'git for-each-ref' learned a new "--format" atom to find the branch
that the history leading to a given commit "%(is-base:<commit>)" is
likely based on.
* ds/for-each-ref-is-base:
p1500: add is-base performance tests
for-each-ref: add 'is-base' token
commit: add gentle reference lookup method
commit-reach: add get_branch_base_for_tip
Add a new reachability algorithm that intends to discover (from a heuristic)
which branch was used as the starting point for a given commit. Add focused
tests using the 'test-tool reach' command.
In repositories that use pull requests (or merge requests) to advance one or
more "protected" branches, the history of that reference can be recovered by
following the first-parent history in most cases. Most are completed using
no-fast-forward merges, though squash merges are quite common. Less common
is rebase-and-merge, which still validates this assumption. Finally, the
case that breaks this assumption is the fast-forward update (with potential
rebasing). Even in this case, the previous commit commonly appears in the
first-parent history of the branch.
Similar assumptions can be made for a topic branch created by a single user
with the intention to merge back into another branch. Using 'git commit',
'git merge', and 'git cherry-pick' from HEAD will default to having the
first-parent commit be the previous commit at HEAD. This history changes
only with commands such as 'git reset' or 'git rebase', where the command
names also imply that the branch is starting from a new location.
With this movement of branches in mind, the following heuristic is proposed
as a way to determine the base branch for a given source branch:
Among a list of candidate base branches, select the candidate that
minimizes the number of commits in the first-parent history of the source
that are not in the first-parent history of the candidate.
Prior third-party solutions to this problem have used this optimization
criteria, but have relied upon extracting the first-parent history and
comparing those lists as tables instead of using commit-graph walks.
Given current command-line interface options, this optimization criteria is
not easy to detect directly. Even using the command
git rev-list --count --first-parent <base>..<source>
does not measure this count, as it uses full reachability from <base> to
determine which commits to remove from the range '<base>..<source>'. This
may lead to one asking if we should instead be using the full reachability
of the candidate and only the first-parent history of the source. This,
unfortunately, does not work for repositories that use long-lived branches
and automation to merge across those branches.
In extremely large repositories, merging into a single trunk may not be
feasible. This is usually due to the desired frequency of updates
(thousands of engineers doing daily work) combined with the time required to
perform a validation build. These factors combine to create significant
risk of semantic merge conflicts, leading to build breaks on the trunk. In
response, repository maintainers can create a single Level Zero (L0) trunk
and multiple Level One (L1) branches. By partitioning the engineers by
organization, these engineers may see lower risk of semantic merge conflicts
as well as be protected against build breaks in other L1 branches. The key
to making this system work is a semi-automated process of merging L1
branches into the L0 trunk and vice-versa. In a large enough organization,
these L1 branches may further split into L2 or L3 branches, but the same
principles apply for merging across deeper levels.
If these automated merges use a typical merge with the second parent
bringing in the "new" content, then each L0 and L1 branch can track its
previous positions by following first-parent history, which appear as
parallel paths (until reaching the first place where the branches diverged).
If we also walk to second parents, then the histories overlap significantly
and cannot be distinguished except for very-recent changes.
For this reason, the first-parent condition should be symmetrical across the
base and source branches.
Another common case for desiring the result of this optimization method is
the use of release branches. When releasing a version of a repository, a
branch can be used to track that release. Any updates that are worth fixing
in that release can be merged to the release branch and shipped with only
the necessary fixes without any new features introduced in the trunk branch.
The 'maint-2.<X>' branches represent this pattern in the Git project. The
microsoft/git fork uses 'vfs-2.<X>.<Y>' branches to track the changes that
are custom to that fork on top of each upstream Git release 2.<X>.<Y>. This
application doesn't need the symmetrical first-parent condition, but the use
of first-parent histories does not change the results for these branches.
To determine the base branch from a list of candidates, create a new method
in commit-reach.c that performs a single* commit-graph walk. The core
concept is to walk first-parents starting at the candidate bases and the
source, tracking the "best" base to reach a given commit. Use generation
numbers to ensure that a commit is walked at most once and all children have
been explored before visiting it. When reaching a commit that is reachable
from both a base and the source, we will then have a guarantee that this is
the closest intersection of first-parent histories. Track the best base to
reach that commit and return it as a result. In rare cases involving
multiple root commits, the first-parent history of the source may never
intersect any of the candidates and thus a null result is returned.
* There are up to two walks, since we require all commits to have a computed
generation number in order to avoid incorrect results. This is similar to
the need for computed generation numbers in ahead_behind() as implemented
in fd67d149bd (commit-reach: implement ahead_behind() logic, 2023-03-20).
