git/commit-graph.h, branch v2.40.2

Merge branch 'tb/commit-graph-genv2-upgrade-fix'

2022-08-03T20:36:08Z

There was a bug in the codepath to upgrade generation information in commit-graph from v1 to v2 format, which has been corrected. * tb/commit-graph-genv2-upgrade-fix: commit-graph: fix corrupt upgrade from generation v1 to v2 commit-graph: introduce `repo_find_commit_pos_in_graph()` t5318: demonstrate commit-graph generation v2 corruption

commit-graph: introduce `repo_find_commit_pos_in_graph()`

2022-07-15T23:51:39Z

Low-level callers in systems that are adjacent to the commit-graph (like the changed-path Bloom filter code) could benefit from being able to call a function like `parse_commit_in_graph()` without modifying the corresponding commit slab data. This is useful in contexts where that slab data is being used to prepare for an upcoming commit-graph write, where Git must be careful to avoid clobbering any of that data during a read operation. Introduce a low-level variant of `parse_commit_in_graph()` which returns the graph position of a given commit only, without modifying any of the slab data. Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano

commit-graph: pass repo_settings instead of repository

2022-07-14T22:42:17Z

The parse_commit_graph() function takes a 'struct repository *' pointer, but it only ever accesses config settings (either directly or through the .settings field of the repo struct). Move all relevant config settings into the repo_settings struct, and update parse_commit_graph() and its existing callers so that it takes 'struct repo_settings *' instead. Callers of parse_commit_graph() will now need to call prepare_repo_settings() themselves, or initialize a 'struct repo_settings' directly. Prior to ab14d0676c (commit-graph: pass a 'struct repository *' in more places, 2020-09-09), parsing a commit-graph was a pure function depending only on the contents of the commit-graph itself. Commit ab14d0676c introduced a dependency on a `struct repository` pointer, and later commits such as b66d84756f (commit-graph: respect 'commitGraph.readChangedPaths', 2020-09-09) added dependencies on config settings, which were accessed through the `settings` field of the repository pointer. This field was initialized via a call to `prepare_repo_settings()`. Additionally, this fixes an issue in fuzz-commit-graph: In 44c7e62 (2021-12-06, repo-settings:prepare_repo_settings only in git repos), prepare_repo_settings was changed to issue a BUG() if it is called by a process whose CWD is not a Git repository. The combination of commits mentioned above broke fuzz-commit-graph, which attempts to parse arbitrary fuzzing-engine-provided bytes as a commit graph file. Prior to this change, parse_commit_graph() called prepare_repo_settings(), but since we run the fuzz tests without a valid repository, we are hitting the BUG() from 44c7e62 for every test case. Signed-off-by: Taylor Blau Signed-off-by: Josh Steadmon Signed-off-by: Junio C Hamano

commit-graph: fix memory leak in misused string_list API

2022-03-04T21:24:18Z

When this code was migrated to the string_list API in d88b14b3fd6 (commit-graph: use string-list API for input, 2018-06-27) it was made to use used both STRING_LIST_INIT_NODUP and a strbuf_detach() pattern. Those should not be used together if string_list_clear() is expected to free the memory, instead we need to either use STRING_LIST_INIT_DUP with a string_list_append_nodup(), or a STRING_LIST_INIT_NODUP and manually fiddle with the "strdup_strings" member before calling string_list_clear(). Let's do the former. Since "strdup_strings = 1" is set now other code might be broken by relying on "pack_indexes" not to duplicate it strings, but that doesn't happen. When we pass this down to write_commit_graph() that code uses the "struct string_list" without modifying it. Let's add a "const" to the variable to have the compiler enforce that assumption. Signed-off-by: Ævar Arnfjörð Bjarmason Signed-off-by: Junio C Hamano

