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diff --git a/Documentation/maintainer/messy-diffstat.rst b/Documentation/maintainer/messy-diffstat.rst new file mode 100644 index 000000000..c015f66d7 --- /dev/null +++ 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 +understand. + +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 +why. + +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. |