git/reachable.c
Jeff King d3038d22f9 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>
2014-10-16 10:10:42 -07:00

250 lines
5.9 KiB
C

#include "cache.h"
#include "refs.h"
#include "tag.h"
#include "commit.h"
#include "blob.h"
#include "diff.h"
#include "revision.h"
#include "reachable.h"
#include "cache-tree.h"
#include "progress.h"
#include "list-objects.h"
struct connectivity_progress {
struct progress *progress;
unsigned long count;
};
static void update_progress(struct connectivity_progress *cp)
{
cp->count++;
if ((cp->count & 1023) == 0)
display_progress(cp->progress, cp->count);
}
static int add_one_ref(const char *path, const unsigned char *sha1, int flag, void *cb_data)
{
struct object *object = parse_object_or_die(sha1, path);
struct rev_info *revs = (struct rev_info *)cb_data;
add_pending_object(revs, object, "");
return 0;
}
static void add_one_tree(const unsigned char *sha1, struct rev_info *revs)
{
struct tree *tree = lookup_tree(sha1);
if (tree)
add_pending_object(revs, &tree->object, "");
}
static void add_cache_tree(struct cache_tree *it, struct rev_info *revs)
{
int i;
if (it->entry_count >= 0)
add_one_tree(it->sha1, revs);
for (i = 0; i < it->subtree_nr; i++)
add_cache_tree(it->down[i]->cache_tree, revs);
}
static void add_cache_refs(struct rev_info *revs)
{
int i;
read_cache();
for (i = 0; i < active_nr; i++) {
struct blob *blob;
/*
* The index can contain blobs and GITLINKs, GITLINKs are hashes
* that don't actually point to objects in the repository, it's
* almost guaranteed that they are NOT blobs, so we don't call
* lookup_blob() on them, to avoid populating the hash table
* with invalid information
*/
if (S_ISGITLINK(active_cache[i]->ce_mode))
continue;
blob = lookup_blob(active_cache[i]->sha1);
if (blob)
blob->object.flags |= SEEN;
/*
* We could add the blobs to the pending list, but quite
* frankly, we don't care. Once we've looked them up, and
* added them as objects, we've really done everything
* there is to do for a blob
*/
}
if (active_cache_tree)
add_cache_tree(active_cache_tree, revs);
}
/*
* The traversal will have already marked us as SEEN, so we
* only need to handle any progress reporting here.
*/
static void mark_object(struct object *obj, const struct name_path *path,
const char *name, void *data)
{
update_progress(data);
}
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;
/*
* Set up revision parsing, and mark us as being interested
* in all object types, not just commits.
*/
revs->tag_objects = 1;
revs->blob_objects = 1;
revs->tree_objects = 1;
/* Add all refs from the index file */
add_cache_refs(revs);
/* Add all external refs */
for_each_ref(add_one_ref, revs);
/* detached HEAD is not included in the list above */
head_ref(add_one_ref, revs);
/* Add all reflog info */
if (mark_reflog)
add_reflogs_to_pending(revs, 0);
cp.progress = progress;
cp.count = 0;
/*
* Set up the revision walk - this will move all commits
* from the pending list to the commit walking list.
*/
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);
}