The existing reachability algorithms in commit-reach.c focus on
finding merge-bases or determining if all commits in a set X can
reach at least one commit in a set Y. However, for two commits sets
X and Y, we may also care about which commits in Y are reachable
from at least one commit in X.
Implement get_reachable_subset() which answers this question. Given
two arrays of commits, 'from' and 'to', return a commit_list with
every commit from the 'to' array that is reachable from at least
one commit in the 'from' array.
The algorithm is a simple walk starting at the 'from' commits, using
the PARENT2 flag to indicate "this commit has already been added to
the walk queue". By marking the 'to' commits with the PARENT1 flag,
we can determine when we see a commit from the 'to' array. We remove
the PARENT1 flag as we add that commit to the result list to avoid
duplicates.
The order of the resulting list is a reverse of the order that the
commits are discovered in the walk.
There are a couple shortcuts to avoid walking more than we need:
1. We determine the minimum generation number of commits in the
'to' array. We do not walk commits with generation number
below this minimum.
2. We count how many distinct commits are in the 'to' array, and
decrement this count when we discover a 'to' commit during the
walk. If this number reaches zero, then we can terminate the
walk.
Tests will be added using the 'test-tool reach' helper in a
subsequent commit.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The elements of the array to be sorted are commit pointers, so the
comparison function gets handed references to these pointers, not
pointers to commit objects. Cast to the right type and dereference
once to correctly get the commit reference.
Found using Clang's ASan and t5500.
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The algorithm in can_all_from_reach_with_flags() performs a depth-
first-search, terminated by generation number, intending to use
a hueristic that "important" commits are found in the first-parent
history. This heuristic is valuable in scenarios like fetch
negotiation.
However, there is a problem! After the search finds a target commit,
it should pop all commits off the stack and mark them as "can reach".
This logic is incorrect, so the algorithm instead walks all reachable
commits above the generation-number cutoff.
The existing algorithm is still an improvement over the previous
algorithm, as the worst-case complexity went from quadratic to linear.
The performance measurement at the time was good, but not dramatic.
By fixing this heuristic, we reduce the number of walked commits.
We can also re-run the performance tests from commit 4fbcca4e
"commit-reach: make can_all_from_reach... linear".
Performance was measured on the Linux repository using
'test-tool reach can_all_from_reach'. The input included rows seeded by
tag values. The "small" case included X-rows as v4.[0-9]* and Y-rows as
v3.[0-9]*. This mimics a (very large) fetch that says "I have all major
v3 releases and want all major v4 releases." The "large" case included
X-rows as "v4.*" and Y-rows as "v3.*". This adds all release-candidate
tags to the set, which does not greatly increase the number of objects
that are considered, but does increase the number of 'from' commits,
demonstrating the quadratic nature of the previous code.
Small Case:
4fbcca4e~1: 0.85 s
4fbcca4e: 0.26 s (num_walked: 1,011,035)
HEAD: 0.14 s (num_walked: 8,601)
Large Case:
4fbcca4e~1: 24.0 s
4fbcca4e: 0.12 s (num_walked: 503,925)
HEAD: 0.06 s (num_walked: 217,243)
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We added faster equality-comparison functions for hashes in
14438c4497 (introduce hasheq() and oideq(), 2018-08-28). A
few topics were in-flight at the time, and can now be
converted. This covers all spots found by "make coccicheck"
in master (the coccicheck results were tweaked by hand for
style).
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Due to a regression introduced by 4fbcca4e "commit-reach: make
can_all_from_reach... linear" the series including b67f6b26
"commit-reach: properly peel tags" was merged to master quickly.
There were a few more cleanups left to apply in the series, which
are included by this change:
1. Clean up a comment that is in the incorrect style.
2. Replace multiple calls to clear_commit_marks() with one call to
clear_commit_marks_many().
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Recent update broke the reachability algorithm when refs (e.g.
tags) that point at objects that are not commit were involved,
which has been fixed.
* ds/reachable:
commit-reach: fix memory and flag leaks
commit-reach: properly peel tags
The can_all_from_reach_with_flag() method uses 'assign_flag' as a
value we can use to mark objects temporarily during our commit walk.
The intent is that these flags are removed from all objects before
returning. However, this is not the case.
