97 lines
4.7 KiB
ReStructuredText
97 lines
4.7 KiB
ReStructuredText
|
.. 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.
|