Differences

This shows you the differences between two versions of the page.

--- git_bisect [2019/02/26 21:26] – [Bisection with feature branches (WARNING: INCOMPLETE!)] rpjday
+++ git_bisect [2019/02/26 21:54] (current) – [Bisection while ignoring feature branches] rpjday
@@ Line 228: / Line 228: @@
 ===== Bisection while ignoring feature branches =====
-If we have two commits directly on the ''master'' branch, and we try to bisect using them as good and bad endpoints, it's entirely possible that we could end up processing many, many commits based on the number of feature branches that were merged into ''master'' between the two commits.
+NOTE: This explanation is based on the writeup [[https://blog.smart.ly/2015/02/03/git-bisect-debugging-with-feature-branches/|here]].
-Rather than bisecting completely on lengthy feature branches, we can restrict the search to just those commits where feature branches were merged into ''master'', so the ultimate goal is to say, "//That// merged feature branch is the one that caused the problem," possibly inspiring another round of more focused bisection.
+If we have two commits directly on the ''master'' branch, and we try to bisect using those two commits as good and bad endpoints, it's entirely possible that we could end up processing many, many, many commits based on the number of (potentially lengthy) feature branches that were merged into ''master'' between the two commits.
-Let's demonstrate this using the Linux kernel source repository, defining ''v4.18'' as the good commit, and ''v4.19'' as the bad commit.
+Rather than bisecting completely on those lengthy feature branches, we can restrict the search to just those commits where feature branches were merged into ''master'', so the ultimate goal is to say, "//That// merged feature branch is the one that caused the problem," possibly inspiring another round of more focused bisection. (This also considers the possibility of a stand-alone commit directly on the ''master'' branch as well.) Let's demonstrate this using the Linux kernel source repository, defining ''v4.18'' as the good commit, and ''v4.19'' as the bad commit.
-First, let's count the number of commits we would need to process if we used basic bisection on those two endpoints:
+First, let's count the number of commits we would normally need to process if we used basic bisection based on those two endpoints:
 <code>
@@ Line 242: / Line 242: @@
 </code>
-The basis for ignoring all of the commits on feature branches is to generate a list of commits to //ignore//, and feed that list to ''git bisect skip'' to cut down on the work.
+That's a lot of commits so let's see if we can reduce that number. The basis for ignoring all of the commits on feature branches is to generate a list of commits to //ignore//, and feed that list to ''git bisect skip'' to cut down on the work. The first step is to generate a list of merge commits between the two endpoints
+<code>
+$ git rev-list v4.18..v4.19 --merges | wc -l
+
+$
+</code>
+So there are 1161 merge commits in the testable range, but we need to further disqualify merge commits exclusively on feature branches, so let's restrict ourselves to just the //first parents// of those merge commits, which guarantees that all of those merge commits are on the ''master'' branch:
+<code>
+$ git rev-list v4.18..v4.19 --merges --first-parent | wc -l
+
+$
+</code>
+What the above set of merge commits represents are those merge commits for which we want to totally ignore all commits on the feature branch that contributed to each of those commits. For each merge commit above (call it ''rev''), we list all commits reachable from the second parent (the one on the feature branch, ''rev^2''), but not reachable from the parent on the master branch (''rev^''), and pass //all// such commits to ''git bisect skip''.
+<code>
+for rev in $(git rev-list v4.18..v4.19 --merges --first-parent); do
+  git rev-list $rev^2 --not $rev^
+done | xargs git bisect skip
+</code>
+Given that our basic search would have processed 15204 commits, we can count how many commits will now be pruned from the bisection:
+<code>
+for rev in $(git rev-list v4.18..v4.19 --merges --first-parent); do
+  git rev-list $rev^2 --not $rev^
+done | sort | uniq | wc -l
+
+$
+</code>