The 'from' array could also contain objects that are not commits, and
we mark those objects with 'assign_flag'. Add a loop to the 'cleanup'
section that removes these markers.
Also, we forgot to free() the memory for 'list', so add that to the
'cleanup' section.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The can_all_from_reach_with_flag() algorithm was refactored in 4fbcca4e
"commit-reach: make can_all_from_reach... linear" but incorrectly
assumed that all objects provided were commits. During a fetch
negotiation, ok_to_give_up() in upload-pack.c may provide unpeeled tags
to the 'from' array. The current code creates a segfault.
Add a direct call to can_all_from_reach_with_flag() in 'test-tool reach'
and add a test in t6600-test-reach.sh that demonstrates this segfault.
Correct the issue by peeling tags when investigating the initial list
of objects in the 'from' array.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The code for computing history reachability has been shuffled,
obtained a bunch of new tests to cover them, and then being
improved.
* ds/reachable:
commit-reach: correct accidental #include of C file
commit-reach: use can_all_from_reach
commit-reach: make can_all_from_reach... linear
commit-reach: replace ref_newer logic
test-reach: test commit_contains
test-reach: test can_all_from_reach_with_flags
test-reach: test reduce_heads
test-reach: test get_merge_bases_many
test-reach: test is_descendant_of
test-reach: test in_merge_bases
test-reach: create new test tool for ref_newer
commit-reach: move can_all_from_reach_with_flags
upload-pack: generalize commit date cutoff
upload-pack: refactor ok_to_give_up()
upload-pack: make reachable() more generic
commit-reach: move commit_contains from ref-filter
commit-reach: move ref_newer from remote.c
commit.h: remove method declarations
commit-reach: move walk methods from commit.c
Without this change, the build breaks with clang:
libgit/ref-filter.pic.o: multiple definition of 'filter_refs'
libgit/commit-reach.pic.o: previous definition here
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The is_descendant_of method previously used in_merge_bases() to check if
the commit can reach any of the commits in the provided list. This had
two performance problems:
1. The performance is quadratic in worst-case.
2. A single in_merge_bases() call requires walking beyond the target
commit in order to find the full set of boundary commits that may be
merge-bases.
The can_all_from_reach method avoids this quadratic behavior and can
limit the search beyond the target commits using generation numbers. It
requires a small prototype adjustment to stop using commit-date as a
cutoff, as that optimization is no longer appropriate here.
Since in_merge_bases() uses paint_down_to_common(), is_descendant_of()
naturally found cutoffs to avoid walking the entire commit graph. Since
we want to always return the correct result, we cannot use the
min_commit_date cutoff in can_all_from_reach. We then rely on generation
numbers to provide the cutoff.
Since not all repos will have a commit-graph file, nor will we always
have generation numbers computed for a commit-graph file, create a new
method, generation_numbers_enabled(), that checks for a commit-graph
file and sees if the first commit in the file has a non-zero generation
number. In the case that we do not have generation numbers, use the old
logic for is_descendant_of().
Performance was meausured on a copy of the Linux repository using the
'test-tool reach is_descendant_of' command using this input:
A:v4.9
X:v4.10
X:v4.11
X:v4.12
X:v4.13
X:v4.14
X:v4.15
X:v4.16
X:v4.17
X.v3.0
Note that this input is tailored to demonstrate the quadratic nature of
the previous method, as it will compute merge-bases for v4.9 versus all
of the later versions before checking against v4.1.
Before: 0.26 s
After: 0.21 s
Since we previously used the is_descendant_of method in the ref_newer
method, we also measured performance there using
'test-tool reach ref_newer' with this input:
A:v4.9
B:v3.19
Before: 0.10 s
After: 0.08 s
By adding a new commit with parent v3.19, we test the non-reachable case
of ref_newer:
Before: 0.09 s
After: 0.08 s
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The can_all_from_reach_with_flags() algorithm is currently quadratic in
the worst case, because it calls the reachable() method for every 'from'
without tracking which commits have already been walked or which can
already reach a commit in 'to'.
Rewrite the algorithm to walk each commit a constant number of times.
We also add some optimizations that should work for the main consumer of
this method: fetch negotitation (haves/wants).