In order to track the "best" base, use a new commit slab that stores an
integer. This value defaults to zero upon initialization, so use -1 to
track that the source commit can reach this commit and use 'i + 1' to track
that the ith base can reach this commit. When multiple bases can reach a
commit, minimize the index to break ties. This allows the caller to specify
an order to the bases that determines some amount of preference when the
heuristic does not result in a unique result.
The trickiest part of the integer slab is what happens when reaching a
collision among the histories of the bases and the history of the source.
This is noticed when viewing the first parent and seeing that it has a slab
value that differs in sign (negative or positive). In this case, the
collision commit is stored in the method variable 'branch_point' and its
slab value is set to -1. The index of the best base (so far) is stored in
the method variable 'best_index'. It is possible that there are multiple
commits that have the branch_point as its first parent, leading to multiple
updates of best_index. The result is determined when 'branch_point' is
visited in the commit walk, giving the guarantee that all commits that could
reach 'branch_point' were visited.
Several interesting cases of collisions and different results are tested in
the t6600-test-reach.sh script. Recall that this script also tests the
algorithm in three possible states involving the commit-graph file and how
many commits are written in the file. This provides some coverage of the
need (and lack of need) for the ensure_generations_valid() method.
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We don't free the local `stack` commit list that we use to compute
reachability of multiple commits at once. Do so.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Use of the `the_repository` variable is deprecated nowadays, and we
slowly but steadily convert the codebase to not use it anymore. Instead,
callers should be passing down the repository to work on via parameters.
It is hard though to prove that a given code unit does not use this
variable anymore. The most trivial case, merely demonstrating that there
is no direct use of `the_repository`, is already a bit of a pain during
code reviews as the reviewer needs to manually verify claims made by the
patch author. The bigger problem though is that we have many interfaces
that implicitly rely on `the_repository`.
Introduce a new `USE_THE_REPOSITORY_VARIABLE` macro that allows code
units to opt into usage of `the_repository`. The intent of this macro is
to demonstrate that a certain code unit does not use this variable
anymore, and to keep it from new dependencies on it in future changes,
be it explicit or implicit
For now, the macro only guards `the_repository` itself as well as
`the_hash_algo`. There are many more known interfaces where we have an
implicit dependency on `the_repository`, but those are not guarded at
the current point in time. Over time though, we should start to add
guards as required (or even better, just remove them).
Define the macro as required in our code units. As expected, most of our
code still relies on the global variable. Nearly all of our builtins
rely on the variable as there is no way yet to pass `the_repository` to
their entry point. For now, declare the macro in "biultin.h" to keep the
required changes at least a little bit more contained.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We use a priority queue in `ahead_behind()` to compute the ahead/behind
count for commits. We may not iterate through all commits part of that
queue though in case all of its entries are stale. Consequently, as we
never make the effort to release the remaining commits, we end up
leaking bit arrays that we have allocated for each of the contained
commits.
Plug this leak and mark the corresponding test as leak free.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
(Actually, this commit is only about passing on "missing commits"
errors, but adding that to the commit's title would have made it too
long.)
The `merge_bases_many()` function was just taught to indicate parsing
errors, and now the `repo_get_merge_bases_many_dirty()` function is
aware of that, too.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The `merge_bases_many()` function was just taught to indicate parsing
errors, and now the `repo_get_merge_bases_many()` function is aware of
that, too.
Naturally, there are a lot of callers that need to be adjusted now, too.
Next stop: `repo_get_merge_bases_dirty()`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The `merge_bases_many()` function was just taught to indicate parsing
errors, and now the `repo_get_merge_bases()` function (which is also
surfaced via the `get_merge_bases()` macro) is aware of that, too.
Naturally, the callers need to be adjusted now, too.
Next step: adjust `repo_get_merge_bases_many()`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The `merge_bases_many()` function was just taught to indicate parsing
errors, and now the `repo_get_merge_bases()` function (which is also
surfaced via the `repo_get_merge_bases()` macro) is aware of that, too.
Naturally, there are a lot of callers that need to be adjusted now, too.
Next step: adjust the callers of `get_octopus_merge_bases()`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The `merge_bases_many()` function was just taught to indicate
parsing errors, and now the `get_merge_bases_many_0()` function is aware
of that, too.
Next step: adjust the callers of `get_merge_bases_many_0()`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The `paint_down_to_common()` function was just taught to indicate
parsing errors, and now the `merge_bases_many()` function is aware of
that, too.
One tricky aspect is that `merge_bases_many()` parses commits of its
own, but wants to gracefully handle the scenario where NULL is passed as
a merge head, returning the empty list of merge bases. The way this was
handled involved calling `repo_parse_commit(NULL)` and relying on it to
return an error. This has to be done differently now so that we can
handle missing commits correctly by producing a fatal error.