revision: avoid hitting packfiles when commits are in commit-graph

2021-08-09T16:51:12Z

When queueing references in git-rev-list(1), we try to optimize parsing of commits via the commit-graph. To do so, we first look up the object's type, and if it is a commit we call `repo_parse_commit()` instead of `parse_object()`. This is quite inefficient though given that we're always uncompressing the object header in order to determine the type. Instead, we can opportunistically search the commit-graph for the object ID: in case it's found, we know it's a commit and can directly fill in the commit object without having to uncompress the object header. Expose a new function `lookup_commit_in_graph()`, which tries to find a commit in the commit-graph by ID, and convert `get_reference()` to use this function. This provides a big performance win in cases where we load references in a repository with lots of references pointing to commits. The following has been executed in a real-world repository with about 2.2 million refs: Benchmark #1: HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev Time (mean ± σ): 4.458 s ± 0.044 s [User: 4.115 s, System: 0.342 s] Range (min … max): 4.409 s … 4.534 s 10 runs Benchmark #2: HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev Time (mean ± σ): 3.089 s ± 0.015 s [User: 2.768 s, System: 0.321 s] Range (min … max): 3.061 s … 3.105 s 10 runs Summary 'HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev' ran 1.44 ± 0.02 times faster than 'HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev' Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano

commit-graph: use config to specify generation type

2021-02-25T23:10:41Z

We have two established generation number versions: 1: topological levels 2: corrected commit dates The corrected commit dates are enabled by default, but they also write extra data in the GDAT and GDOV chunks. Services that host Git data might want to have more control over when this feature rolls out than just updating the Git binaries. Add a new "commitGraph.generationVersion" config option that specifies the intended generation number version. If this value is less than 2, then the GDAT chunk is never written _or read_ from an existing file. This can replace our use of the GIT_TEST_COMMIT_GRAPH_NO_GDAT environment variable in the test suite. Remove it. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano

commit-reach: use corrected commit dates in paint_down_to_common()

2021-01-19T00:21:18Z

091f4cf (commit: don't use generation numbers if not needed, 2018-08-30) changed paint_down_to_common() to use commit dates instead of generation numbers v1 (topological levels) as the performance regressed on certain topologies. With generation number v2 (corrected commit dates) implemented, we no longer have to rely on commit dates and can use generation numbers. For example, the command `git merge-base v4.8 v4.9` on the Linux repository walks 167468 commits, taking 0.135s for committer date and 167496 commits, taking 0.157s for corrected committer date respectively. While using corrected commit dates, Git walks nearly the same number of commits as commit date, the process is slower as for each comparision we have to access a commit-slab (for corrected committer date) instead of accessing struct member (for committer date). This change incidentally broke the fragile t6404-recursive-merge test. t6404-recursive-merge sets up a unique repository where all commits have the same committer date without a well-defined merge-base. While running tests with GIT_TEST_COMMIT_GRAPH unset, we use committer date as a heuristic in paint_down_to_common(). 6404.1 'combined merge conflicts' merges commits in the order: - Merge C with B to form an intermediate commit. - Merge the intermediate commit with A. With GIT_TEST_COMMIT_GRAPH=1, we write a commit-graph and subsequently use the corrected committer date, which changes the order in which commits are merged: - Merge A with B to form an intermediate commit. - Merge the intermediate commit with C. While resulting repositories are equivalent, 6404.4 'virtual trees were processed' fails with GIT_TEST_COMMIT_GRAPH=1 as we are selecting different merge-bases and thus have different object ids for the intermediate commits. As this has already causes problems (as noted in 859fdc0 (commit-graph: define GIT_TEST_COMMIT_GRAPH, 2018-08-29)), we disable commit graph within t6404-recursive-merge. Signed-off-by: Abhishek Kumar Reviewed-by: Taylor Blau Reviewed-by: Derrick Stolee Signed-off-by: Junio C Hamano

