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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. |