Next step: adjust the caller of `merge_bases_many()`:
`get_merge_bases_many_0()`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
If a commit cannot be parsed, it is currently ignored when looking for
merge bases. That's undesirable as the operation can pretend success in
a corrupt repository, even though the command should fail with an error
message.
Let's start at the bottom of the stack by teaching the
`paint_down_to_common()` function to return an `int`: if negative, it
indicates fatal error, if 0 success.
This requires a couple of callers to be adjusted accordingly.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When `git fetch --update-shallow` needs to test for commit ancestry, it
can naturally run into a missing object (e.g. if it is a parent of a
shallow commit). For the purpose of `--update-shallow`, this needs to be
treated as if the child commit did not even have that parent, i.e. the
commit history needs to be clamped.
For all other scenarios, clamping the commit history is actually a bug,
as it would hide repository corruption (for an analysis regarding
shallow and partial clones, see the analysis further down).
Add a flag to optionally ask the function to ignore missing commits, as
`--update-shallow` needs it to, while detecting missing objects as a
repository corruption error by default.
This flag is needed, and cannot be replaced by `is_repository_shallow()`
to indicate that situation, because that function would return 0 in the
`--update-shallow` scenario: There is not actually a `shallow` file in
that scenario, as demonstrated e.g. by t5537.10 ("add new shallow root
with receive.updateshallow on") and t5538.4 ("add new shallow root with
receive.updateshallow on").
Note: shallow commits' parents are set to `NULL` internally already,
therefore there is no need to special-case shallow repositories here, as
the merge-base logic will not try to access parent commits of shallow
commits.
Likewise, partial clones aren't an issue either: If a commit is missing
during the revision walk in the merge-base logic, it is fetched via
`promisor_remote_get_direct()`. And not only the single missing commit
object: Due to the way the "promised" objects are fetched (in
`fetch_objects()` in `promisor-remote.c`, using `fetch
--filter=blob:none`), there is no actual way to fetch a single commit
object, as the remote side will pass that commit OID to `pack-objects
--revs [...]` which in turn passes it to `rev-list` which interprets
this as a commit _range_ instead of a single object. Therefore, in
partial clones (unless they are shallow in addition), all commits
reachable from a commit that is in the local object database are also
present in that local database.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Some functions in Git's source code follow the convention that returning
a negative value indicates a fatal error, e.g. repository corruption.
Let's use this convention in `repo_in_merge_bases()` to report when one
of the specified commits is missing (i.e. when `repo_parse_commit()`
reports an error).
Also adjust the callers of `repo_in_merge_bases()` to handle such
negative return values.
Note: As of this patch, errors are returned only if any of the specified
merge heads is missing. Over the course of the next patches, missing
commits will also be reported by the `paint_down_to_common()` function,
which is called by `repo_in_merge_bases_many()`, and those errors will
be properly propagated back to the caller at that stage.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Currently this function treats unrelated commit histories the same way
as commit histories with missing commit objects.
Typically, missing commit objects constitute a corrupt repository,
though, and should be reported as such. The next commits will make it
so, but there is one exception: In `git fetch --update-shallow` we
_expect_ commit objects to be missing, and we do want to treat the
now-incomplete commit histories as unrelated.
To allow for that, let's introduce an additional parameter that is
passed to `repo_in_merge_bases_many()` to trigger this behavior, and use
it in the two callers in `shallow.c`.
This commit changes behavior slightly: unless called from the
`shallow.c` functions that set the `ignore_missing_commits` bit, any
non-existing tip commit that is passed to `repo_in_merge_bases_many()`
will now result in an error.
Note: When encountering missing commits while traversing the commit
history in search for merge bases, with this commit there won't be a
change in behavior just yet, their children will still be interpreted as
root commits. This bug will get fixed by follow-up commits.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When a commit is missing, we return early (currently pretending that no
merge basis could be found in that case). At that stage, it is possible
that a merge base could have been found already, and added to the
`result`, which is now leaked.
The priority queue has a similar issue: There might still be a commit in
that queue.
Let's release both, to address the potential memory leaks.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Each of these were checked with
gcc -E -I. ${SOURCE_FILE} | grep ${HEADER_FILE}
to ensure that removing the direct inclusion of the header actually
resulted in that header no longer being included at all (i.e. that
no other header pulled it in transitively).
...except for a few cases where we verified that although the header
was brought in transitively, nothing from it was directly used in
that source file. These cases were:
* builtin/credential-cache.c
* builtin/pull.c
* builtin/send-pack.c
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We loop over the set of commits to merge, and for each one compute the
merge base against the existing set of merge base candidates we've
found. Then we replace the candidate set with a simple assignment of the
list head, leaking the old list. We should free it first before
assignment.
