tdb2: make head of free list point to last entry.

This allows for access to the end of the list, in case we need it
later (eg. moving free list entries to the end of the list after
failed coalescing?).

ccan/tdb2/check.c

@@ -492,7 +492,7 @@ static enum TDB_ERROR check_free(struct tdb_context *tdb,
 				  (long long)off,
 				  bucket, size_to_bucket(frec_len(frec)));
 	}
-	if (prev != frec_prev(frec)) {
+	if (prev && prev != frec_prev(frec)) {
 		return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
 				  "tdb_check: offset %llu bad prev"
 				  " (%llu vs %llu)",
@@ -528,11 +528,13 @@ static enum TDB_ERROR check_free_table(struct tdb_context *tdb,
 	}
 
 	for (i = 0; i < TDB_FREE_BUCKETS; i++) {
-		tdb_off_t off, prev = 0, *p;
+		tdb_off_t off, prev = 0, *p, first = 0;
 		struct tdb_free_record f;
 
 		h = bucket_off(ftable_off, i);
 		for (off = tdb_read_off(tdb, h); off; off = f.next) {
+			if (!first)
+				first = off;
 			if (TDB_OFF_IS_ERR(off)) {
 				return off;
 			}
@@ -559,6 +561,18 @@ static enum TDB_ERROR check_free_table(struct tdb_context *tdb,
 			(*num_found)++;
 			prev = off;
 		}
+
+		if (first) {
+			/* Now we can check first back pointer. */
+			ecode = tdb_read_convert(tdb, first, &f, sizeof(f));
+			if (ecode != TDB_SUCCESS) {
+				return ecode;
+			}
+			ecode = check_free(tdb, first, &f, prev, ftable_num, i);
+			if (ecode != TDB_SUCCESS) {
+				return ecode;
+			}
+		}
 	}
 	return TDB_SUCCESS;
 }

ccan/tdb2/free.c

@@ -103,55 +103,96 @@ static tdb_off_t find_free_head(struct tdb_context *tdb,
 				    bucket, TDB_FREE_BUCKETS);
 }
 
+static void check_list(struct tdb_context *tdb, tdb_off_t b_off)
+{
+#ifdef CCAN_TDB2_DEBUG
+	tdb_off_t off, prev = 0, first;
+	struct tdb_free_record r;
+
+	first = off = tdb_read_off(tdb, b_off);
+	while (off != 0) {
+		tdb_read_convert(tdb, off, &r, sizeof(r));
+		if (frec_magic(&r) != TDB_FREE_MAGIC)
+			abort();
+		if (prev && frec_prev(&r) != prev)
+			abort();
+		prev = off;
+		off = r.next;
+	}
+
+	if (first) {
+		tdb_read_convert(tdb, first, &r, sizeof(r));
+		if (frec_prev(&r) != prev)
+			abort();
+	}
+#endif
+}
+
 /* Remove from free bucket. */
 static enum TDB_ERROR remove_from_list(struct tdb_context *tdb,
 				       tdb_off_t b_off, tdb_off_t r_off,
 				       const struct tdb_free_record *r)
 {
-	tdb_off_t off;
+	tdb_off_t off, prev_next, head;
 	enum TDB_ERROR ecode;
 
-	/* Front of list? */
-	if (frec_prev(r) == 0) {
-		off = b_off;
-	} else {
-		off = frec_prev(r) + offsetof(struct tdb_free_record, next);
-	}
+	/* Is this only element in list?  Zero out bucket, and we're done. */
+	if (frec_prev(r) == r_off)
+		return tdb_write_off(tdb, b_off, 0);
 
-#ifdef CCAN_TDB2_DEBUG
-	if (tdb_read_off(tdb, off) != r_off) {
-		return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
-				  "remove_from_list:"
-				  " %llu bad prev in list %llu",
-				  (long long)r_off, (long long)b_off);
-	}
-#endif
-
-	/* r->prev->next = r->next */
-	ecode = tdb_write_off(tdb, off, r->next);
-	if (ecode != TDB_SUCCESS) {
-		return ecode;
-	}
+	/* off = &r->prev->next */
+	off = frec_prev(r) + offsetof(struct tdb_free_record, next);
 
-	if (r->next != 0) {
-		off = r->next + offsetof(struct tdb_free_record,magic_and_prev);
-		/* r->next->prev = r->prev */
+	/* Get prev->next */
+	prev_next = tdb_read_off(tdb, off);
+	if (TDB_OFF_IS_ERR(prev_next))
+		return prev_next;
 
