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/include/access/hash.h

@@ -468,7 +468,7 @@ extern uint32 _hash_get_totalbuckets(uint32 splitpoint_phase);
 extern void _hash_checkpage(Relation rel, Buffer buf, int flags);
 extern uint32 _hash_get_indextuple_hashkey(IndexTuple itup);
 extern 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);
 extern OffsetNumber _hash_binsearch(Page page, uint32 hash_value);
 extern OffsetNumber _hash_binsearch_last(Page page, uint32 hash_value);

src/include/access/heaptoast.h

@@ -133,8 +133,8 @@ extern Datum toast_flatten_tuple_to_datum(HeapTupleHeader tup,
  * ----------
  */
 extern HeapTuple toast_build_flattened_tuple(TupleDesc tupleDesc,
-											 Datum *values,
-											 bool *isnull);
+											 const Datum *values,
+											 const bool *isnull);
 
 /* ----------
  * heap_fetch_toast_slice

src/include/access/spgist_private.h

@@ -541,7 +541,7 @@ extern void spgPageIndexMultiDelete(SpGistState *state, Page page,
 									int firststate, int reststate,
 									BlockNumber blkno, OffsetNumber offnum);
 extern bool spgdoinsert(Relation index, SpGistState *state,
-						const ItemPointerData *heapPtr, Datum *datums, bool *isnulls);
+						const ItemPointerData *heapPtr, const Datum *datums, const bool *isnulls);
 
 /* spgproc.c */
 extern double *spg_key_orderbys_distances(Datum key, bool isLeaf,

src/include/executor/spi.h

@@ -111,7 +111,7 @@ extern int	SPI_finish(void);
 extern int	SPI_execute(const char *src, bool read_only, long tcount);
 extern int	SPI_execute_extended(const char *src,
 								 const SPIExecuteOptions *options);
-extern int	SPI_execute_plan(SPIPlanPtr plan, Datum *Values, const char *Nulls,
+extern int	SPI_execute_plan(SPIPlanPtr plan, const Datum *Values, const char *Nulls,
 							 bool read_only, long tcount);
 extern int	SPI_execute_plan_extended(SPIPlanPtr plan,
 									  const SPIExecuteOptions *options);
@@ -122,13 +122,13 @@ extern int	SPI_exec(const char *src, long tcount);
 extern int	SPI_execp(SPIPlanPtr plan, Datum *Values, const char *Nulls,
 					  long tcount);
 extern int	SPI_execute_snapshot(SPIPlanPtr plan,
-								 Datum *Values, const char *Nulls,
+								 const Datum *Values, const char *Nulls,
 								 Snapshot snapshot,
 								 Snapshot crosscheck_snapshot,
 								 bool read_only, bool fire_triggers, long tcount);
 extern int	SPI_execute_with_args(const char *src,
 								  int nargs, Oid *argtypes,
-								  Datum *Values, const char *Nulls,
+								  const Datum *Values, const char *Nulls,
 								  bool read_only, long tcount);
 extern SPIPlanPtr SPI_prepare(const char *src, int nargs, Oid *argtypes);
 extern SPIPlanPtr SPI_prepare_cursor(const char *src, int nargs, Oid *argtypes,
@@ -172,7 +172,7 @@ extern void SPI_freetuple(HeapTuple tuple);
 extern void SPI_freetuptable(SPITupleTable *tuptable);
 
 extern Portal SPI_cursor_open(const char *name, SPIPlanPtr plan,
-							  Datum *Values, const char *Nulls, bool read_only);
+							  const Datum *Values, const char *Nulls, bool read_only);
 extern Portal SPI_cursor_open_with_args(const char *name,
 										const char *src,
 										int nargs, Oid *argtypes,

src/include/partitioning/partbounds.h

@@ -130,8 +130,8 @@ extern void check_default_partition_contents(Relation parent,
 