This makes t5521 leak-free, so mark it as such.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
alloc_nr, ALLOC_GROW, and ALLOC_GROW_BY are commonly used macros for
dynamic array allocation. Moving these macros to git-compat-util.h with
the other alloc macros focuses alloc.[ch] to allocation for Git objects
and additionally allows us to remove inclusions to alloc.h from files
that solely used the above macros.
Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This is a leak that has existed since the method was first created
in fcb2c0769d (commit-reach: implement get_reachable_subset,
2018-11-02).
Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Code clean-up around the use of the_repository.
* ab/remove-implicit-use-of-the-repository:
libs: use "struct repository *" argument, not "the_repository"
post-cocci: adjust comments for recent repo_* migration
cocci: apply the "revision.h" part of "the_repository.pending"
cocci: apply the "rerere.h" part of "the_repository.pending"
cocci: apply the "refs.h" part of "the_repository.pending"
cocci: apply the "promisor-remote.h" part of "the_repository.pending"
cocci: apply the "packfile.h" part of "the_repository.pending"
cocci: apply the "pretty.h" part of "the_repository.pending"
cocci: apply the "object-store.h" part of "the_repository.pending"
cocci: apply the "diff.h" part of "the_repository.pending"
cocci: apply the "commit.h" part of "the_repository.pending"
cocci: apply the "commit-reach.h" part of "the_repository.pending"
cocci: apply the "cache.h" part of "the_repository.pending"
cocci: add missing "the_repository" macros to "pending"
cocci: sort "the_repository" rules by header
cocci: fix incorrect & verbose "the_repository" rules
cocci: remove dead rule from "the_repository.pending.cocci"
"git for-each-ref" learns '%(ahead-behind:<base>)' that computes the
distances from a single reference point in the history with bunch
of commits in bulk.
* ds/ahead-behind:
commit-reach: add tips_reachable_from_bases()
for-each-ref: add ahead-behind format atom
commit-reach: implement ahead_behind() logic
commit-graph: introduce `ensure_generations_valid()`
commit-graph: return generation from memory
commit-graph: simplify compute_generation_numbers()
commit-graph: refactor compute_topological_levels()
for-each-ref: explicitly test no matches
for-each-ref: add --stdin option
As can easily be seen from grepping in our sources, we had these uses
of "the_repository" in various library code in cases where the
function in question was already getting a "struct repository *"
argument. Let's use that argument instead.
Out of these changes only the changes to "cache-tree.c",
"commit-reach.c", "shallow.c" and "upload-pack.c" would have cleanly
applied before the migration away from the "repo_*()" wrapper macros
in the preceding commits.
The rest aren't new, as we'd previously implicitly refer to
"the_repository", but it's now more obvious that we were doing the
wrong thing all along, and should have used the parameter instead.
The change to change "get_index_format_default(the_repository)" in
"read-cache.c" to use the "r" variable instead should arguably have
been part of [1], or in the subsequent cleanup in [2]. Let's do it
here, as can be seen from the initial code in [3] it's not important
that we use "the_repository" there, but would prefer to always use the
current repository.
This change excludes the "the_repository" use in "upload-pack.c"'s
upload_pack_advertise(), as the in-flight [4] makes that change.
1. ee1f0c242e (read-cache: add index.skipHash config option,
2023-01-06)
2. 6269f8eaad (treewide: always have a valid "index_state.repo"
member, 2023-01-17)
3. 7211b9e753 (repo-settings: consolidate some config settings,
2019-08-13)
4. <Y/hbUsGPVNAxTdmS@coredump.intra.peff.net>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Apply the part of "the_repository.pending.cocci" pertaining to
"commit.h".
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Apply the part of "the_repository.pending.cocci" pertaining to
"commit-reach.h".
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Both 'git for-each-ref --merged=<X>' and 'git branch --merged=<X>' use
the ref-filter machinery to select references or branches (respectively)
that are reachable from a set of commits presented by one or more
--merged arguments. This happens within reach_filter(), which uses the
revision-walk machinery to walk history in a standard way.
However, the commit-reach.c file is full of custom searches that are
more efficient, especially for reachability queries that can terminate
early when reachability is discovered. Add a new
tips_reachable_from_bases() method to commit-reach.c and call it from
within reach_filter() in ref-filter.c. This affects both 'git branch'
and 'git for-each-ref' as tested in p1500-graph-walks.sh.
For the Linux kernel repository, we take an already-fast algorithm and
make it even faster:
Test HEAD~1 HEAD
-------------------------------------------------------------------
1500.5: contains: git for-each-ref --merged 0.13 0.02 -84.6%
1500.6: contains: git branch --merged 0.14 0.02 -85.7%
1500.7: contains: git tag --merged 0.15 0.03 -80.0%
(Note that we remove the iterative 'git rev-list' test from p1500
because it no longer makes sense as a comparison to 'git for-each-ref'
and would just waste time running it for these comparisons.)
