git/midx.c, branch v2.50.1

midx: stop repeatedly looking up nonexistent packfiles

2025-05-28T14:56:29Z

The multi-pack index acts as a cache across a set of packfiles so that we can quickly look up which of those packfiles contains a given object. As such, the multi-pack index naturally needs to be updated every time one of the packfiles goes away, or otherwise the multi-pack index has grown stale. A stale multi-pack index should be handled gracefully by Git though, and in fact it is: if the indexed pack cannot be found we simply ignore it and eventually we fall back to doing the object lookup by just iterating through all packs, even if those aren't indexed. But while this fallback works, it has one significant downside: we don't cache the fact that a pack has vanished. This leads to us repeatedly trying to look up the same pack only to realize that it (still) doesn't exist. This issue can be easily demonstrated by creating a repository with a stale multi-pack index and a couple of objects. We do so by creating a repository with two packfiles, both of which are indexed by the multi-pack index, and then repack those two packfiles. Note that we have to move the multi-pack-index before doing the final repack, as Git knows to delete it otherwise. $ git init repo $ cd repo/ $ git config set maintenance.auto false $ for i in $(seq 1000); do printf "%d-original" $i >file-$i; done $ git add . $ git commit -moriginal $ git repack -dl $ for i in $(seq 1000); do printf "%d-modified" $i >file-$i; done $ git commit -a -mmodified $ git repack -dl $ git multi-pack-index write $ mv .git/objects/pack/multi-pack-index . $ git repack -Adl $ mv multi-pack-index .git/objects/pack/ Commands that cause a lot of objects lookups will now repeatedly invoke `add_packed_git()`, which leads to three failed access(3p) calls as well as one failed stat(3p) call. The following strace for example is done for `git log --patch` in the above repository: % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 74.67 0.024693 1 18038 18031 access 25.33 0.008378 1 6045 6017 newfstatat ------ ----------- ----------- --------- --------- ---------------- 100.00 0.033071 1 24083 24048 total Fix the issue by introducing a negative lookup cache for indexed packs. This cache works by simply storing an invalid pointer for a missing pack when `prepare_midx_pack()` fails to look up the pack. Most users of the `packs` array don't need to be adjusted, either, as they all know to call `prepare_midx_pack()` before accessing the array. With this change in place we can now see a significantly reduced number of syscalls: % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 73.58 0.000323 5 60 28 newfstatat 26.42 0.000116 5 23 16 access ------ ----------- ----------- --------- --------- ---------------- 100.00 0.000439 5 83 44 total Furthermore, this change also results in a speedup: Benchmark 1: git log --patch (revision = HEAD~) Time (mean ± σ): 50.4 ms ± 2.5 ms [User: 22.0 ms, System: 24.4 ms] Range (min … max): 45.4 ms … 54.9 ms 53 runs Benchmark 2: git log --patch (revision = HEAD) Time (mean ± σ): 12.7 ms ± 0.4 ms [User: 11.1 ms, System: 1.6 ms] Range (min … max): 12.4 ms … 15.0 ms 191 runs Summary git log --patch (revision = HEAD) ran 3.96 ± 0.22 times faster than git log --patch (revision = HEAD~) In the end, it should in theory never be necessary to have this negative lookup cache given that we know to update the multi-pack index together with repacks. But as the change is quite contained and as the speedup can be significant as demonstrated above, it does feel sensible to have the negative lookup cache regardless. Based-on-patch-by: Jeff King Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano

object-file: move `git_open_cloexec()` to "compat/open.c"

