path: root/Documentation/maintainer
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authorJonathan Corbet <corbet@lwn.net>2022-03-25 15:51:35 -0600
committerJonathan Corbet <corbet@lwn.net>2022-03-30 13:45:15 -0600
commit022bb490c79799229ef467d9ccc3e5966001b0ae (patch)
treed05de6664c429dab38608b391bb0990e8fa7a43b /Documentation/maintainer
parentdocs: sphinx/requirements: Limit jinja2<3.1 (diff)
docs: Add a document on how to fix a messy diffstat
A branch with merges in will sometimes create a diffstat containing a lot of unrelated work at "git request-pull" time. Create a document based on Linus's advice (found in the links below) and add it to the maintainer manual in the hope of saving some wear on Linus's keyboard going forward. Link: https://lore.kernel.org/lkml/CAHk-=wg3wXH2JNxkQi+eLZkpuxqV+wPiHhw_Jf7ViH33Sw7PHA@mail.gmail.com/ Link: https://lore.kernel.org/lkml/CAHk-=wgXbSa8yq8Dht8at+gxb_idnJ7X5qWZQWRBN4_CUPr=eQ@mail.gmail.com/ Acked-by: Borislav Petkov <bp@suse.de> Reviewed-by: Akira Yokosawa <akiyks@gmail.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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diff --git a/Documentation/maintainer/index.rst b/Documentation/maintainer/index.rst
index f0a60435b124..3e03283c144e 100644
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@@ -12,6 +12,7 @@ additions to this manual.
+ messy-diffstat
diff --git a/Documentation/maintainer/messy-diffstat.rst b/Documentation/maintainer/messy-diffstat.rst
new file mode 100644
index 000000000000..c015f66d7621
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+++ b/Documentation/maintainer/messy-diffstat.rst
@@ -0,0 +1,96 @@
+.. SPDX-License-Identifier: GPL-2.0
+Handling messy pull-request diffstats
+Subsystem maintainers routinely use ``git request-pull`` as part of the
+process of sending work upstream. Normally, the result includes a nice
+diffstat that shows which files will be touched and how much of each will
+be changed. Occasionally, though, a repository with a relatively
+complicated development history will yield a massive diffstat containing a
+great deal of unrelated work. The result looks ugly and obscures what the
+pull request is actually doing. This document describes what is happening
+and how to fix things up; it is derived from The Wisdom of Linus Torvalds,
+found in Linus1_ and Linus2_.
+.. _Linus1: https://lore.kernel.org/lkml/CAHk-=wg3wXH2JNxkQi+eLZkpuxqV+wPiHhw_Jf7ViH33Sw7PHA@mail.gmail.com/
+.. _Linus2: https://lore.kernel.org/lkml/CAHk-=wgXbSa8yq8Dht8at+gxb_idnJ7X5qWZQWRBN4_CUPr=eQ@mail.gmail.com/
+A Git development history proceeds as a series of commits. In a simplified
+manner, mainline kernel development looks like this::
+ ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+If one wants to see what has changed between two points, a command like
+this will do the job::
+ $ git diff --stat --summary vN-rc2..vN-rc3
+Here, there are two clear points in the history; Git will essentially
+"subtract" the beginning point from the end point and display the resulting
+differences. The requested operation is unambiguous and easy enough to
+When a subsystem maintainer creates a branch and commits changes to it, the
+result in the simplest case is a history that looks like::
+ ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+ |
+ +-- c1 --- c2 --- ... --- cN
+If that maintainer now uses ``git diff`` to see what has changed between
+the mainline branch (let's call it "linus") and cN, there are still two
+clear endpoints, and the result is as expected. So a pull request
+generated with ``git request-pull`` will also be as expected. But now
+consider a slightly more complex development history::
+ ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+ | |
+ | +-- c1 --- c2 --- ... --- cN
+ | /
+ +-- x1 --- x2 --- x3
+Our maintainer has created one branch at vN-rc1 and another at vN-rc2; the
+two were then subsequently merged into c2. Now a pull request generated
+for cN may end up being messy indeed, and developers often end up wondering
+What is happening here is that there are no longer two clear end points for
+the ``git diff`` operation to use. The development culminating in cN
+started in two different places; to generate the diffstat, ``git diff``
+ends up having pick one of them and hoping for the best. If the diffstat
+starts at vN-rc1, it may end up including all of the changes between there
+and the second origin end point (vN-rc2), which is certainly not what our
+maintainer had in mind. With all of that extra junk in the diffstat, it
+may be impossible to tell what actually happened in the changes leading up
+to cN.
+Maintainers often try to resolve this problem by, for example, rebasing the
+branch or performing another merge with the linus branch, then recreating
+the pull request. This approach tends not to lead to joy at the receiving
+end of that pull request; rebasing and/or merging just before pushing
+upstream is a well-known way to get a grumpy response.
+So what is to be done? The best response when confronted with this
+situation is to indeed to do a merge with the branch you intend your work
+to be pulled into, but to do it privately, as if it were the source of
+shame. Create a new, throwaway branch and do the merge there::
+ ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+ | | |
+ | +-- c1 --- c2 --- ... --- cN |
+ | / | |
+ +-- x1 --- x2 --- x3 +------------+-- TEMP
+The merge operation resolves all of the complications resulting from the
+multiple beginning points, yielding a coherent result that contains only
+the differences from the mainline branch. Now it will be possible to
+generate a diffstat with the desired information::
+ $ git diff -C --stat --summary linus..TEMP
+Save the output from this command, then simply delete the TEMP branch;
+definitely do not expose it to the outside world. Take the saved diffstat
+output and edit it into the messy pull request, yielding a result that
+shows what is really going on. That request can then be sent upstream.