The first step includes using a depth-first-search (DFS) from each
'from' commit, sorted by ascending generation number. We do not walk
beyond the minimum generation number or the minimum commit date. This
DFS is likely to be faster than the existing reachable() method because
we expect previous ref values to be along the first-parent history.
If we find a target commit, then we mark everything in the DFS stack as
a RESULT. This expands the set of targets for the other 'from' commits.
We also mark the visited commits using 'assign_flag' to prevent re-
walking the same commits.
We still need to clear our flags at the end, which is why we will have a
total of three visits to each commit.
Performance was measured on the Linux repository using
'test-tool reach can_all_from_reach'. The input included rows seeded by
tag values. The "small" case included X-rows as v4.[0-9]* and Y-rows as
v3.[0-9]*. This mimics a (very large) fetch that says "I have all major
v3 releases and want all major v4 releases." The "large" case included
X-rows as "v4.*" and Y-rows as "v3.*". This adds all release-candidate
tags to the set, which does not greatly increase the number of objects
that are considered, but does increase the number of 'from' commits,
demonstrating the quadratic nature of the previous code.
Small Case:
Before: 1.52 s
After: 0.26 s
Large Case:
Before: 3.50 s
After: 0.27 s
Note how the time increases between the two cases in the two versions.
The new code increases relative to the number of commits that need to be
walked, but not directly relative to the number of 'from' commits.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The ref_newer method is used by 'git push' to check if a force-push is
required. This method does not use any kind of cutoff when walking, so
in the case of a force-push will walk all reachable commits.
The is_descendant_of method already uses paint_down_to_common along with
cutoffs. By translating the ref_newer arguments into the commit and
commit_list required by is_descendant_of, we can have one fewer commit
walk and also improve our performance!
For a copy of the Linux repository, 'test-tool reach ref_newer' presents
the following improvements with the specified input. In the case that
ref_newer returns 1, there is no improvement. The improvement is in the
second case where ref_newer returns 0.
Input:
A:v4.9
B:v3.19
Before: 0.09 s
After: 0.09 s
To test the negative case, add a new commit with parent v3.19,
regenerate the commit-graph, and then run with B pointing at that
commit.
Before: 0.43 s
After: 0.09 s
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The can_all_from_reach_with_flags method is used by ok_to_give_up in
upload-pack.c to see if we have done enough negotiation during a fetch.
This method is intentionally created to preserve state between calls to
assist with stateful negotiation, such as over SSH.
To make this method testable, add a new can_all_from_reach method that
does the initial setup and final tear-down. We will later use this
method in production code. Call the method from 'test-tool reach' for
now.
Since this is a many-to-many reachability query, add a new type of input
to the 'test-tool reach' input format. Lines "Y:<committish>" create a
list of commits to be the reachability targets from the commits in the
'X' list. In the context of fetch negotiation, the 'X' commits are the
'want' commits and the 'Y' commits are the 'have' commits.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are several commit walks in the codebase. Group them together into
a new commit-reach.c file and corresponding header. After we group these
walks into one place, we can reduce duplicate logic by calling
equivalent methods.
The can_all_from_reach_with_flags method is used in a stateful way by
upload-pack.c. The parameters are very flexible, so we will be able to
use its commit walking logic for many other callers.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are several commit walks in the codebase. Group them together into
a new commit-reach.c file and corresponding header. After we group these
walks into one place, we can reduce duplicate logic by calling
equivalent methods.
All methods are direct moves, except we also make the commit_contains()
method public so its consumers in ref-filter.c can still call it. We can
also test this method in a test-tool in a later commit.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are several commit walks in the codebase. Group them together into
a new commit-reach.c file and corresponding header. After we group these
walks into one place, we can reduce duplicate logic by calling
equivalent methods.
The ref_newer() method is used by 'git push -f' to check if a force-push
is necessary. By making the method public, we make it possible to test
the method directly without setting up an envieronment where a 'git
push' call makes sense.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are several commit walks in the codebase. Group them together into
a new commit-reach.c file and corresponding header. After we group these
walks into one place, we can reduce duplicate logic by calling
equivalent methods.
The method declarations in commit.h are not touched by this commit and
will be moved in a following commit. Many consumers need to point to
commit-reach.h and that would bloat this commit.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