2025-04-15T15:24:35Z

The `git_open_cloexec()` wrapper function provides the ability to open a file with `O_CLOEXEC` in a platform-agnostic way. This function is provided by "object-file.c" even though it is not specific to the object subsystem at all. Move the file into "compat/open.c". This file already exists before this commit, but has only been compiled conditionally depending on whether or not open(3p) may return EINTR. With this change we now unconditionally compile the object, but wrap `git_open_with_retry()` in an ifdef. Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano

csum-file: stop depending on `the_repository`

2025-03-10T20:16:18Z

There are multiple sites in "csum-file.c" where we use the global `the_repository` variable, either explicitly or implicitly by using `the_hash_algo`. Refactor the code to stop using `the_repository` by adapting functions to receive required data as parameters. Adapt callsites accordingly by either using `the_repository->hash_algo`, or by using a context-provided hash algorithm in case the subsystem already got rid of its dependency on `the_repository`. Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano

progress: stop using `the_repository`

2024-12-18T18:44:30Z

Stop using `the_repository` in the "progress" subsystem by passing in a repository when initializing `struct progress`. Furthermore, store a pointer to the repository in that struct so that we can pass it to the trace2 API when logging information. Adjust callers accordingly by using `the_repository`. While there may be some callers that have a repository available in their context, this trivial conversion allows for easier verification and bubbles up the use of `the_repository` by one level. Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano

Merge branch 'ps/build-sign-compare' into ps/the-repository

2024-12-18T18:43:16Z

* ps/build-sign-compare: t/helper: don't depend on implicit wraparound scalar: address -Wsign-compare warnings builtin/patch-id: fix type of `get_one_patchid()` builtin/blame: fix type of `length` variable when emitting object ID gpg-interface: address -Wsign-comparison warnings daemon: fix type of `max_connections` daemon: fix loops that have mismatching integer types global: trivial conversions to fix `-Wsign-compare` warnings pkt-line: fix -Wsign-compare warning on 32 bit platform csum-file: fix -Wsign-compare warning on 32-bit platform diff.h: fix index used to loop through unsigned integer config.mak.dev: drop `-Wno-sign-compare` global: mark code units that generate warnings with `-Wsign-compare` compat/win32: fix -Wsign-compare warning in "wWinMain()" compat/regex: explicitly ignore "-Wsign-compare" warnings git-compat-util: introduce macros to disable "-Wsign-compare" warnings

global: mark code units that generate warnings with `-Wsign-compare`

2024-12-06T11:20:02Z

Mark code units that generate warnings with `-Wsign-compare`. This allows for a structured approach to get rid of all such warnings over time in a way that can be easily measured. Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano

midx: inline the `MIDX_MIN_SIZE` definition

2024-12-04T01:32:21Z

The `MIDX_MIN_SIZE` definition is used to check the midx_size in `local_multi_pack_index_one`. This definition relies on the `the_hash_algo` global variable. Inline this and remove the global variable usage. With this, remove `USE_THE_REPOSITORY_VARIABLE` usage from `midx.c`. Signed-off-by: Karthik Nayak Signed-off-by: Junio C Hamano

midx: pass down `hash_algo` to functions using global variables

2024-12-04T01:32:21Z

The functions `get_split_midx_filename_ext()`, `get_midx_filename()` and `get_midx_filename_ext()` use `hash_to_hex()` which internally uses the `the_hash_algo` global variable. Remove this dependency on global variables by passing down the `hash_algo` through to the functions mentioned and instead calling `hash_to_hex_algop()` along with the obtained `hash_algo`. Signed-off-by: Karthik Nayak Signed-off-by: Junio C Hamano

midx: pass `repository` to `load_multi_pack_index`

2024-12-04T01:32:20Z

The `load_multi_pack_index` function in midx uses `the_repository` variable to access the `repository` struct. Modify the function and its callee's to send the `repository` field. This moves usage of `the_repository` to the `test-read-midx.c` file. While that is not optimal, it is okay, since the upcoming commits will slowly move the usage of `the_repository` up the layers and remove it eventually. Signed-off-by: Karthik Nayak Signed-off-by: Junio C Hamano

midx: cleanup internal usage of `the_repository` and `the_hash_algo`

2024-12-04T01:32:20Z

In the `midx.c` file, there are multiple usages of `the_repository` and `the_hash_algo` within static functions of the file. Some of the usages can be simply swapped out with the available `repository` struct. While some of them can be swapped out by passing the repository to the required functions. This leaves out only some other usages of `the_repository` and `the_hash_algo` in the file in non-static functions, which we'll tackle in upcoming commits. Signed-off-by: Karthik Nayak Signed-off-by: Junio C Hamano