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authorDerrick Stolee <dstolee@microsoft.com>2020-04-10 12:19:43 (GMT)
committerJunio C Hamano <gitster@pobox.com>2020-04-10 16:58:55 (GMT)
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revision: --show-pulls adds helpful merges
The default file history simplification of "git log -- <path>" or "git rev-list -- <path>" focuses on providing the smallest set of commits that first contributed a change. The revision walk greatly restricts the set of walked commits by visiting only the first TREESAME parent of a merge commit, when one exists. This means that portions of the commit-graph are not walked, which can be a performance benefit, but can also "hide" commits that added changes but were ignored by a merge resolution. The --full-history option modifies this by walking all commits and reporting a merge commit as "interesting" if it has _any_ parent that is not TREESAME. This tends to be an over-representation of important commits, especially in an environment where most merge commits are created by pull request completion. Suppose we have a commit A and we create a commit B on top that changes our file. When we merge the pull request, we create a merge commit M. If no one else changed the file in the first-parent history between M and A, then M will not be TREESAME to its first parent, but will be TREESAME to B. Thus, the simplified history will be "B". However, M will appear in the --full-history mode. However, suppose that a number of topics T1, T2, ..., Tn were created based on commits C1, C2, ..., Cn between A and M as follows: A----C1----C2--- ... ---Cn----M------P1---P2--- ... ---Pn \ \ \ \ / / / / \ \__.. \ \/ ..__T1 / Tn \ \__.. /\ ..__T2 / \_____________________B \____________________/ If the commits T1, T2, ... Tn did not change the file, then all of P1 through Pn will be TREESAME to their first parent, but not TREESAME to their second. This means that all of those merge commits appear in the --full-history view, with edges that immediately collapse into the lower history without introducing interesting single-parent commits. The --simplify-merges option was introduced to remove these extra merge commits. By noticing that the rewritten parents are reachable from their first parents, those edges can be simplified away. Finally, the commits now look like single-parent commits that are TREESAME to their "only" parent. Thus, they are removed and this issue does not cause issues anymore. However, this also ends up removing the commit M from the history view! Even worse, the --simplify-merges option requires walking the entire history before returning a single result. Many Git users are using Git alongside a Git service that provides code storage alongside a code review tool commonly called "Pull Requests" or "Merge Requests" against a target branch. When these requests are accepted and merged, they typically create a merge commit whose first parent is the previous branch tip and the second parent is the tip of the topic branch used for the request. This presents a valuable order to the parents, but also makes that merge commit slightly special. Users may want to see not only which commits changed a file, but which pull requests merged those commits into their branch. In the previous example, this would mean the users want to see the merge commit "M" in addition to the single- parent commit "C". Users are even more likely to want these merge commits when they use pull requests to merge into a feature branch before merging that feature branch into their trunk. In some sense, users are asking for the "first" merge commit to bring in the change to their branch. As long as the parent order is consistent, this can be handled with the following rule: Include a merge commit if it is not TREESAME to its first parent, but is TREESAME to a later parent. These merges look like the merge commits that would result from running "git pull <topic>" on a main branch. Thus, the option to show these commits is called "--show-pulls". This has the added benefit of showing the commits created by closing a pull request or merge request on any of the Git hosting and code review platforms. To test these options, extend the standard test example to include a merge commit that is not TREESAME to its first parent. It is surprising that that option was not already in the example, as it is instructive. In particular, this extension demonstrates a common issue with file history simplification. When a user resolves a merge conflict using "-Xours" or otherwise ignoring one side of the conflict, they create a TREESAME edge that probably should not be TREESAME. This leads users to become frustrated and complain that "my change disappeared!" In my experience, showing them history with --full-history and --simplify-merges quickly reveals the problematic merge. As mentioned, this option is expensive to compute. The --show-pulls option _might_ show the merge commit (usually titled "resolving conflicts") more quickly. Of course, this depends on the user having the correct parent order, which is backwards when using "git pull master" from a topic branch. There are some special considerations when combining the --show-pulls option with --simplify-merges. This requires adding a new PULL_MERGE object flag to store the information from the initial TREESAME comparisons. This helps avoid dropping those commits in later filters. This is covered by a test, including how the parents can be simplified. Since "struct object" has already ruined its 32-bit alignment by using 33 bits across parsed, type, and flags member, let's not make it worse. PULL_MERGE is used in revision.c with the same value (1u<<15) as REACHABLE in commit-graph.c. The REACHABLE flag is only used when writing a commit-graph file, and a revision walk using --show-pulls does not happen in the same process. Care must be taken in the future to ensure this remains the case. Update Documentation/rev-list-options.txt with significant details around this option. This requires updating the example in the History Simplification section to demonstrate some of the problems with TREESAME second parents. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 't/t6012-rev-list-simplify.sh')
-rwxr-xr-xt/t6012-rev-list-simplify.sh120
1 files changed, 120 insertions, 0 deletions
diff --git a/t/t6012-rev-list-simplify.sh b/t/t6012-rev-list-simplify.sh
index a10f0df..b6fa43a 100755
--- a/t/t6012-rev-list-simplify.sh
+++ b/t/t6012-rev-list-simplify.sh
@@ -154,4 +154,124 @@ test_expect_success '--full-diff is not affected by --parents' '
test_cmp expected actual
'
+#
+# Create a new history to demonstrate the value of --show-pulls
+# with respect to the subtleties of simplified history, --full-history,
+# and --simplify-merges.
