r21722: Add the dead record functionality presented on samba-technical@samba.org. If

you do a tdb_set_max_dead(tdb, n), then for this tdb a delete operation will
only mark a record as dead and re-use it if a new record is created. The
parameter n allows for at most n dead records per hash chain. If this number
is exceeded, all dead records are put on the central freelist.

Volker
(This used to be commit 98a27ab28a3cd554e370a9a0e3652f4dea8749e9)

3 files changed, 186 insertions, 6 deletions

diff --git a/source4/lib/tdb/common/open.c b/source4/lib/tdb/common/open.c
index 3d6e222b96..47e3258c09 100644
--- a/source4/lib/tdb/common/open.c
+++ b/source4/lib/tdb/common/open.c
@@ -263,6 +263,7 @@ struct tdb_context *tdb_open_ex(const char *name, int hash_size, int tdb_flags,
 	tdb->map_size = st.st_size;
 	tdb->device = st.st_dev;
 	tdb->inode = st.st_ino;
+	tdb->max_dead_records = 0;
 	tdb_mmap(tdb);
 	if (locked) {
 		if (tdb->methods->tdb_brlock(tdb, ACTIVE_LOCK, F_UNLCK, F_SETLK, 0, 1) == -1) {
@@ -321,6 +322,15 @@ struct tdb_context *tdb_open_ex(const char *name, int hash_size, int tdb_flags,
 	}
 }
 
+/*
+ * Set the maximum number of dead records per hash chain
+ */
+
+void tdb_set_max_dead(struct tdb_context *tdb, int max_dead)
+{
+	tdb->max_dead_records = max_dead;
+}
+
 /**
  * Close a database.
  *
diff --git a/source4/lib/tdb/common/tdb.c b/source4/lib/tdb/common/tdb.c
index 03a66804b3..a6b472ae94 100644
--- a/source4/lib/tdb/common/tdb.c
+++ b/source4/lib/tdb/common/tdb.c
@@ -258,6 +258,66 @@ int tdb_do_delete(struct tdb_context *tdb, tdb_off_t rec_ptr, struct list_struct
 	return 0;
 }
 
+static int tdb_count_dead(struct tdb_context *tdb, u32 hash)
+{
+	int res = 0;
+	tdb_off_t rec_ptr;
+	struct list_struct rec;
+	
+	/* read in the hash top */
+	if (tdb_ofs_read(tdb, TDB_HASH_TOP(hash), &rec_ptr) == -1)
+		return 0;
+
+	while (rec_ptr) {
+		if (tdb_rec_read(tdb, rec_ptr, &rec) == -1)
+			return 0;
+
+		if (rec.magic == TDB_DEAD_MAGIC) {
+			res += 1;
+		}
+		rec_ptr = rec.next;
+	}
+	return res;
+}
+
+/*
+ * Purge all DEAD records from a hash chain
+ */
+static int tdb_purge_dead(struct tdb_context *tdb, u32 hash)
+{
+	int res = -1;
+	struct list_struct rec;
+	tdb_off_t rec_ptr;
+
+	if (tdb_lock(tdb, -1, F_WRLCK) == -1) {
+		return -1;
+	}
+	
+	/* read in the hash top */
+	if (tdb_ofs_read(tdb, TDB_HASH_TOP(hash), &rec_ptr) == -1)
+		goto fail;
+
+	while (rec_ptr) {
+		tdb_off_t next;
+
+		if (tdb_rec_read(tdb, rec_ptr, &rec) == -1) {
+			goto fail;
+		}
+
+		next = rec.next;
+
+		if (rec.magic == TDB_DEAD_MAGIC
+		    && tdb_do_delete(tdb, rec_ptr, &rec) == -1) {
+			goto fail;
+		}
+		rec_ptr = next;
+	}
+	res = 0;
+ fail:
+	tdb_unlock(tdb, -1, F_WRLCK);
+	return res;
+}
+
 /* delete an entry in the database given a key */
 static int tdb_delete_hash(struct tdb_context *tdb, TDB_DATA key, u32 hash)
 {
@@ -265,9 +325,42 @@ static int tdb_delete_hash(struct tdb_context *tdb, TDB_DATA key, u32 hash)
 	struct list_struct rec;
 	int ret;
 
-	if (!(rec_ptr = tdb_find_lock_hash(tdb, key, hash, F_WRLCK, &rec)))
-		return -1;
-	ret = tdb_do_delete(tdb, rec_ptr, &rec);
+	if (tdb->max_dead_records != 0) {
+
+		/*
+		 * Allow for some dead records per hash chain, mainly for
+		 * tdb's with a very high create/delete rate like locking.tdb.
+		 */
+
+		if (tdb_lock(tdb, BUCKET(hash), F_WRLCK) == -1)
+			return -1;
+
+		if (tdb_count_dead(tdb, hash) >= tdb->max_dead_records) {
+			/*
+			 * Don't let the per-chain freelist grow too large,
+			 * delete all existing dead records
+			 */
+			tdb_purge_dead(tdb, hash);
+		}
+
+		if (!(rec_ptr = tdb_find(tdb, key, hash, &rec))) {
+			tdb_unlock(tdb, BUCKET(hash), F_WRLCK);
+			return -1;
+		}
+
+		/*
+		 * Just mark the record as dead.
+		 */
+		rec.magic = TDB_DEAD_MAGIC;
+		ret = tdb_rec_write(tdb, rec_ptr, &rec);
+	}
+	else {
+		if (!(rec_ptr = tdb_find_lock_hash(tdb, key, hash, F_WRLCK,
+						   &rec)))
+			return -1;
+
+		ret = tdb_do_delete(tdb, rec_ptr, &rec);
+	}
 
 	if (ret == 0) {
 		tdb_increment_seqnum(tdb);
@@ -284,6 +377,35 @@ int tdb_delete(struct tdb_context *tdb, TDB_DATA key)
 	return tdb_delete_hash(tdb, key, hash);
 }
 
