Re: [PATCH] commit: Add -f, --fixes <commit> option to add Fixes:line

From: Jim Hill
Date: Sun Oct 27 2013 - 20:49:44 EST

On 10/26/13 18:34, Josh Triplett wrote:
Linux Kernel ... "Fixes:" line ... containing an abbreviated commit hash

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This helps people (or automated tools) determine how far to backport

I beg pardon if I'm rehearsing an old debate, but it seems to me it would be better and worthwhile to bring more of git to bear by adding `reference` links as follows from considering this proposed sequence:

# ...G---B---... history-with-bug-at-B

Gprime=`git commit-tree --reference G`
Bprime=`git commit-tree --reference B -p $Gprime`

# ...G---B---... history-with-bug-at-B
# : : # <-- `:`'s are `reference` links
# G'--B' $Bprime is a mergeable cherry-pick for B

`reference` links have no enforced semantics. Teach all current logic to ignore them (fetch doesn't fetch through them, fsck doesn't care, etc.). Elaborating some of the good parts:

* If the author and committer data are left untouched when `commit-tree`'s tree and message arguments are defaulted, as above, to the referenced commit's tree and message, the resulting commit is unique.

* Bullet-proof cherry-pick creation becomes easy and idempotent:

git-make-cherry-pick() {
local picked=$1
set -- `git rev-list --parents $picked^!`
local parents
local parent
local p2
for parent; do
p2="$p2 -p `git commit-tree --reference $parent`"
git commit-tree --reference $picked $parents`

* Which makes the created commit id a fully-implemented _change-id_ for the referenced commit:

git merge $(git-make-cherry-pick $B)

can be done from anywhere, merge won't have to rely on patch-id's to detect cherry-picks done this way.

* A bugged commit gets fixed by fixing its reference commit and merging normally, worry-free:

...G---B ... -F Merge fix X for a bug in B
: : /
G'--B'---X X's commit message is the `Fixes:` equivalent

Bugfix commit X can be safely merged anywhere. Worst case, `git merge -s ours --no-commit X` and do whatever you would have done otherwise.

`merge` might usefully be updated to warn about merging from a commit with only a reference parent, I think merging from `G'` would probably be a mistake.

So, this is as far as I've gotten with this, is there reason to think it should or shouldn't be pursued?
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