+#
+# .-A---M-----C--N---O---P
+# / / \ \ \/ / /
+# I B \ R-'`-Z' /
+# \ / \/ /
+# \ / /\ /
+# `---X--' `---Y--'
+#
+# This example is explained in Documentation/rev-list-options.txt
+
+test_expect_success 'rebuild repo' '
+ rm -rf .git * &&
+ git init &&
+ git switch -c main &&
+
+ echo base >file &&
+ git add file &&
+ test_commit I &&
+
+ echo A >file &&
+ git add file &&
+ test_commit A &&
+
+ git switch -c branchB I &&
+ echo B >file &&
+ git add file &&
+ test_commit B &&
+
+ git switch main &&
+ test_must_fail git merge -m "M" B &&
+ echo A >file &&
+ echo B >>file &&
+ git add file &&
+ git merge --continue &&
+ note M &&
+
+ echo C >other &&
+ git add other &&
+ test_commit C &&
+
+ git switch -c branchX I &&
+ echo X >file &&
+ git add file &&
+ test_commit X &&
+
+ git switch -c branchR M &&
+ git merge -m R -Xtheirs X &&
+ note R &&
+
+ git switch main &&
+ git merge -m N R &&
+ note N &&
+
+ git switch -c branchY M &&
+ echo Y >y &&
+ git add y &&
+ test_commit Y &&
+
+ git switch -c branchZ C &&
+ echo Z >z &&
+ git add z &&
+ test_commit Z &&
+
+ git switch main &&
+ git merge -m O Z &&
+ note O &&
+
+ git merge -m P Y &&
+ note P
+'
+
+check_result 'X I' -- file
+check_result 'N R X I' --show-pulls -- file
+
+check_result 'P O N R X M B A I' --full-history --topo-order -- file
+check_result 'N R X M B A I' --simplify-merges --topo-order --show-pulls -- file
+check_result 'R X M B A I' --simplify-merges --topo-order -- file
+check_result 'N M A I' --first-parent -- file
+check_result 'N M A I' --first-parent --show-pulls -- file
+
+# --ancestry-path implies --full-history
+check_result 'P O N R M' --topo-order \
+ --ancestry-path A..HEAD -- file
+check_result 'P O N R M' --topo-order \
+ --show-pulls \
+ --ancestry-path A..HEAD -- file
+check_result 'P O N R M' --topo-order \
+ --full-history \
+ --ancestry-path A..HEAD -- file
+check_result 'R M' --topo-order \
+ --simplify-merges \
+ --ancestry-path A..HEAD -- file
+check_result 'N R M' --topo-order \
+ --simplify-merges --show-pulls \
+ --ancestry-path A..HEAD -- file
+
+test_expect_success 'log --graph --simplify-merges --show-pulls' '
+ cat >expect <<-\EOF &&
+ * N
+ * R
+ |\
+ | * X
+ * | M
+ |\ \
+ | * | B
+ | |/
+ * / A
+ |/
+ * I
+ EOF
+ git log --graph --pretty="%s" \
+ --simplify-merges --show-pulls \
+ -- file >actual &&
+ test_cmp expect actual
+'
+
test_done