97 lines
4.7 KiB
ReStructuredText
97 lines
4.7 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=====================================
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Handling messy pull-request diffstats
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=====================================
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Subsystem maintainers routinely use ``git request-pull`` as part of the
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process of sending work upstream. Normally, the result includes a nice
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diffstat that shows which files will be touched and how much of each will
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be changed. Occasionally, though, a repository with a relatively
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complicated development history will yield a massive diffstat containing a
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great deal of unrelated work. The result looks ugly and obscures what the
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pull request is actually doing. This document describes what is happening
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and how to fix things up; it is derived from The Wisdom of Linus Torvalds,
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found in Linus1_ and Linus2_.
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.. _Linus1: https://lore.kernel.org/lkml/CAHk-=wg3wXH2JNxkQi+eLZkpuxqV+wPiHhw_Jf7ViH33Sw7PHA@mail.gmail.com/
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.. _Linus2: https://lore.kernel.org/lkml/CAHk-=wgXbSa8yq8Dht8at+gxb_idnJ7X5qWZQWRBN4_CUPr=eQ@mail.gmail.com/
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A Git development history proceeds as a series of commits. In a simplified
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manner, mainline kernel development looks like this::
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... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
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If one wants to see what has changed between two points, a command like
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this will do the job::
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$ git diff --stat --summary vN-rc2..vN-rc3
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Here, there are two clear points in the history; Git will essentially
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"subtract" the beginning point from the end point and display the resulting
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differences. The requested operation is unambiguous and easy enough to
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understand.
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When a subsystem maintainer creates a branch and commits changes to it, the
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result in the simplest case is a history that looks like::
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... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
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+-- c1 --- c2 --- ... --- cN
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If that maintainer now uses ``git diff`` to see what has changed between
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the mainline branch (let's call it "linus") and cN, there are still two
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clear endpoints, and the result is as expected. So a pull request
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generated with ``git request-pull`` will also be as expected. But now
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consider a slightly more complex development history::
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... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
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| +-- c1 --- c2 --- ... --- cN
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| /
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+-- x1 --- x2 --- x3
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Our maintainer has created one branch at vN-rc1 and another at vN-rc2; the
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two were then subsequently merged into c2. Now a pull request generated
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for cN may end up being messy indeed, and developers often end up wondering
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why.
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What is happening here is that there are no longer two clear end points for
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the ``git diff`` operation to use. The development culminating in cN
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started in two different places; to generate the diffstat, ``git diff``
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ends up having pick one of them and hoping for the best. If the diffstat
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starts at vN-rc1, it may end up including all of the changes between there
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and the second origin end point (vN-rc2), which is certainly not what our
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maintainer had in mind. With all of that extra junk in the diffstat, it
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may be impossible to tell what actually happened in the changes leading up
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to cN.
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Maintainers often try to resolve this problem by, for example, rebasing the
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branch or performing another merge with the linus branch, then recreating
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the pull request. This approach tends not to lead to joy at the receiving
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end of that pull request; rebasing and/or merging just before pushing
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upstream is a well-known way to get a grumpy response.
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So what is to be done? The best response when confronted with this
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situation is to indeed to do a merge with the branch you intend your work
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to be pulled into, but to do it privately, as if it were the source of
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shame. Create a new, throwaway branch and do the merge there::
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... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
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| +-- c1 --- c2 --- ... --- cN |
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| / | |
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+-- x1 --- x2 --- x3 +------------+-- TEMP
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The merge operation resolves all of the complications resulting from the
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multiple beginning points, yielding a coherent result that contains only
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the differences from the mainline branch. Now it will be possible to
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generate a diffstat with the desired information::
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$ git diff -C --stat --summary linus..TEMP
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Save the output from this command, then simply delete the TEMP branch;
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definitely do not expose it to the outside world. Take the saved diffstat
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output and edit it into the messy pull request, yielding a result that
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shows what is really going on. That request can then be sent upstream.
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