+	/* If prev->next == 0, we were head: update bucket to point to next. */
+	if (prev_next == 0) {
 #ifdef CCAN_TDB2_DEBUG
-		if (tdb_read_off(tdb, off) & TDB_OFF_MASK != r_off) {
+		if (tdb_read_off(tdb, b_off) != r_off) {
 			return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
 					  "remove_from_list:"
-					  " %llu bad list %llu",
-					  (long long)r_off, (long long)b_off);
+					  " %llu head %llu on list %llu",
+					  (long long)r_off,
+					  (long long)tdb_read_off(tdb, b_off),
+					  (long long)b_off);
 		}
 #endif
-
-		ecode = tdb_write_off(tdb, off, r->magic_and_prev);
-		if (ecode != TDB_SUCCESS) {
+		ecode = tdb_write_off(tdb, b_off, r->next);
+		if (ecode != TDB_SUCCESS)
+			return ecode;
+	} else {
+		/* r->prev->next = r->next */
+		ecode = tdb_write_off(tdb, off, r->next);
+		if (ecode != TDB_SUCCESS)
 			return ecode;
-		}
 	}
-	return TDB_SUCCESS;
+
+	/* If we were the tail, off = &head->prev. */
+	if (r->next == 0) {
+		head = tdb_read_off(tdb, b_off);
+		if (TDB_OFF_IS_ERR(head))
+			return head;
+		off = head + offsetof(struct tdb_free_record, magic_and_prev);
+	} else {
+		/* off = &r->next->prev */
+		off = r->next + offsetof(struct tdb_free_record,
+					 magic_and_prev);
+	}
+
+#ifdef CCAN_TDB2_DEBUG
+	/* *off == r */
+	if ((tdb_read_off(tdb, off) & TDB_OFF_MASK) != r_off) {
+		return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
+				  "remove_from_list:"
+				  " %llu bad prev in list %llu",
+				  (long long)r_off, (long long)b_off);
+	}
+#endif
+	/* r->next->prev = r->prev */
+	return tdb_write_off(tdb, off, r->magic_and_prev);
 }
 
 /* Enqueue in this free bucket. */
@@ -162,13 +203,12 @@ static enum TDB_ERROR enqueue_in_free(struct tdb_context *tdb,
 {
 	struct tdb_free_record new;
 	enum TDB_ERROR ecode;
+	tdb_off_t prev;
 	uint64_t magic = (TDB_FREE_MAGIC << (64 - TDB_OFF_UPPER_STEAL));
 
 	/* We only need to set ftable_and_len; rest is set in enqueue_in_free */
 	new.ftable_and_len = ((uint64_t)tdb->ftable << (64 - TDB_OFF_UPPER_STEAL))
 		| len;
-	/* prev = 0. */
-	new.magic_and_prev = magic;
 
 	/* new->next = head. */
 	new.next = tdb_read_off(tdb, b_off);
@@ -176,19 +216,22 @@ static enum TDB_ERROR enqueue_in_free(struct tdb_context *tdb,
 		return new.next;
 	}
 
-	if (new.next) {
-#ifdef CCAN_TDB2_DEBUG
-		if (tdb_read_off(tdb,
-				 new.next + offsetof(struct tdb_free_record,
-						     magic_and_prev))
-		    != magic) {
+	/* First element?  Prev points to ourselves. */
+	if (!new.next) {
+		new.magic_and_prev = (magic | off);
+	} else {
+		/* new->prev = next->prev */
+		prev = tdb_read_off(tdb,
+				    new.next + offsetof(struct tdb_free_record,
+							magic_and_prev));
+		new.magic_and_prev = prev;
+		if (frec_magic(&new) != TDB_FREE_MAGIC) {
 			return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
 					  "enqueue_in_free: %llu bad head"
 					  " prev %llu",
 					  (long long)new.next,
-					  (long long)b_off);
+					  (long long)prev);
 		}
-#endif
 		/* next->prev = new. */
 		ecode = tdb_write_off(tdb, new.next
 				      + offsetof(struct tdb_free_record,
@@ -197,6 +240,20 @@ static enum TDB_ERROR enqueue_in_free(struct tdb_context *tdb,
 		if (ecode != TDB_SUCCESS) {
 			return ecode;
 		}
+
+#ifdef CCAN_TDB2_DEBUG
+		prev = tdb_read_off(tdb, frec_prev(&new)
+				    + offsetof(struct tdb_free_record, next));
+		if (prev != 0) {
+			return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
+					  "enqueue_in_free:"
+					  " %llu bad tail next ptr %llu",
+					  (long long)frec_prev(&new)
+					  + offsetof(struct tdb_free_record,
+						     next),
+					  (long long)prev);
+		}
+#endif
 	}
 	/* head = new */
 	ecode = tdb_write_off(tdb, b_off, off);
@@ -226,6 +283,7 @@ enum TDB_ERROR add_free_record(struct tdb_context *tdb,
 	}
 
 	ecode = enqueue_in_free(tdb, b_off, off, len);
+	check_list(tdb, b_off);
 	tdb_unlock_free_bucket(tdb, b_off);
 	return ecode;
 }
@@ -343,6 +401,7 @@ static tdb_bool_err coalesce(struct tdb_context *tdb,
 		}
 
 		ecode = remove_from_list(tdb, nb_off, end, &rec);
+		check_list(tdb, nb_off);
 		if (ecode != TDB_SUCCESS) {
 			tdb_unlock_free_bucket(tdb, nb_off);
 			goto err;
@@ -371,6 +430,7 @@ static tdb_bool_err coalesce(struct tdb_context *tdb,
 	}
 
 	ecode = remove_from_list(tdb, b_off, off, &rec);
+	check_list(tdb, b_off);
 	if (ecode != TDB_SUCCESS) {
 		goto err;
 	}
@@ -502,6 +562,7 @@ again:
 
 		/* We're happy with this size: take it. */
 		ecode = remove_from_list(tdb, b_off, best_off, &best);
+		check_list(tdb, b_off);
 		if (ecode != TDB_SUCCESS) {
 			goto unlock_err;
 		}