The algorithm is implemented in commit-reach.c in the method
tips_reachable_from_base(). This method takes a string_list of tips and
assigns the 'util' for each item with the value 1 if the base commit can
reach those tips.
Like other reachability queries in commit-reach.c, the fastest way to
search for "can A reach B?" is to do a depth-first search up to the
generation number of B, preferring to explore first parents before later
parents. While we must walk all reachable commits up to that generation
number when the answer is "no", the depth-first search can answer "yes"
much faster than other approaches in most cases.
This search becomes trickier when there are multiple targets for the
depth-first search. The commits with lower generation number are more
likely to be within the history of the start commit, but we don't want
to waste time searching commits of low generation number if the commit
target with lowest generation number has already been found.
The trick here is to take the input commits and sort them by generation
number in ascending order. Track the index within this order as
min_generation_index. When we find a commit, if its index in the list is
equal to min_generation_index, then we can increase the generation
number boundary of our search to the next-lowest value in the list.
With this mechanism, the number of commits to search is minimized with
respect to the depth-first search heuristic. We will walk all commits up
to the minimum generation number of a commit that is _not_ reachable
from the start, but we will walk only the necessary portion of the
depth-first search for the reachable commits of lower generation.
Add extra tests for this behavior in t6600-test-reach.sh as the
interesting data shape of that repository can sometimes demonstrate
corner case bugs.
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Fully implement the commit-counting logic required to determine
ahead/behind counts for a batch of commit pairs. This is a new library
method within commit-reach.h. This method will be linked to the
for-each-ref builtin in the next change.
The interface for ahead_behind() uses two arrays. The first array of
commits contains the list of all starting points for the walk. This
includes all tip commits _and_ base commits. The second array specifies
base/tip pairs by pointing to commits within the first array, by index.
The second array also stores the resulting ahead/behind counts for each
of these pairs.
This implementation of ahead_behind() allows multiple bases, if desired.
Even with multiple bases, there is only one commit walk used for
counting the ahead/behind values, saving time when the base/tip ranges
overlap significantly.
This interface for ahead_behind() also makes it very easy to call
ensure_generations_valid() on the entire array of bases and tips. This
call is necessary because it is critical that the walk that counts
ahead/behind values never walks a commit more than once. Without
generation numbers on every commit, there is a possibility that a
commit date skew could cause the walk to revisit a commit and then
double-count it. For this reason, it is strongly recommended that 'git
ahead-behind' is only run in a repository with a commit-graph file that
covers most of the reachable commits, storing precomputed generation
numbers. If no commit-graph exists, this walk will be much slower as it
must walk all reachable commits in ensure_generations_valid() before
performing the counting logic.
It is possible to detect if generation numbers are available at run time
and redirect the implementation to another algorithm that does not
require this property. However, that implementation requires a commit
walk per base/tip pair _and_ can be slower due to the commit date
heuristics required. Such an implementation could be considered in the
future if there is a reason to include it, but most Git hosts should
already be generating a commit-graph file as part of repository
maintenance. Most Git clients should also be generating commit-graph
files as part of background maintenance or automatic GCs.
Now, let's discuss the ahead/behind counting algorithm.
The first array of commits are considered the starting commits. The
index within that array will play a critical role.
We create a new commit slab that maps commits to a bitmap. For a given
commit (anywhere in the history), its bitmap stores information relative
to which of the input commits can reach that commit. The ith bit will be
on if the ith commit from the starting list can reach that commit. It is
important to notice that these bitmaps are not the typical "reachability
bitmaps" that are stored in .bitmap files. Instead of signalling which
objects are reachable from the current commit, they instead signal
"which starting commits can reach me?" It is also important to know that
the bitmap is not necessarily "complete" until we walk that commit. We
will perform a commit walk by generation number in such a way that we
can guarantee the bitmap is correct when we visit that commit.
At the beginning of the ahead_behind() method, we initialize the bitmaps
for each of the starting commits. By enabling the ith bit for the ith
starting commit, we signal "the ith commit can reach itself."
We walk commits by popping the commit with maximum generation number out
of the queue, guaranteeing that we will never walk a child of that
commit in any future steps.
As we walk, we load the bitmap for the current commit and perform two
main steps. The _second_ step examines each parent of the current commit
and adds the current commit's bitmap bits to each parent's bitmap. (We
create a new bitmap for the parent if this is our first time seeing that
parent.) After adding the bits to the parent's bitmap, the parent is
added to the walk queue. Due to this passing of bits to parents, the
current commit has a guarantee that the ith bit is enabled on its bitmap
if and only if the ith commit can reach the current commit.
