When writing both loose and packed references to disk we first create a lockfile, write the updated values into that lockfile, and on commit we rename the file into place. According to filesystem developers, this behaviour is broken because applications should always sync data to disk before doing the final rename to ensure data consistency [1][2][3]. If applications fail to do this correctly, a hard crash of the machine can easily result in corrupted on-disk data. This kind of corruption can in fact be easily observed with Git when the machine hard-resets shortly after writing references to disk. On machines with ext4, this will likely lead to the "empty files" problem: the file has been renamed, but its data has not been synced to disk. The result is that the reference is corrupt, and in the worst case this can lead to data loss. Implement a new option to harden references so that users and admins can avoid this scenario by syncing locked loose and packed references to disk before we rename them into place. [1]: https://thunk.org/tytso/blog/2009/03/15/dont-fear-the-fsync/ [2]: https://btrfs.wiki.kernel.org/index.php/FAQ (What are the crash guarantees of overwrite-by-rename) [3]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/ext4.rst (see auto_da_alloc) Signed-off-by: Patrick Steinhardt <ps@xxxxxx> --- Hi, here's my updated patch series which implements syncing of refs. It applies on top of Neeraj's v6 of "A design for future-proofing fsync() configuration". I've simplified these patches a bit: - I don't distinguishing between "loose" and "packed" refs anymore. I agree with Junio that it's probably not worth it, but we can still reintroduce the split at a later point without breaking backwards compatibility if the need comes up. - I've simplified the way loose refs are written to disk so that we now sync them when before we close their files. The previous implementation I had was broken because we tried to sync after closing. Because this really only changes a few lines of code I've also decided to squash together the patches into a single one. Patrick Documentation/config/core.txt | 1 + cache.h | 7 +++++-- config.c | 1 + refs/files-backend.c | 1 + refs/packed-backend.c | 3 ++- 5 files changed, 10 insertions(+), 3 deletions(-) diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt index 37105a7be4..812cca7de7 100644 --- a/Documentation/config/core.txt +++ b/Documentation/config/core.txt @@ -575,6 +575,7 @@ but risks losing recent work in the event of an unclean system shutdown. * `index` hardens the index when it is modified. * `objects` is an aggregate option that is equivalent to `loose-object,pack`. +* `reference` hardens references modified in the repo. * `derived-metadata` is an aggregate option that is equivalent to `pack-metadata,commit-graph`. * `committed` is an aggregate option that is currently equivalent to diff --git a/cache.h b/cache.h index cde0900d05..033e5b0779 100644 --- a/cache.h +++ b/cache.h @@ -1005,6 +1005,7 @@ enum fsync_component { FSYNC_COMPONENT_PACK_METADATA = 1 << 2, FSYNC_COMPONENT_COMMIT_GRAPH = 1 << 3, FSYNC_COMPONENT_INDEX = 1 << 4, + FSYNC_COMPONENT_REFERENCE = 1 << 5, }; #define FSYNC_COMPONENTS_OBJECTS (FSYNC_COMPONENT_LOOSE_OBJECT | \ @@ -1017,7 +1018,8 @@ enum fsync_component { FSYNC_COMPONENTS_DERIVED_METADATA | \ ~FSYNC_COMPONENT_LOOSE_OBJECT) -#define FSYNC_COMPONENTS_COMMITTED (FSYNC_COMPONENTS_OBJECTS) +#define FSYNC_COMPONENTS_COMMITTED (FSYNC_COMPONENTS_OBJECTS | \ + FSYNC_COMPONENT_REFERENCE) #define FSYNC_COMPONENTS_ADDED (FSYNC_COMPONENTS_COMMITTED | \ FSYNC_COMPONENT_INDEX) @@ -1026,7 +1028,8 @@ enum fsync_component { FSYNC_COMPONENT_PACK | \ FSYNC_COMPONENT_PACK_METADATA | \ FSYNC_COMPONENT_COMMIT_GRAPH | \ - FSYNC_COMPONENT_INDEX) + FSYNC_COMPONENT_INDEX | \ + FSYNC_COMPONENT_REFERENCE) /* * A bitmask indicating which components of the repo should be fsynced. diff --git a/config.c b/config.c index eb75f65338..3c9b6b589a 100644 --- a/config.c +++ b/config.c @@ -1333,6 +1333,7 @@ static const struct fsync_component_name { { "commit-graph", FSYNC_COMPONENT_COMMIT_GRAPH }, { "index", FSYNC_COMPONENT_INDEX }, { "objects", FSYNC_COMPONENTS_OBJECTS }, + { "reference", FSYNC_COMPONENT_REFERENCE }, { "derived-metadata", FSYNC_COMPONENTS_DERIVED_METADATA }, { "committed", FSYNC_COMPONENTS_COMMITTED }, { "added", FSYNC_COMPONENTS_ADDED }, diff --git a/refs/files-backend.c b/refs/files-backend.c index f59589d6cc..6521ee8af5 100644 --- a/refs/files-backend.c +++ b/refs/files-backend.c @@ -1787,6 +1787,7 @@ static int write_ref_to_lockfile(struct ref_lock *lock, fd = get_lock_file_fd(&lock->lk); if (write_in_full(fd, oid_to_hex(oid), the_hash_algo->hexsz) < 0 || write_in_full(fd, &term, 1) < 0 || + fsync_component(FSYNC_COMPONENT_REFERENCE, get_lock_file_fd(&lock->lk)) < 0 || close_ref_gently(lock) < 0) { strbuf_addf(err, "couldn't write '%s'", get_lock_file_path(&lock->lk)); diff --git a/refs/packed-backend.c b/refs/packed-backend.c index 27dd8c3922..9d704ccd3e 100644 --- a/refs/packed-backend.c +++ b/refs/packed-backend.c @@ -1262,7 +1262,8 @@ static int write_with_updates(struct packed_ref_store *refs, goto error; } - if (close_tempfile_gently(refs->tempfile)) { + if (fsync_component(FSYNC_COMPONENT_REFERENCE, get_tempfile_fd(refs->tempfile)) || + close_tempfile_gently(refs->tempfile)) { strbuf_addf(err, "error closing file %s: %s", get_tempfile_path(refs->tempfile), strerror(errno)); -- 2.35.1
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