htable: restore perfect bit when resizing.

If the lower bit differs between pointers, we can lose our "perfect"
bit.  Since we're rehashing the entire thing anyway, we can pick
another bit when we double the hash table.

ccan/htable/htable.c

@@ -2,6 +2,7 @@
 #include <ccan/compiler/compiler.h>
 #include <stdint.h>
 #include <stdlib.h>
+#include <limits.h>
 #include <stdbool.h>
 #include <assert.h>
 
@@ -170,7 +171,16 @@ static COLD_ATTRIBUTE bool double_table(struct htable *ht)
 	ht->max *= 2;
 	ht->max_with_deleted *= 2;
 
-	/* FIXME: If we lost our perfect bit, we could reclaim it here! */
+	/* If we lost our "perfect bit", get it back now. */
+	if (!ht->perfect_bit && ht->common_mask) {
+		for (i = 0; i < sizeof(ht->common_mask) * CHAR_BIT; i++) {
+			if (ht->common_mask & ((size_t)1 << i)) {
+				ht->perfect_bit = (size_t)1 << i;
+				break;
+			}
+		}
+	}
+
 	for (i = 0; i < oldnum; i++) {
 		if (entry_is_valid(e = oldtable[i])) {
 			void *p = get_raw_ptr(ht, e);

ccan/htable/test/run.c

@@ -96,13 +96,14 @@ static bool check_mask(struct htable *ht, uint64_t val[], unsigned num)
 int main(int argc, char *argv[])
 {
 	unsigned int i;
+	uintptr_t perfect_bit;
 	struct htable *ht;
 	uint64_t val[NUM_VALS];
 	uint64_t dne;
 	void *p;
 	struct htable_iter iter;
 
-	plan_tests(19);
+	plan_tests(23);
 	for (i = 0; i < NUM_VALS; i++)
 		val[i] = i;
 	dne = i;
@@ -143,6 +144,8 @@ int main(int argc, char *argv[])
 	htable_add(ht, hash(&val[NUM_VALS-1], NULL), &val[NUM_VALS-1]);
 	ok1(ht->common_mask == 0);
 	ok1(ht->common_bits == 0);
+	/* Get rid of bogus pointer before we trip over it! */
+	htable_del(ht, 0, (void *)~(uintptr_t)&val[NUM_VALS-1]);
 
 	/* Add the rest. */
 	add_vals(ht, val, NUM_VALS-1);
@@ -151,5 +154,23 @@ int main(int argc, char *argv[])
 	find_vals(ht, val, NUM_VALS);
 	ok1(!htable_get(ht, hash(&dne, NULL), objcmp, &dne));
 
+	htable_free(ht);
+
+	/* Corner cases: wipe out the perfect bit using bogus pointer. */
+	ht = htable_new(hash, NULL);
+	htable_add(ht, 0, (void *)((uintptr_t)&val[NUM_VALS-1]));
+	ok1(ht->perfect_bit);
+	perfect_bit = ht->perfect_bit;
+	htable_add(ht, 0, (void *)((uintptr_t)&val[NUM_VALS-1]
+				   | perfect_bit));
+	ok1(ht->perfect_bit == 0);
+	htable_del(ht, 0, (void *)((uintptr_t)&val[NUM_VALS-1] | perfect_bit));
+
+	/* Enlarging should restore it... */
+	add_vals(ht, val, NUM_VALS-1);
+
+	ok1(ht->perfect_bit != 0);
+	htable_free(ht);
+
 	return exit_status();
 }