t-reftable-pq: make merged_iter_pqueue_check() callable by reference

merged_iter_pqueue_check() checks the validity of a priority queue
represented by a merged_iter_pqueue struct by asserting the
parent-child relation in the struct's heap. Explicity passing a
struct to this function means a copy of the entire struct is created,
which is inefficient.

Make the function accept a pointer to the struct instead. This is
safe to do since the function doesn't modify the struct in any way.
Make the function parameter 'const' to assert immutability.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>

Author: Chandra Pratap

t/unit-tests/t-reftable-pq.c

@@ -10,11 +10,11 @@ license that can be found in the LICENSE file or at
 #include "reftable/constants.h"
 #include "reftable/pq.h"
 
-static void merged_iter_pqueue_check(struct merged_iter_pqueue pq)
+static void merged_iter_pqueue_check(const struct merged_iter_pqueue *pq)
 {
-	for (size_t i = 1; i < pq.len; i++) {
+	for (size_t i = 1; i < pq->len; i++) {
 		size_t parent = (i - 1) / 2;
-		check(pq_less(&pq.heap[parent], &pq.heap[i]));
+		check(pq_less(&pq->heap[parent], &pq->heap[i]));
 	}
 }
 
@@ -40,13 +40,13 @@ static void t_pq(void)
 		};
 
 		merged_iter_pqueue_add(&pq, &e);
-		merged_iter_pqueue_check(pq);
+		merged_iter_pqueue_check(&pq);
 		i = (i * 7) % N;
 	} while (i != 1);
 
 	while (!merged_iter_pqueue_is_empty(pq)) {
 		struct pq_entry e = merged_iter_pqueue_remove(&pq);
-		merged_iter_pqueue_check(pq);
+		merged_iter_pqueue_check(&pq);
 
 		check(reftable_record_type(e.rec) == BLOCK_TYPE_REF);
 		if (last)