1 .. SPDX-License-Identifier: GPL-2.0 2 3 ===================================== 4 Handling messy pull-request diffstats 5 ===================================== 6 7 Subsystem maintainers routinely use ``git request-pull`` as part of the 8 process of sending work upstream. Normally, the result includes a nice 9 diffstat that shows which files will be touched and how much of each will 10 be changed. Occasionally, though, a repository with a relatively 11 complicated development history will yield a massive diffstat containing a 12 great deal of unrelated work. The result looks ugly and obscures what the 13 pull request is actually doing. This document describes what is happening 14 and how to fix things up; it is derived from The Wisdom of Linus Torvalds, 15 found in Linus1_ and Linus2_. 16 17 .. _Linus1: https://lore.kernel.org/lkml/CAHk-=wg3wXH2JNxkQi+eLZkpuxqV+wPiHhw_Jf7ViH33Sw7PHA@mail.gmail.com/ 18 .. _Linus2: https://lore.kernel.org/lkml/CAHk-=wgXbSa8yq8Dht8at+gxb_idnJ7X5qWZQWRBN4_CUPr=eQ@mail.gmail.com/ 19 20 A Git development history proceeds as a series of commits. In a simplified 21 manner, mainline kernel development looks like this:: 22 23 ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN 24 25 If one wants to see what has changed between two points, a command like 26 this will do the job:: 27 28 $ git diff --stat --summary vN-rc2..vN-rc3 29 30 Here, there are two clear points in the history; Git will essentially 31 "subtract" the beginning point from the end point and display the resulting 32 differences. The requested operation is unambiguous and easy enough to 33 understand. 34 35 When a subsystem maintainer creates a branch and commits changes to it, the 36 result in the simplest case is a history that looks like:: 37 38 ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN 39 | 40 +-- c1 --- c2 --- ... --- cN 41 42 If that maintainer now uses ``git diff`` to see what has changed between 43 the mainline branch (let's call it "linus") and cN, there are still two 44 clear endpoints, and the result is as expected. So a pull request 45 generated with ``git request-pull`` will also be as expected. But now 46 consider a slightly more complex development history:: 47 48 ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN 49 | | 50 | +-- c1 --- c2 --- ... --- cN 51 | / 52 +-- x1 --- x2 --- x3 53 54 Our maintainer has created one branch at vN-rc1 and another at vN-rc2; the 55 two were then subsequently merged into c2. Now a pull request generated 56 for cN may end up being messy indeed, and developers often end up wondering 57 why. 58 59 What is happening here is that there are no longer two clear end points for 60 the ``git diff`` operation to use. The development culminating in cN 61 started in two different places; to generate the diffstat, ``git diff`` 62 ends up having pick one of them and hoping for the best. If the diffstat 63 starts at vN-rc1, it may end up including all of the changes between there 64 and the second origin end point (vN-rc2), which is certainly not what our 65 maintainer had in mind. With all of that extra junk in the diffstat, it 66 may be impossible to tell what actually happened in the changes leading up 67 to cN. 68 69 Maintainers often try to resolve this problem by, for example, rebasing the 70 branch or performing another merge with the linus branch, then recreating 71 the pull request. This approach tends not to lead to joy at the receiving 72 end of that pull request; rebasing and/or merging just before pushing 73 upstream is a well-known way to get a grumpy response. 74 75 So what is to be done? The best response when confronted with this 76 situation is to indeed to do a merge with the branch you intend your work 77 to be pulled into, but to do it privately, as if it were the source of 78 shame. Create a new, throwaway branch and do the merge there:: 79 80 ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN 81 | | | 82 | +-- c1 --- c2 --- ... --- cN | 83 | / | | 84 +-- x1 --- x2 --- x3 +------------+-- TEMP 85 86 The merge operation resolves all of the complications resulting from the 87 multiple beginning points, yielding a coherent result that contains only 88 the differences from the mainline branch. Now it will be possible to 89 generate a diffstat with the desired information:: 90 91 $ git diff -C --stat --summary linus..TEMP 92 93 Save the output from this command, then simply delete the TEMP branch; 94 definitely do not expose it to the outside world. Take the saved diffstat 95 output and edit it into the messy pull request, yielding a result that 96 shows what is really going on. That request can then be sent upstream.
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