[PATCH v2 1/3] Don't cast away const where possible

Add const to read only local variables, preserving the const qualifiers from the
function signatures.

This does not change all such instances, but only those hand-picked by the author.

The ones that are not changed:

- are just thin wrappers
- would require public API changes
- rely on external functions (such as LZ4F_compressUpdate())
- would require complex subsystem changes

Discussion: https://postgr.es/m/aUQHy/MmWq7c97wK%40ip-10-97-1-34.eu-west-3.compute.internal
---
 src/backend/access/brin/brin_minmax_multi.c          | 4 ++--
 src/backend/access/heap/pruneheap.c                  | 4 ++--
 src/backend/access/spgist/spgkdtreeproc.c            | 8 ++++----
 src/backend/statistics/mcv.c                         | 4 ++--
 src/backend/tsearch/spell.c                          | 4 ++--
 src/test/modules/injection_points/injection_points.c | 8 ++++----
 6 files changed, 16 insertions(+), 16 deletions(-)
  10.2% src/backend/access/brin/
  12.5% src/backend/access/heap/
  18.7% src/backend/access/spgist/
   8.3% src/backend/statistics/
  11.6% src/backend/tsearch/
  38.4% src/test/modules/injection_points/

diff --git a/src/backend/access/brin/brin_minmax_multi.c b/src/backend/access/brin/brin_minmax_multi.c
index 0298a9da8ba..0e87ce759f1 100644
--- a/src/backend/access/brin/brin_minmax_multi.c
+++ b/src/backend/access/brin/brin_minmax_multi.c
@@ -1304,8 +1304,8 @@ merge_overlapping_ranges(FmgrInfo *cmp, Oid colloid,
 static int
 compare_distances(const void *a, const void *b)
 {
-	DistanceValue *da = (DistanceValue *) a;
-	DistanceValue *db = (DistanceValue *) b;
+	const DistanceValue *da = a;
+	const DistanceValue *db = b;
 
 	if (da->value < db->value)
 		return 1;
diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index 07aa08cfe14..92aab1d6723 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -2021,8 +2021,8 @@ heap_log_freeze_eq(xlhp_freeze_plan *plan, HeapTupleFreeze *frz)
 static int
 heap_log_freeze_cmp(const void *arg1, const void *arg2)
 {
-	HeapTupleFreeze *frz1 = (HeapTupleFreeze *) arg1;
-	HeapTupleFreeze *frz2 = (HeapTupleFreeze *) arg2;
+	const HeapTupleFreeze *frz1 = arg1;
+	const HeapTupleFreeze *frz2 = arg2;
 
 	if (frz1->xmax < frz2->xmax)
 		return -1;
diff --git a/src/backend/access/spgist/spgkdtreeproc.c b/src/backend/access/spgist/spgkdtreeproc.c
index f0167d6ffa6..c02de835847 100644
--- a/src/backend/access/spgist/spgkdtreeproc.c
+++ b/src/backend/access/spgist/spgkdtreeproc.c
@@ -84,8 +84,8 @@ typedef struct SortedPoint
 static int
 x_cmp(const void *a, const void *b)
 {
-	SortedPoint *pa = (SortedPoint *) a;
-	SortedPoint *pb = (SortedPoint *) b;
+	const SortedPoint *pa = a;
+	const SortedPoint *pb = b;
 
 	if (pa->p->x == pb->p->x)
 		return 0;
@@ -95,8 +95,8 @@ x_cmp(const void *a, const void *b)
 static int
 y_cmp(const void *a, const void *b)
 {
-	SortedPoint *pa = (SortedPoint *) a;
-	SortedPoint *pb = (SortedPoint *) b;
+	const SortedPoint *pa = a;
+	const SortedPoint *pb = b;
 
 	if (pa->p->y == pb->p->y)
 		return 0;
diff --git a/src/backend/statistics/mcv.c b/src/backend/statistics/mcv.c
index ec650ba029f..dc577b9a1e4 100644
--- a/src/backend/statistics/mcv.c
+++ b/src/backend/statistics/mcv.c
@@ -402,8 +402,8 @@ count_distinct_groups(int numrows, SortItem *items, MultiSortSupport mss)
 static int
 compare_sort_item_count(const void *a, const void *b, void *arg)
 {
-	SortItem   *ia = (SortItem *) a;
-	SortItem   *ib = (SortItem *) b;
+	const SortItem *ia = a;
+	const SortItem *ib = b;
 
 	if (ia->count == ib->count)
 		return 0;
diff --git a/src/backend/tsearch/spell.c b/src/backend/tsearch/spell.c
index e5badb6b43f..3b0abfd9c35 100644
--- a/src/backend/tsearch/spell.c
+++ b/src/backend/tsearch/spell.c
@@ -210,8 +210,8 @@ cmpspellaffix(const void *s1, const void *s2)
 static int
 cmpcmdflag(const void *f1, const void *f2)
 {
-	CompoundAffixFlag *fv1 = (CompoundAffixFlag *) f1,
-			   *fv2 = (CompoundAffixFlag *) f2;
+	const CompoundAffixFlag *fv1 = f1;
+	const CompoundAffixFlag *fv2 = f2;
 
 	Assert(fv1->flagMode == fv2->flagMode);
 
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index 25340e8d81b..12ea81a030c 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -189,7 +189,7 @@ injection_init_shmem(void)
  * otherwise.
  */
 static bool
-injection_point_allowed(InjectionPointCondition *condition)
+injection_point_allowed(const InjectionPointCondition *condition)
 {
 	bool		result = true;
 
@@ -232,7 +232,7 @@ injection_points_cleanup(int code, Datum arg)
 void
 injection_error(const char *name, const void *private_data, void *arg)
 {
-	InjectionPointCondition *condition = (InjectionPointCondition *) private_data;
+	const InjectionPointCondition *condition = private_data;
 	char	   *argstr = (char *) arg;
 
 	if (!injection_point_allowed(condition))
@@ -248,7 +248,7 @@ injection_error(const char *name, const void *private_data, void *arg)
 void
 injection_notice(const char *name, const void *private_data, void *arg)
 {
-	InjectionPointCondition *condition = (InjectionPointCondition *) private_data;
+	const InjectionPointCondition *condition = private_data;
 	char	   *argstr = (char *) arg;
 
 	if (!injection_point_allowed(condition))
@@ -268,7 +268,7 @@ injection_wait(const char *name, const void *private_data, void *arg)
 	uint32		old_wait_counts = 0;
 	int			index = -1;
 	uint32		injection_wait_event = 0;
-	InjectionPointCondition *condition = (InjectionPointCondition *) private_data;
+	const InjectionPointCondition *condition = private_data;
 
 	if (inj_state == NULL)
 		injection_init_shmem();
-- 
2.34.1


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