 extern int32 partition_rbound_datum_cmp(FmgrInfo *partsupfunc,
 										Oid *partcollation,
-										Datum *rb_datums, PartitionRangeDatumKind *rb_kind,
-										Datum *tuple_datums, int n_tuple_datums);
+										const Datum *rb_datums, PartitionRangeDatumKind *rb_kind,
+										const Datum *tuple_datums, int n_tuple_datums);
 extern int	partition_list_bsearch(FmgrInfo *partsupfunc,
 								   Oid *partcollation,
 								   PartitionBoundInfo boundinfo,
@@ -139,7 +139,7 @@ extern int	partition_list_bsearch(FmgrInfo *partsupfunc,
 extern int	partition_range_datum_bsearch(FmgrInfo *partsupfunc,
 										  Oid *partcollation,
 										  PartitionBoundInfo boundinfo,
-										  int nvalues, Datum *values, bool *is_equal);
+										  int nvalues, const Datum *values, bool *is_equal);
 extern int	partition_hash_bsearch(PartitionBoundInfo boundinfo,
 								   int modulus, int remainder);
 

src/include/utils/array.h

@@ -352,8 +352,8 @@ extern PGDLLIMPORT bool Array_nulls;
  * prototypes for functions defined in arrayfuncs.c
  */
 extern void CopyArrayEls(ArrayType *array,
-						 Datum *values,
-						 bool *nulls,
+						 const Datum *values,
+						 const bool *nulls,
 						 int nitems,
 						 int typlen,
 						 bool typbyval,
@@ -405,14 +405,14 @@ extern ArrayType *construct_empty_array(Oid elmtype);
 extern ExpandedArrayHeader *construct_empty_expanded_array(Oid element_type,
 														   MemoryContext parentcontext,
 														   ArrayMetaState *metacache);
-extern void deconstruct_array(ArrayType *array,
+extern void deconstruct_array(const ArrayType *array,
 							  Oid elmtype,
 							  int elmlen, bool elmbyval, char elmalign,
 							  Datum **elemsp, bool **nullsp, int *nelemsp);
-extern void deconstruct_array_builtin(ArrayType *array,
+extern void deconstruct_array_builtin(const ArrayType *array,
 									  Oid elmtype,
 									  Datum **elemsp, bool **nullsp, int *nelemsp);
-extern bool array_contains_nulls(ArrayType *array);
+extern bool array_contains_nulls(const ArrayType *array);
 
 extern ArrayBuildState *initArrayResult(Oid element_type,
 										MemoryContext rcontext, bool subcontext);

src/include/utils/jsonb.h

@@ -426,9 +426,9 @@ extern char *JsonbUnquote(Jsonb *jb);
 extern bool JsonbExtractScalar(JsonbContainer *jbc, JsonbValue *res);
 extern const char *JsonbTypeName(JsonbValue *val);
 
-extern Datum jsonb_set_element(Jsonb *jb, Datum *path, int path_len,
+extern Datum jsonb_set_element(Jsonb *jb, const Datum *path, int path_len,
 							   JsonbValue *newval);
-extern Datum jsonb_get_element(Jsonb *jb, Datum *path, int npath,
+extern Datum jsonb_get_element(Jsonb *jb, const Datum *path, int npath,
 							   bool *isnull, bool as_text);
 extern bool to_jsonb_is_immutable(Oid typoid);
 extern Datum jsonb_build_object_worker(int nargs, const Datum *args, const bool *nulls,

src/include/utils/xml.h

@@ -71,8 +71,8 @@ extern void xml_ereport(PgXmlErrorContext *errcxt, int level, int sqlcode,
 
 extern xmltype *xmlconcat(List *args);
 extern xmltype *xmlelement(XmlExpr *xexpr,
-						   Datum *named_argvalue, bool *named_argnull,
-						   Datum *argvalue, bool *argnull);
+						   const Datum *named_argvalue, const bool *named_argnull,
+						   const Datum *argvalue, const bool *argnull);
 extern xmltype *xmlparse(text *data, XmlOptionType xmloption_arg, bool preserve_whitespace);
 extern xmltype *xmlpi(const char *target, text *arg, bool arg_is_null, bool *result_is_null);
 extern xmltype *xmlroot(xmltype *data, text *version, int standalone);