The first step of the walk is to examine the bitmask on the current
commit and decide which ranges the commit is in or not. Due to the "bit
pushing" in the second step, we have a guarantee that the ith bit of the
current commit's bitmap is on if and only if the ith starting commit can
reach it. For each ahead_behind_count struct, check the base_index and
tip_index to see if those bits are enabled on the current bitmap. If
exactly one bit is enabled, then increment the corresponding 'ahead' or
'behind' count. This increment is the reason we _absolutely need_ to
walk commits at most once.
The only subtle thing to do with this walk is to check to see if a
parent has all bits on in its bitmap, in which case it becomes "stale"
and is marked with the STALE bit. This allows queue_has_nonstale() to be
the terminating condition of the walk, which greatly reduces the number
of commits walked if all of the commits are nearby in history. It avoids
walking a large number of common commits when there is a deep history.
We also use the helper method insert_no_dup() to add commits to the
priority queue without adding them multiple times. This uses the PARENT2
flag. Thus, we must clear both the STALE and PARENT2 bits of all
commits, in case ahead_behind() is called multiple times in the same
process.
Co-authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Fix a segfaulting loop. The function and its caller may need
further clean-up.
* ew/commit-reach-clean-up-flags-fix:
commit-reach: avoid NULL dereference
This allows us to replace includes of cache.h with includes of the much
smaller alloc.h in many places. It does mean that we also need to add
includes of alloc.h in a number of C files.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The loop at the top of can_all_from_reach_with_flag() already
accounts for `from->objects[i].item' being NULL, so it follows
the cleanup loop should also account for a NULL `from_one'.
I managed to segfault here on one of my giant, many-remote repos
using `git fetch --negotiation-tip=... --negotiation-only'
where the --negotiation-tip= argument was a glob which (inadvertently)
captured more refs than I wanted. I have not reproduced this
in a standalone test case.
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add a semantic patch for replace ALLOC_ARRAY+COPY_ARRAY with DUP_ARRAY
to reduce code duplication and apply its results.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add and apply a semantic patch for converting code that open-codes
CALLOC_ARRAY to use it instead. It shortens the code and infers the
element size automatically.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The remove_redundant_with_gen() algorithm performs a depth-first-search
to find commits in the 'array' list, starting at the parents of each
commit in 'array'. The result is that commits in 'array' are marked
STALE when they are reachable from another commit in 'array'.
This depth-first-search is fast when commits lie on or near the
first-parent history of the higher commits. The search terminates early
if all but one commit becomes marked STALE.
However, it is possible that there are two independent commits with high
generation number. In that case, the depth-first-search might languish
by searching in lower generations due to the fixed min_generation used
throughout the method.
With the expectation that commits with lower generation are expected to
become STALE more often, we can optimize further by increasing that
min_generation boundary upon discovery of the commit with minimum
generation.
We must first sort the commits in 'array' by generation. We cannot sort
'array' itself since it must preserve relative order among the returned
results (see revision.c:mark_redundant_parents() for an example).
This simplifies the initialization of min_generation, but it also allows
us to increase the new min_generation when we find the commit with
smallest generation remaining.
This requires more than two commits in order to test, so I used the
Linux kernel repository with a few commits that are slightly off of the
first-parent history. I timed the following command:
git merge-base --independent 2ecedd756908 d2360a398f0b \
1253935ad801 160bab43419e 0e2209629fec 1d0e16ac1a9e
The first two commits have similar generation and are near the v5.10
tag. Commit 160bab43419e is off of the first-parent history behind v5.5,
while the others are scattered somewhere reachable from v5.9. This is
designed to demonstrate the optimization, as that commit within v5.5
would normally cause a lot of extra commit walking.
Since remove_redundant_with_alg() is called only when at least one of
the input commits has a finite generation number, this algorithm is
tested with a commit-graph generated starting at a number of different
tags, the earliest being v5.5.
commit-graph at v5.5:
| Method | Time |
|-----------------------+-------|
| *_no_gen() | 864ms |
| *_with_gen() (before) | 858ms |
| *_with_gen() (after) | 810ms |
commit-graph at v5.7:
| Method | Time |
|-----------------------+-------|
| *_no_gen() | 625ms |
| *_with_gen() (before) | 572ms |
| *_with_gen() (after) | 517ms |
commit-graph at v5.9:
| Method | Time |
|-----------------------+-------|
| *_no_gen() | 268ms |
| *_with_gen() (before) | 224ms |
| *_with_gen() (after) | 202ms |
commit-graph at v5.10:
| Method | Time |
|-----------------------+-------|
| *_no_gen() | 72ms |
| *_with_gen() (before) | 37ms |
| *_with_gen() (after) | 9ms |
Note that these are only modest improvements for the case where the two
independent commits are not in the commit-graph (not until v5.10). All
algorithms get faster as more commits are indexed, which is not a
surprise. However, the cost of walking extra commits is more and more
prevalent in relative terms as more commits are indexed. Finally, the
last case allows us to jump to the minimum generation between the last
two commits (that are actually independent) so we greatly reduce the
cost in that case.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reachability algorithms in commit-reach.c frequently benefit from using
the first-parent history as a heuristic for satisfying reachability
queries. The most obvious example was implemented in 4fbcca4e
(commit-reach: make can_all_from_reach... linear, 2018-07-20).