+/*
+ * See if we have a dead record around with enough space
+ */
+static tdb_off_t tdb_find_dead(struct tdb_context *tdb, u32 hash,
+			       struct list_struct *r, tdb_len_t length)
+{
+	tdb_off_t rec_ptr;
+	
+	/* read in the hash top */
+	if (tdb_ofs_read(tdb, TDB_HASH_TOP(hash), &rec_ptr) == -1)
+		return 0;
+
+	/* keep looking until we find the right record */
+	while (rec_ptr) {
+		if (tdb_rec_read(tdb, rec_ptr, r) == -1)
+			return 0;
+
+		if (TDB_DEAD(r) && r->rec_len >= length) {
+			/*
+			 * First fit for simple coding, TODO: change to best
+			 * fit
+			 */
+			return rec_ptr;
+		}
+		rec_ptr = r->next;
+	}
+	return 0;
+}
+
 /* store an element in the database, replacing any existing element
    with the same key 
 
@@ -316,8 +438,7 @@ int tdb_store(struct tdb_context *tdb, TDB_DATA key, TDB_DATA dbuf, int flag)
 	} else {
 		/* first try in-place update, on modify or replace. */
 		if (tdb_update_hash(tdb, key, hash, dbuf) == 0) {
-			ret = 0;
-			goto fail; /* Well, not really failed */
+			goto done;
 		}
 		if (tdb->ecode == TDB_ERR_NOEXIST &&
 		    flag == TDB_MODIFY) {
@@ -347,9 +468,56 @@ int tdb_store(struct tdb_context *tdb, TDB_DATA key, TDB_DATA dbuf, int flag)
 	if (dbuf.dsize)
 		memcpy(p+key.dsize, dbuf.dptr, dbuf.dsize);
 
+	if (tdb->max_dead_records != 0) {
+		/*
+		 * Allow for some dead records per hash chain, look if we can
+		 * find one that can hold the new record. We need enough space
+		 * for key, data and tailer. If we find one, we don't have to
+		 * consult the central freelist.
+		 */
+		rec_ptr = tdb_find_dead(
+			tdb, hash, &rec,
+			key.dsize + dbuf.dsize + sizeof(tdb_off_t));
+
+		if (rec_ptr != 0) {
+			rec.key_len = key.dsize;
+			rec.data_len = dbuf.dsize;
+			rec.full_hash = hash;
+			rec.magic = TDB_MAGIC;
+			if (tdb_rec_write(tdb, rec_ptr, &rec) == -1
+			    || tdb->methods->tdb_write(
+				    tdb, rec_ptr + sizeof(rec),
+				    p, key.dsize + dbuf.dsize) == -1) {
+				goto fail;
+			}
+			goto done;
+		}
+	}
+
+	/*
+	 * We have to allocate some space from the freelist, so this means we
+	 * have to lock it. Use the chance to purge all the DEAD records from
+	 * the hash chain under the freelist lock.
+	 */
+
+	if (tdb_lock(tdb, -1, F_WRLCK) == -1) {
+		goto fail;
+	}
+
+	if ((tdb->max_dead_records != 0)
+	    && (tdb_purge_dead(tdb, hash) == -1)) {
+		tdb_unlock(tdb, -1, F_WRLCK);
+		goto fail;
+	}
+
 	/* we have to allocate some space */
-	if (!(rec_ptr = tdb_allocate(tdb, key.dsize + dbuf.dsize, &rec)))
+	rec_ptr = tdb_allocate(tdb, key.dsize + dbuf.dsize, &rec);
+
+	tdb_unlock(tdb, -1, F_WRLCK);
+
+	if (rec_ptr == 0) {
 		goto fail;
+	}
 
 	/* Read hash top into next ptr */
 	if (tdb_ofs_read(tdb, TDB_HASH_TOP(hash), &rec.next) == -1)
@@ -368,6 +536,7 @@ int tdb_store(struct tdb_context *tdb, TDB_DATA key, TDB_DATA dbuf, int flag)
 		goto fail;
 	}
 
+ done:
 	ret = 0;
  fail:
 	if (ret == 0) {
diff --git a/source4/lib/tdb/common/tdb_private.h b/source4/lib/tdb/common/tdb_private.h
index a05f4da0cf..7219a1a2e2 100644
--- a/source4/lib/tdb/common/tdb_private.h
+++ b/source4/lib/tdb/common/tdb_private.h
@@ -169,6 +169,7 @@ struct tdb_context {
 	const struct tdb_methods *methods;
 	struct tdb_transaction *transaction;
 	int page_size;
+	int max_dead_records;
 };