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authorJeff King <peff@peff.net>2014-10-15 22:41:35 (GMT)
committerJunio C Hamano <gitster@pobox.com>2014-10-16 17:10:42 (GMT)
commitd3038d22f91aad9620bd8e6fc43fc67c16219738 (patch)
tree1f078433287962db85aa5cd8d61003037e989d02 /reachable.c
parent660c889e46d185dc98ba78963528826728b0a55d (diff)
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prune: keep objects reachable from recent objects
Our current strategy with prune is that an object falls into one of three categories: 1. Reachable (from ref tips, reflogs, index, etc). 2. Not reachable, but recent (based on the --expire time). 3. Not reachable and not recent. We keep objects from (1) and (2), but prune objects in (3). The point of (2) is that these objects may be part of an in-progress operation that has not yet updated any refs. However, it is not always the case that objects for an in-progress operation will have a recent mtime. For example, the object database may have an old copy of a blob (from an abandoned operation, a branch that was deleted, etc). If we create a new tree that points to it, a simultaneous prune will leave our tree, but delete the blob. Referencing that tree with a commit will then work (we check that the tree is in the object database, but not that all of its referred objects are), as will mentioning the commit in a ref. But the resulting repo is corrupt; we are missing the blob reachable from a ref. One way to solve this is to be more thorough when referencing a sha1: make sure that not only do we have that sha1, but that we have objects it refers to, and so forth recursively. The problem is that this is very expensive. Creating a parent link would require traversing the entire object graph! Instead, this patch pushes the extra work onto prune, which runs less frequently (and has to look at the whole object graph anyway). It creates a new category of objects: objects which are not recent, but which are reachable from a recent object. We do not prune these objects, just like the reachable and recent ones. This lets us avoid the recursive check above, because if we have an object, even if it is unreachable, we should have its referent. We can make a simple inductive argument that with this patch, this property holds (that there are no objects with missing referents in the repository): 0. When we have no objects, we have nothing to refer or be referred to, so the property holds. 1. If we add objects to the repository, their direct referents must generally exist (e.g., if you create a tree, the blobs it references must exist; if you create a commit to point at the tree, the tree must exist). This is already the case before this patch. And it is not 100% foolproof (you can make bogus objects using `git hash-object`, for example), but it should be the case for normal usage. Therefore for any sequence of object additions, the property will continue to hold. 2. If we remove objects from the repository, then we will not remove a child object (like a blob) if an object that refers to it is being kept. That is the part implemented by this patch. Note, however, that our reachability check and the actual pruning are not atomic. So it _is_ still possible to violate the property (e.g., an object becomes referenced just as we are deleting it). This patch is shooting for eliminating problems where the mtimes of dependent objects differ by hours or days, and one is dropped without the other. It does nothing to help with short races. Naively, the simplest way to implement this would be to add all recent objects as tips to the reachability traversal. However, this does not perform well. In a recently-packed repository, all reachable objects will also be recent, and therefore we have to look at each object twice. This patch instead performs the reachability traversal, then follows up with a second traversal for recent objects, skipping any that have already been marked. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'reachable.c')
-rw-r--r--reachable.c112
1 files changed, 112 insertions, 0 deletions
diff --git a/reachable.c b/reachable.c
index d03f829..55589a0 100644
--- a/reachable.c
+++ b/reachable.c
@@ -97,7 +97,109 @@ static void mark_commit(struct commit *c, void *data)
mark_object(&c->object, NULL, NULL, data);
}
+struct recent_data {
+ struct rev_info *revs;
+ unsigned long timestamp;
+};
+
+static void add_recent_object(const unsigned char *sha1,
+ unsigned long mtime,
+ struct recent_data *data)
+{
+ struct object *obj;
+ enum object_type type;
+
+ if (mtime <= data->timestamp)
+ return;
+
+ /*
+ * We do not want to call parse_object here, because
+ * inflating blobs and trees could be very expensive.
+ * However, we do need to know the correct type for
+ * later processing, and the revision machinery expects
+ * commits and tags to have been parsed.
+ */
+ type = sha1_object_info(sha1, NULL);
+ if (type < 0)
+ die("unable to get object info for %s", sha1_to_hex(sha1));
+
+ switch (type) {
+ case OBJ_TAG:
+ case OBJ_COMMIT:
+ obj = parse_object_or_die(sha1, NULL);
+ break;
+ case OBJ_TREE:
+ obj = (struct object *)lookup_tree(sha1);
+ break;
+ case OBJ_BLOB:
+ obj = (struct object *)lookup_blob(sha1);
+ break;
+ default:
+ die("unknown object type for %s: %s",
+ sha1_to_hex(sha1), typename(type));
+ }
+
+ if (!obj)
+ die("unable to lookup %s", sha1_to_hex(sha1));
+
+ add_pending_object(data->revs, obj, "");
+}
+
+static int add_recent_loose(const unsigned char *sha1,
+ const char *path, void *data)
+{
+ struct stat st;
+ struct object *obj = lookup_object(sha1);
+
+ if (obj && obj->flags & SEEN)
+ return 0;
+
+ if (stat(path, &st) < 0) {
+ /*
+ * It's OK if an object went away during our iteration; this
+ * could be due to a simultaneous repack. But anything else
+ * we should abort, since we might then fail to mark objects
+ * which should not be pruned.
+ */
+ if (errno == ENOENT)
+ return 0;
+ return error("unable to stat %s: %s",
+ sha1_to_hex(sha1), strerror(errno));
+ }
+
+ add_recent_object(sha1, st.st_mtime, data);
+ return 0;
+}
+
+static int add_recent_packed(const unsigned char *sha1,
+ struct packed_git *p, uint32_t pos,
+ void *data)
+{
+ struct object *obj = lookup_object(sha1);
+
+ if (obj && obj->flags & SEEN)
+ return 0;
+ add_recent_object(sha1, p->mtime, data);
+ return 0;
+}
+
+static int add_unseen_recent_objects_to_traversal(struct rev_info *revs,
+ unsigned long timestamp)
+{
+ struct recent_data data;
+ int r;
+
+ data.revs = revs;
+ data.timestamp = timestamp;
+
+ r = for_each_loose_object(add_recent_loose, &data);
+ if (r)
+ return r;
+ return for_each_packed_object(add_recent_packed, &data);
+}
+
void mark_reachable_objects(struct rev_info *revs, int mark_reflog,
+ unsigned long mark_recent,
struct progress *progress)
{
struct connectivity_progress cp;
@@ -133,5 +235,15 @@ void mark_reachable_objects(struct rev_info *revs, int mark_reflog,
if (prepare_revision_walk(revs))
die("revision walk setup failed");
traverse_commit_list(revs, mark_commit, mark_object, &cp);
+
+ if (mark_recent) {
+ revs->ignore_missing_links = 1;
+ if (add_unseen_recent_objects_to_traversal(revs, mark_recent))
+ die("unable to mark recent objects");
+ if (prepare_revision_walk(revs))
+ die("revision walk setup failed");
+ traverse_commit_list(revs, mark_commit, mark_object, &cp);
+ }
+
display_progress(cp.progress, cp.count);
}