull)
 {
 	HeapTuple	new_tuple;
 	int			numAttrs = tupleDesc->natts;

src/backend/access/nbtree/nbtpreprocesskeys.c

@@ -67,7 +67,7 @@ static int	_bt_num_array_keys(IndexScanDesc scan, Oid *skip_eq_ops_out,
 							   int *numSkipArrayKeys_out);
 static Datum _bt_find_extreme_element(IndexScanDesc scan, ScanKey skey,
 									  Oid elemtype, StrategyNumber strat,
-									  Datum *elems, int nelems);
+									  const Datum *elems, int nelems);
 static void _bt_setup_array_cmp(IndexScanDesc scan, ScanKey skey, Oid elemtype,
 								FmgrInfo *orderproc, FmgrInfo **sortprocp);
 static int	_bt_sort_array_elements(ScanKey skey, FmgrInfo *sortproc,
@@ -2569,7 +2569,7 @@ _bt_num_array_keys(IndexScanDesc scan, Oid *skip_eq_ops_out,
 static Datum
 _bt_find_extreme_element(IndexScanDesc scan, ScanKey skey, Oid elemtype,
 						 StrategyNumber strat,
-						 Datum *elems, int nelems)
+						 const Datum *elems, int nelems)
 {
 	Relation	rel = scan->indexRelation;
 	Oid			cmp_op;

src/backend/access/nbtree/nbtsort.c

@@ -258,7 +258,7 @@ static double _bt_spools_heapscan(Relation heap, Relation index,
 								  BTBuildState *buildstate, IndexInfo *indexInfo);
 static void _bt_spooldestroy(BTSpool *btspool);
 static void _bt_spool(BTSpool *btspool, const ItemPointerData *self,
-					  Datum *values, bool *isnull);
+					  const Datum *values, const bool *isnull);
 static void _bt_leafbuild(BTSpool *btspool, BTSpool *btspool2);
 static void _bt_build_callback(Relation index, ItemPointer tid, Datum *values,
 							   bool *isnull, bool tupleIsAlive, void *state);
@@ -525,7 +525,7 @@ _bt_spooldestroy(BTSpool *btspool)
  * spool an index entry into the sort file.
  */
 static void
-_bt_spool(BTSpool *btspool, const ItemPointerData *self, Datum *values, bool *isnull)
+_bt_spool(BTSpool *btspool, const ItemPointerData *self, const Datum *values, const bool *isnull)
 {
 	tuplesort_putindextuplevalues(btspool->sortstate, btspool->index,
 								  self, values, isnull);

src/backend/access/spgist/spgdoinsert.c

@@ -1908,7 +1908,7 @@ spgSplitNodeAction(Relation index, SpGistState *state,
  */
 bool
 spgdoinsert(Relation index, SpGistState *state,
-			const ItemPointerData *heapPtr, Datum *datums, bool *isnulls)
+			const ItemPointerData *heapPtr, const Datum *datums, const bool *isnulls)
 {
 	bool		result = true;
 	TupleDesc	leafDescriptor = state->leafTupDesc;

src/backend/access/spgist/spgtextproc.c

@@ -155,7 +155,7 @@ commonPrefix(const char *a, const char *b, int lena, int lenb)
  * On success, *i gets the match location; on failure, it gets where to insert
  */
 static bool
-searchChar(Datum *nodeLabels, int nNodes, int16 c, int *i)
+searchChar(const Datum *nodeLabels, int nNodes, int16 c, int *i)
 {
 	int			StopLow = 0,
 				StopHigh = nNodes;