commit-graph: use generation v2 only if entire chain does

2021-01-19T00:21:18Z

Since there are released versions of Git that understand generation numbers in the commit-graph's CDAT chunk but do not understand the GDAT chunk, the following scenario is possible: 1. "New" Git writes a commit-graph with the GDAT chunk. 2. "Old" Git writes a split commit-graph on top without a GDAT chunk. If each layer of split commit-graph is treated independently, as it was the case before this commit, with Git inspecting only the current layer for chunk_generation_data pointer, commits in the lower layer (one with GDAT) whould have corrected commit date as their generation number, while commits in the upper layer would have topological levels as their generation. Corrected commit dates usually have much larger values than topological levels. This means that if we take two commits, one from the upper layer, and one reachable from it in the lower layer, then the expectation that the generation of a parent is smaller than the generation of a child would be violated. It is difficult to expose this issue in a test. Since we _start_ with artificially low generation numbers, any commit walk that prioritizes generation numbers will walk all of the commits with high generation number before walking the commits with low generation number. In all the cases I tried, the commit-graph layers themselves "protect" any incorrect behavior since none of the commits in the lower layer can reach the commits in the upper layer. This issue would manifest itself as a performance problem in this case, especially with something like "git log --graph" since the low generation numbers would cause the in-degree queue to walk all of the commits in the lower layer before allowing the topo-order queue to write anything to output (depending on the size of the upper layer). Therefore, When writing the new layer in split commit-graph, we write a GDAT chunk only if the topmost layer has a GDAT chunk. This guarantees that if a layer has GDAT chunk, all lower layers must have a GDAT chunk as well. Rewriting layers follows similar approach: if the topmost layer below the set of layers being rewritten (in the split commit-graph chain) exists, and it does not contain GDAT chunk, then the result of rewrite does not have GDAT chunks either. Signed-off-by: Derrick Stolee Signed-off-by: Abhishek Kumar Reviewed-by: Taylor Blau Reviewed-by: Derrick Stolee Signed-off-by: Junio C Hamano

commit-graph: implement generation data chunk

2021-01-19T00:21:18Z

As discovered by Ævar, we cannot increment graph version to distinguish between generation numbers v1 and v2 [1]. Thus, one of pre-requistes before implementing generation number v2 was to distinguish between graph versions in a backwards compatible manner. We are going to introduce a new chunk called Generation DATa chunk (or GDAT). GDAT will store corrected committer date offsets whereas CDAT will still store topological level. Old Git does not understand GDAT chunk and would ignore it, reading topological levels from CDAT. New Git can parse GDAT and take advantage of newer generation numbers, falling back to topological levels when GDAT chunk is missing (as it would happen with a commit-graph written by old Git). We introduce a test environment variable 'GIT_TEST_COMMIT_GRAPH_NO_GDAT' which forces commit-graph file to be written without generation data chunk to emulate a commit-graph file written by old Git. To minimize the space required to store corrrected commit date, Git stores corrected commit date offsets into the commit-graph file, instea of corrected commit dates. This saves us 4 bytes per commit, decreasing the GDAT chunk size by half, but it's possible for the offset to overflow the 4-bytes allocated for storage. As such overflows are and should be exceedingly rare, we use the following overflow management scheme: We introduce a new commit-graph chunk, Generation Data OVerflow ('GDOV') to store corrected commit dates for commits with offsets greater than GENERATION_NUMBER_V2_OFFSET_MAX. If the offset is greater than GENERATION_NUMBER_V2_OFFSET_MAX, we set the MSB of the offset and the other bits store the position of corrected commit date in GDOV chunk, similar to how Extra Edge List is maintained. We test the overflow-related code with the following repo history: F - N - U / \ U - N - U N \ / N - F - N Where the commits denoted by U have committer date of zero seconds since Unix epoch, the commits denoted by N have committer date of 1112354055 (default committer date for the test suite) seconds since Unix epoch and the commits denoted by F have committer date of (2 ^ 31 - 2) seconds since Unix epoch. The largest offset observed is 2 ^ 31, just large enough to overflow. [1]: https://lore.kernel.org/git/87a7gdspo4.fsf@evledraar.gmail.com/ Signed-off-by: Abhishek Kumar Reviewed-by: Taylor Blau Reviewed-by: Derrick Stolee Signed-off-by: Junio C Hamano

commit-graph: return 64-bit generation number

2021-01-19T00:21:18Z

In a preparatory step for introducing corrected commit dates, let's return timestamp_t values from commit_graph_generation(), use timestamp_t for local variables and define GENERATION_NUMBER_INFINITY as (2 ^ 63 - 1) instead. We rename GENERATION_NUMBER_MAX to GENERATION_NUMBER_V1_MAX to represent the largest topological level we can store in the commit data chunk. With corrected commit dates implemented, we will have two such *_MAX variables to denote the largest offset and largest topological level that can be stored. Signed-off-by: Abhishek Kumar Reviewed-by: Taylor Blau Reviewed-by: Derrick Stolee Signed-off-by: Junio C Hamano