Update the walk in remove_redundant() to use this same heuristic. Here,
we are walking starting at the parents of the input commits. Sort those
parents and walk from the highest generation to lower. Each time, use
the heuristic of searching the first parent history before continuing to
expand the walk.
The order in which we explore the commits matters, so update
compare_commits_by_gen to break generation number ties with commit date.
This has no effect when the commits are in a commit-graph file with
corrected commit dates computed, but it will assist when the commits are
in the region "above" the commit-graph with "infinite" generation
number. Note that we cannot shift to use
compare_commits_by_gen_then_commit_date as the method prototype is
different. We use compare_commits_by_gen for QSORT() as opposed to as a
priority function.
The important piece is to ensure we short-circuit the walk when we find
that there is a single non-redundant commit. This happens frequently
when looking for merge-bases or comparing several tags with 'git
merge-base --independent'. Use a new count 'count_still_independent' and
if that hits 1 we can stop walking.
To update 'count_still_independent' properly, we add use of the RESULT
flag on the input commits. Then we can detect when we reach one of these
commits and decrease the count. We need to remove the RESULT flag at
that moment because we might re-visit that commit when popping the
stack.
We use the STALE flag to mark parents that have been added to the new
walk_start list, but we need to clear that flag before we start walking
so those flags don't halt our depth-first-search walk.
On my copy of the Linux kernel repository, the performance of 'git
merge-base --independent <all-tags>' goes from 1.1 seconds to 0.11
seconds.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Move this earlier in the file so it can be used by more methods.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The current implementation of remove_redundant() uses several calls to
paint_down_to_common() to determine that commits are independent of each
other. This leads to quadratic behavior when many inputs are passed to
commands such as 'git merge-base'.
For example, in the Linux kernel repository, I tested the performance
by passing all tags:
git merge-base --independent $(git for-each-ref refs/tags --format="$(refname)")
(Note: I had to delete the tags v2.6.11-tree and v2.6.11 as they do
not point to commits.)
Here is the performance improvement introduced by this change:
Before: 16.4s
After: 1.1s
This performance improvement requires the commit-graph file to be
present. We keep the old algorithm around as remove_redundant_no_gen()
and use it when generation_numbers_enabled() is false. This is similar
to other algorithms within commit-reach.c. The new algorithm is
implemented in remove_redundant_with_gen().
The basic approach is to do one commit walk instead of many. First, scan
all commits in the list and mark their _parents_ with the STALE flag.
This flag will indicate commits that are reachable from one of the
inputs, except not including themselves. Then, walk commits until
covering all commits up to the minimum generation number pushing the
STALE flag throughout.
At the end, we need to clear the STALE bit from all of the commits
we walked. We move the non-stale commits in 'array' to the beginning of
the list, and this might overwrite stale commits. However, we store an
array of commits that started the walk, and use clear_commit_marks() on
each of those starting commits. That method will walk the reachable
commits with the STALE bit and clear them all. This makes the algorithm
safe for re-entry or for other uses of those commits after this walk.
This logic is covered by tests in t6600-test-reach.sh, so the behavior
does not change. This is tested both in the case with a commit-graph and
without.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Remove a comment at the beggining of remove_redundant() that mentions a
reordering of the input array to have the initial segment be the
independent commits and the final segment be the redundant commits.
While this behavior is followed in remove_redundant(), no callers rely
on that behavior.
Remove the final loop that copies this final segment and update the
comment to match the new behavior.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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 <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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 <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Way back in f9b8908b (commit.c: use generation numbers for
in_merge_bases(), 2018-05-01), a heuristic was used to short-circuit
the in_merge_bases() walk. This works just fine as long as the
caller is checking only two commits, but when there are multiple,
there is a possibility that this heuristic is _very wrong_.
Some code moves since then has changed this method to
repo_in_merge_bases_many() inside commit-reach.c. The heuristic
computes the minimum generation number of the "reference" list, then
compares this number to the generation number of the "commit".
In a recent topic, a test was added that used in_merge_bases_many()
to test if a commit was reachable from a number of commits pulled
from a reflog. However, this highlighted the problem: if any of the
reference commits have a smaller generation number than the given
commit, then the walk is skipped _even if there exist some with
higher generation number_.
