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| 3 | =====================================
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| 4 | Handling messy pull-request diffstats
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| 5 | =====================================
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| 6 |
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| 7 | Subsystem maintainers routinely use ``git request-pull`` as part of the
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| 8 | process of sending work upstream. Normally, the result includes a nice
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| 9 | diffstat that shows which files will be touched and how much of each will
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| 10 | be changed. Occasionally, though, a repository with a relatively
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| 11 | complicated development history will yield a massive diffstat containing a
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| 12 | great deal of unrelated work. The result looks ugly and obscures what the
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| 13 | pull request is actually doing. This document describes what is happening
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| 14 | and how to fix things up; it is derived from The Wisdom of Linus Torvalds,
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| 15 | found in Linus1_ and Linus2_.
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| 16 |
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| 17 | .. _Linus1: https://lore.kernel.org/lkml/CAHk-=wg3wXH2JNxkQi+eLZkpuxqV+wPiHhw_Jf7ViH33Sw7PHA@mail.gmail.com/
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| 18 | .. _Linus2: https://lore.kernel.org/lkml/CAHk-=wgXbSa8yq8Dht8at+gxb_idnJ7X5qWZQWRBN4_CUPr=eQ@mail.gmail.com/
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| 19 |
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| 20 | A Git development history proceeds as a series of commits. In a simplified
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| 21 | manner, mainline kernel development looks like this::
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| 22 |
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| 23 | ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
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| 24 |
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| 25 | If one wants to see what has changed between two points, a command like
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| 26 | this will do the job::
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| 27 |
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| 28 | $ git diff --stat --summary vN-rc2..vN-rc3
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| 29 |
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| 30 | Here, there are two clear points in the history; Git will essentially
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| 31 | "subtract" the beginning point from the end point and display the resulting
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| 32 | differences. The requested operation is unambiguous and easy enough to
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| 33 | understand.
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| 34 |
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| 35 | When a subsystem maintainer creates a branch and commits changes to it, the
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| 36 | result in the simplest case is a history that looks like::
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| 37 |
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| 38 | ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
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| 39 | |
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| 40 | +-- c1 --- c2 --- ... --- cN
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| 41 |
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| 42 | If that maintainer now uses ``git diff`` to see what has changed between
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| 43 | the mainline branch (let's call it "linus") and cN, there are still two
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| 44 | clear endpoints, and the result is as expected. So a pull request
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| 45 | generated with ``git request-pull`` will also be as expected. But now
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| 46 | consider a slightly more complex development history::
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| 47 |
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| 48 | ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
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| 49 | | |
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| 50 | | +-- c1 --- c2 --- ... --- cN
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| 51 | | /
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| 52 | +-- x1 --- x2 --- x3
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| 53 |
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| 54 | Our maintainer has created one branch at vN-rc1 and another at vN-rc2; the
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| 55 | two were then subsequently merged into c2. Now a pull request generated
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| 56 | for cN may end up being messy indeed, and developers often end up wondering
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| 57 | why.
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| 58 |
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| 59 | What is happening here is that there are no longer two clear end points for
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| 60 | the ``git diff`` operation to use. The development culminating in cN
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| 61 | started in two different places; to generate the diffstat, ``git diff``
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| 62 | ends up having pick one of them and hoping for the best. If the diffstat
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| 63 | starts at vN-rc1, it may end up including all of the changes between there
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| 64 | and the second origin end point (vN-rc2), which is certainly not what our
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| 65 | maintainer had in mind. With all of that extra junk in the diffstat, it
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| 66 | may be impossible to tell what actually happened in the changes leading up
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| 67 | to cN.
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| 68 |
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| 69 | Maintainers often try to resolve this problem by, for example, rebasing the
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| 70 | branch or performing another merge with the linus branch, then recreating
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| 71 | the pull request. This approach tends not to lead to joy at the receiving
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| 72 | end of that pull request; rebasing and/or merging just before pushing
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| 73 | upstream is a well-known way to get a grumpy response.
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| 74 |
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| 75 | So what is to be done? The best response when confronted with this
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| 76 | situation is to indeed to do a merge with the branch you intend your work
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| 77 | to be pulled into, but to do it privately, as if it were the source of
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| 78 | shame. Create a new, throwaway branch and do the merge there::
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| 79 |
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| 80 | ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
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| 81 | | | |
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| 82 | | +-- c1 --- c2 --- ... --- cN |
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| 83 | | / | |
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| 84 | +-- x1 --- x2 --- x3 +------------+-- TEMP
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| 85 |
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| 86 | The merge operation resolves all of the complications resulting from the
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| 87 | multiple beginning points, yielding a coherent result that contains only
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| 88 | the differences from the mainline branch. Now it will be possible to
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| 89 | generate a diffstat with the desired information::
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| 90 |
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| 91 | $ git diff -C --stat --summary linus..TEMP
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| 92 |
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| 93 | Save the output from this command, then simply delete the TEMP branch;
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| 94 | definitely do not expose it to the outside world. Take the saved diffstat
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| 95 | output and edit it into the messy pull request, yielding a result that
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| 96 | shows what is really going on. That request can then be sent upstream.
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