tdb: tdb_repack() only when it's worthwhile.

tdb_repack() is expensive and consumes memory, so we can spend some
effort to see if it's worthwhile.  In particular, tdbbackup doesn't
need to repack: it started with an empty database!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 31 insertions, 6 deletions

diff --git a/lib/tdb/common/transaction.c b/lib/tdb/common/transaction.c
index 4bc8044219..e110a08dbc 100644
--- a/lib/tdb/common/transaction.c
+++ b/lib/tdb/common/transaction.c
@@ -138,8 +138,8 @@ struct tdb_transaction {
 	/* old file size before transaction */
 	tdb_len_t old_map_size;
 
-	/* we should re-pack on commit */
-	bool need_repack;
+	/* did we expand in this transaction */
+	bool expanded;
 };
 
 
@@ -403,7 +403,7 @@ static int transaction_expand_file(struct tdb_context *tdb, tdb_off_t size,
 		return -1;
 	}
 
-	tdb->transaction->need_repack = true;
+	tdb->transaction->expanded = true;
 
 	return 0;
 }
@@ -983,6 +983,28 @@ _PUBLIC_ int tdb_transaction_prepare_commit(struct tdb_context *tdb)
 	return _tdb_transaction_prepare_commit(tdb);
 }
 
+/* A repack is worthwhile if the largest is less than half total free. */
+static bool repack_worthwhile(struct tdb_context *tdb)
+{
+	tdb_off_t ptr;
+	struct tdb_record rec;
+	tdb_len_t total = 0, largest = 0;
+
+	if (tdb_ofs_read(tdb, FREELIST_TOP, &ptr) == -1) {
+		return false;
+	}
+
+	while (ptr != 0 && tdb_rec_free_read(tdb, ptr, &rec) == 0) {
+		total += rec.rec_len;
+		if (rec.rec_len > largest) {
+			largest = rec.rec_len;
+		}
+		ptr = rec.next;
+	}
+
+	return total > largest * 2;
+}
+
 /*
   commit the current transaction
 */
@@ -990,7 +1012,7 @@ _PUBLIC_ int tdb_transaction_commit(struct tdb_context *tdb)
 {
 	const struct tdb_methods *methods;
 	int i;
-	bool need_repack;
+	bool need_repack = false;
 
 	if (tdb->transaction == NULL) {
 		TDB_LOG((tdb, TDB_DEBUG_ERROR, "tdb_transaction_commit: no transaction\n"));
@@ -1058,6 +1080,11 @@ _PUBLIC_ int tdb_transaction_commit(struct tdb_context *tdb)
 		SAFE_FREE(tdb->transaction->blocks[i]);
 	} 
 
+	/* Do this before we drop lock or blocks. */
+	if (tdb->transaction->expanded) {
+		need_repack = repack_worthwhile(tdb);
+	}
+
 	SAFE_FREE(tdb->transaction->blocks);
 	tdb->transaction->num_blocks = 0;
 
@@ -1081,8 +1108,6 @@ _PUBLIC_ int tdb_transaction_commit(struct tdb_context *tdb)
 	utime(tdb->name, NULL);
 #endif
 
-	need_repack = tdb->transaction->need_repack;
-
 	/* use a transaction cancel to free memory and remove the
 	   transaction locks */
 	_tdb_transaction_cancel(tdb);