This heuristic is wrong! It must check the MAXIMUM generation number
of the reference commits, not the MINIMUM.
This highlights a testing gap. t6600-test-reach.sh covers many
methods in commit-reach.c, including in_merge_bases() and
get_merge_bases_many(), but since these methods either restrict to
two input commits or actually look for the full list of merge bases,
they don't check this heuristic!
Add a possible input to "test-tool reach" that tests
in_merge_bases_many() and add tests to t6600-test-reach.sh that
cover this heuristic. This includes cases for the reference commits
having generation above and below the generation of the input commit,
but also having maximum generation below the generation of the input
commit.
The fix itself is to swap min_generation with a max_generation in
repo_in_merge_bases_many().
Reported-by: Srinidhi Kaushik <shrinidhi.kaushik@gmail.com>
Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
A few fields in "struct commit" that do not have to always be
present have been moved to commit slabs.
* ak/commit-graph-to-slab:
commit-graph: minimize commit_graph_data_slab access
commit: move members graph_pos, generation to a slab
commit-graph: introduce commit_graph_data_slab
object: drop parsed_object_pool->commit_count
d91d6fbf26 (commit-reach: create repo_is_descendant_of(), 2020-06-17)
adds a repository aware version of is_descendant_of() and a backward
compatibility shim that is barely used.
Update all callers to directly use the new repo_is_descendant_of()
function instead; making the codebase simpler and pushing more
the_repository references higher up the stack.
Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
ref_newer() builds a commit_list to pass a single potential ancestor to
is_descendant_of(). The latter leaves the list intact. Release the
allocated memory after the call.
Signed-off-by: René Scharfe <l.s.r@web.de>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In an earlier patch, multiple struct acccesses to `graph_pos` and
`generation` were auto-converted to multiple method calls.
Since the values are fixed and commit-slab access costly, we would be
better off with storing the values as a local variable and reusing it.
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We remove members `graph_pos` and `generation` from the struct commit.
The default assignments in init_commit_node() are no longer valid,
which is fine as the slab helpers return appropriate default values and
the assignments are removed.
We will replace existing use of commit->generation and commit->graph_pos
by commit_graph_data_slab helpers using
`contrib/coccinelle/commit.cocci'.
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The repo_is_descendant_of() method is aware of the existence of the
commit-graph file. It checks for generation_numbers_enabled() before
deciding on using can_all_from_reach() or repo_in_merge_bases()
depending on the situation. The reason here is that can_all_from_reach()
uses a depth-first search that is limited by the minimum generation
number of the target commits, and that algorithm can be very slow when
generation numbers are not present. The alternative uses
paint_down_to_common() which will walk the entire merge-base boundary,
which is typically slower.
This method is used by commands like "git tag --contains" and "git
branch --contains" for very fast results when a commit-graph file
exists. Unfortunately, it is _not_ used in commands like "git merge-base
--is-ancestor" which is doing an even simpler request.
This issue was raised recently [1] with respect to a change to how
generation numbers are stored, but was also reported much earlier [2]
before commit-reach.c existed to simplify these reachability queries.
[1] https://lore.kernel.org/git/20200607195347.GA8232@szeder.dev/
[2] https://lore.kernel.org/git/87608bawoa.fsf@evledraar.gmail.com/
The root cause is that builtin/merge-base.c has a method
handle_is_ancestor() that calls in_merge_bases(), an older version of
repo_in_merge_bases(). It would be better if we have every caller to
in_merge_bases() use the logic in can_all_from_reach() when possible.
This is where things get a little tricky: repo_is_descendant_of() calls
repo_in_merge_bases() in the non-generation numbers enabled case! If we
simply update repo_in_merge_bases() to call repo_is_descendant_of()
instead of repo_in_merge_bases_many(), then we will get a recursive call
loop. Thankfully, this is caught by the test suite in the default mode
(i.e. GIT_TEST_COMMIT_GRAPH=0).
The trick, then, is to make the non-generation number case for
repo_is_descendant_of() call repo_in_merge_bases_many() directly,
skipping the non-_many version. This allows us to take advantage of this
faster code path, when possible.
The easiest way to measure the performance impact is to test the
following command on the Linux kernel repository:
git merge-base --is-ancestor <A> <B>
| A | B | Time Before | Time After |
|------|------|-------------|------------|
| v3.0 | v5.7 | 0.459s | 0.028s |
| v4.0 | v5.7 | 0.267s | 0.021s |
| v5.0 | v5.7 | 0.074s | 0.013s |
Note that each of these samples return success. The old code performed
the same operation when <A> and <B> are swapped. However,
can_all_from_reach() will return immediately if the generation numbers
show that <A> has larger generation number than <B>. Thus, the time for
the swapped case is universally 0.004s in each case.
Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
