From d3038d22f91aad9620bd8e6fc43fc67c16219738 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Wed, 15 Oct 2014 18:41:35 -0400 Subject: 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 Signed-off-by: Junio C Hamano --- reachable.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'reachable.h') diff --git a/reachable.h b/reachable.h index 5d082adfec..141fe30879 100644 --- a/reachable.h +++ b/reachable.h @@ -2,6 +2,7 @@ #define REACHEABLE_H struct progress; -extern void mark_reachable_objects(struct rev_info *revs, int mark_reflog, struct progress *); +extern void mark_reachable_objects(struct rev_info *revs, int mark_reflog, + unsigned long mark_recent, struct progress *); #endif -- cgit v1.3 From abcb86553d3ec4afffa4e3963089dffe0559740e Mon Sep 17 00:00:00 2001 From: Jeff King Date: Wed, 15 Oct 2014 18:42:09 -0400 Subject: pack-objects: match prune logic for discarding objects A recent commit taught git-prune to keep non-recent objects that are reachable from recent ones. However, pack-objects, when loosening unreachable objects, tries to optimize out the write in the case that the object will be immediately pruned. It now gets this wrong, since its rule does not reflect the new prune code (and this can be seen by running t6501 with a strategically placed repack). Let's teach pack-objects similar logic. Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- builtin/pack-objects.c | 39 +++++++++++++++++++ reachable.c | 4 +- reachable.h | 2 + t/t6501-freshen-objects.sh | 93 +++++++++++++++++++++++++++------------------- 4 files changed, 98 insertions(+), 40 deletions(-) (limited to 'reachable.h') diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c index 2fe2ab0603..4df9499040 100644 --- a/builtin/pack-objects.c +++ b/builtin/pack-objects.c @@ -20,6 +20,8 @@ #include "streaming.h" #include "thread-utils.h" #include "pack-bitmap.h" +#include "reachable.h" +#include "sha1-array.h" static const char *pack_usage[] = { N_("git pack-objects --stdout [options...] [< ref-list | < object-list]"), @@ -2407,6 +2409,15 @@ static int has_sha1_pack_kept_or_nonlocal(const unsigned char *sha1) return 0; } +/* + * Store a list of sha1s that are should not be discarded + * because they are either written too recently, or are + * reachable from another object that was. + * + * This is filled by get_object_list. + */ +static struct sha1_array recent_objects; + static int loosened_object_can_be_discarded(const unsigned char *sha1, unsigned long mtime) { @@ -2414,6 +2425,8 @@ static int loosened_object_can_be_discarded(const unsigned char *sha1, return 0; if (mtime > unpack_unreachable_expiration) return 0; + if (sha1_array_lookup(&recent_objects, sha1) >= 0) + return 0; return 1; } @@ -2470,6 +2483,19 @@ static int get_object_list_from_bitmap(struct rev_info *revs) return 0; } +static void record_recent_object(struct object *obj, + const struct name_path *path, + const char *last, + void *data) +{ + sha1_array_append(&recent_objects, obj->sha1); +} + +static void record_recent_commit(struct commit *commit, void *data) +{ + sha1_array_append(&recent_objects, commit->object.sha1); +} + static void get_object_list(int ac, const char **av) { struct rev_info revs; @@ -2517,10 +2543,23 @@ static void get_object_list(int ac, const char **av) mark_edges_uninteresting(&revs, show_edge); traverse_commit_list(&revs, show_commit, show_object, NULL); + if (unpack_unreachable_expiration) { + revs.ignore_missing_links = 1; + if (add_unseen_recent_objects_to_traversal(&revs, + unpack_unreachable_expiration)) + die("unable to add recent objects"); + if (prepare_revision_walk(&revs)) + die("revision walk setup failed"); + traverse_commit_list(&revs, record_recent_commit, + record_recent_object, NULL); + } + if (keep_unreachable) add_objects_in_unpacked_packs(&revs); if (unpack_unreachable) loosen_unused_packed_objects(&revs); + + sha1_array_clear(&recent_objects); } static int option_parse_index_version(const struct option *opt, diff --git a/reachable.c b/reachable.c index 55589a02eb..0176a88b81 100644 --- a/reachable.c +++ b/reachable.c @@ -183,8 +183,8 @@ static int add_recent_packed(const unsigned char *sha1, return 0; } -static int add_unseen_recent_objects_to_traversal(struct rev_info *revs, - unsigned long timestamp) +int add_unseen_recent_objects_to_traversal(struct rev_info *revs, + unsigned long timestamp) { struct recent_data data; int r; diff --git a/reachable.h b/reachable.h index 141fe30879..d23efc36ec 100644 --- a/reachable.h +++ b/reachable.h @@ -2,6 +2,8 @@ #define REACHEABLE_H struct progress; +extern int add_unseen_recent_objects_to_traversal(struct rev_info *revs, + unsigned long timestamp); extern void mark_reachable_objects(struct rev_info *revs, int mark_reflog, unsigned long mark_recent, struct progress *); diff --git a/t/t6501-freshen-objects.sh b/t/t6501-freshen-objects.sh index de941c2cb2..e25c47dd5c 100755 --- a/t/t6501-freshen-objects.sh +++ b/t/t6501-freshen-objects.sh @@ -39,50 +39,67 @@ commit () { git commit -m "$1" } -test_expect_success 'disable reflogs' ' - git config core.logallrefupdates false && - rm -rf .git/logs -' +maybe_repack () { + if test -n "$repack"; then + git repack -ad + fi +} + +for repack in '' true; do + title=${repack:+repack} + title=${title:-loose} + + test_expect_success "make repo completely empty ($title)" ' + rm -rf .git && + git init + ' + + test_expect_success "disable reflogs ($title)" ' + git config core.logallrefupdates false && + rm -rf .git/logs + ' -test_expect_success 'setup basic history' ' - commit base -' + test_expect_success "setup basic history ($title)" ' + commit base + ' -test_expect_success 'create and abandon some objects' ' - git checkout -b experiment && - commit abandon && - git checkout master && - git branch -D experiment -' + test_expect_success "create and abandon some objects ($title)" ' + git checkout -b experiment && + commit abandon && + maybe_repack && + git checkout master && + git branch -D experiment + ' -test_expect_success 'simulate time passing' ' - find .git/objects -type f | - xargs test-chmtime -v -86400 -' + test_expect_success "simulate time passing ($title)" ' + find .git/objects -type f | + xargs test-chmtime -v -86400 + ' -test_expect_success 'start writing new commit with old blob' ' - tree=$( - GIT_INDEX_FILE=index.tmp && - export GIT_INDEX_FILE && - git read-tree HEAD && - add unrelated && - add abandon && - git write-tree - ) -' + test_expect_success "start writing new commit with old blob ($title)" ' + tree=$( + GIT_INDEX_FILE=index.tmp && + export GIT_INDEX_FILE && + git read-tree HEAD && + add unrelated && + add abandon && + git write-tree + ) + ' -test_expect_success 'simultaneous gc' ' - git gc --prune=12.hours.ago -' + test_expect_success "simultaneous gc ($title)" ' + git gc --prune=12.hours.ago + ' -test_expect_success 'finish writing out commit' ' - commit=$(echo foo | git commit-tree -p HEAD $tree) && - git update-ref HEAD $commit -' + test_expect_success "finish writing out commit ($title)" ' + commit=$(echo foo | git commit-tree -p HEAD $tree) && + git update-ref HEAD $commit + ' -# "abandon" blob should have been rescued by reference from new tree -test_expect_success 'repository passes fsck' ' - git fsck -' + # "abandon" blob should have been rescued by reference from new tree + test_expect_success "repository passes fsck ($title)" ' + git fsck + ' +done test_done -- cgit v1.3