Re: [RFC] Rebasing merges: a jorney to the ultimate solution (Road Clear)

Hi Sergey,

On 01/03/2018 06:39, Sergey Organov wrote:

No problem, that`s why we`re discussing it, and I`m glad we`re aligned now, so we can move forward :)

You`re welcome, and I am enjoying it :)

Yes, this might be it...! ;)

To prove myself it works, I`ve assembled a pretty crazy `-s ours` merge interactive rebase scenario, and it seems this passes the test, ticking all the check boxes (I could think of) :P

Let`s see our starting situation:

 (0) ---X8--B2'--X9 (master)
        |\
        | A1---A2---A3 (A)
        |             \
        |              M (topic)
        |             /
        \-B1---B2---B3 (B)

Here, merge commit M is done with `-s ours` (obsoleting branch "B"), plus amended to make it an "evil merge", where a commit B2 from obsoleted branch "B" is cherry picked to "master".

Now, we want to rebase "topic" (M) onto updated "master" (X9), but to make things more interesting, we`ll do it interactively, with some amendments, drops, additions and even more cherry-picks!

This is what the final result looks like:

 (1) ---X8--B2'--X9 (master)
                 |\
                 | A12--A2'---B3' (A)
                 |             \
                 |              M' (topic)
                 |             /
                 \-B1'--B3'---B4  (B)

During interactive rebase, on branch "A", we amended A1 into A12, dropped A3 and cherry-picked B3. On branch "B", B4 is added, B2' being omitted automatically as already present in "master".

So... In comparison to original merge commit M, rebased merge commit M' is expected to:

 - Add X9, from updated "master"
 - Have A1 changed to A12, due to A12 commit amendment
 - Keep A2, rebased as A2'
 - Remove A3, due to dropped A3 commit
 - Keep amendment from original (evil) merge commit M
 - Miss B1' like M does B, due to original `-s ours` merge strategy
 - Add B2, cherry-picked as B2' into "master"
 - Add B3, cherry-picked as B3' into "A"
 - Add B4, added to "B"
 - Most important, provide safety mechanism to "fail loud", being 
   aware of non-trivial things going on, allowing to stop for user 
   inspection/decision

There, I hope I didn`t miss any expectation. And, it _seems_ to work exactly as expected :D

Not to leave this to imagination only, and hopefully helping others to get to speed and possibly discuss this, pointing to still possible flaws, I`m adding a demo script[1], showing how this exact example works.

Note that script _is_ coined to avoid rebase conflicts, as they`re not currently important for the point to be made here.

In real life, except for usual possibility for conflicts during commit rebasing, we might experience _three_ possible conflict situations once "rebased" merge itself is to be created - two when rebasing each of temporary merge helper commits, and one on the "rebased" merge itself. This is something where we might think about user experience, not introducing (too much) confusion...

Regards, Buga

[1] Demonstration script: -- >8 -- #!/bin/sh

# rm -rf ./.git # rm -f ./test.txt

git init

touch ./test.txt git add -- test.txt

# prepare repository
for i in {1..8}
do
	echo X$i >>test.txt
	git commit -am "X$i"
done

# prepare branch A git checkout -b A sed -i '2iA1' test.txt git commit -am "A1" sed -i '4iA2' test.txt git commit -am "A2" sed -i '6iA3' test.txt git commit -am "A3"

# prepare branch B git checkout -b B master sed -i '5iB1' test.txt git commit -am "B1" sed -i '7iB2' test.txt git commit -am "B2" sed -i '9iB3' test.txt git commit -am "B3"

git checkout -b topic A git merge -s ours --no-commit B # merge A and B with `-s ours` sed -i '8iM' test.txt # amend merge commit ("evil merge") git commit -am "M" git tag original-merge

# master moves on... git checkout master git cherry-pick B^ # cherry-pick B2 into master sed -i "1iX9" test.txt # add X9 git commit -am "X9"

# (0) ---X8--B2'--X9 (master) # |\ # | A1---A2---A3 (A) # | \ # | M (topic) # | / # \-B1---B2---B3 (B)

# simple/naive demonstration of proposed merge rebasing logic # using described new approach, preserving merge commit manual # amendments, testing `-s ours` merge with cherry-picking from # obsoleted part, but still respecting interactively rebased # added/modified/dropped/cherry-picked commits :)

git checkout A git cherry-pick -m1 original-merge # prepare temporary helper commit U1 git tag U1 git reset --hard HEAD^^ # drop U1 and A3 from A sed -i '/A1/c\A12' test.txt # amend A1 to A12 git commit -a --amend --no-edit git rebase master # rebase A onto master git cherry-pick B # cherry-pick B3 into A git cherry-pick U1 # "rebase" temporary helper commit U1 git tag U1-prime

git checkout B git cherry-pick -m2 original-merge # prepare temporary helper commit U2 git tag U2 git reset --hard HEAD^ # drop U2 from B git rebase master # rebase B onto master sed -i '12iB4' test.txt # add B4 git commit -am "B4" git cherry-pick U2 # "rebase" temporary helper commit U2 git tag U2-prime

git branch -f topic A git checkout topic # merge rebased temporary commits U1' and U2', # using original merge commit as a merge base, # producing "rebased" merge commit M' git read-tree -m --aggressive original-merge A B git merge-index -o git-merge-one-file -a

# recognize complex stuff going on during rebasing merge commit,
# allowing user to inspect result, edit, and continue or abort
git diff --quiet U1-prime U2-prime
if test $? -ne 0
then
	# PLACEHOLDER
	# chance to inspect result, like:
	git diff original-merge
	# edit if needed, continue or abort
fi

# drop rebased temporary commits U1' and U2' git branch -f A A^ git branch -f B B^

# record branches A and B as parents of "rebased" merge commit M', # updating topic branch git update-ref refs/heads/topic "$(git show -s --format=%B original-merge | git commit-tree "$(git write-tree)" -p "$(git rev-parse A)" -p "$(git rev-parse B)")" git tag angel-merge

# (1) ---X8--B2'--X9 (master) # |\ # | A12--A2'---B3' (A) # | \ # | M' (topic) # | / # \-B1'--B3'---B4 (B)

# show resulting graph # echo # git log --all --decorate --oneline --graph

# in comparison to original merge commit M, rebased merge commit # M' is expected to: # # - Add X9, from updated "master" # - Have A1 changed to A12, due to A12 commit amendment # - Keep A2, rebased as A2' # - Remove A3, due to dropped A3 commit # - Keep amendment from original (evil) merge commit M # - Miss B1' like M does B, due to original `-s ours` merge strategy # - Add B2, cherry-picked as B2' into "master" # - Add B3, cherry-picked as B3' into "A" # - Add B4, added to "B" # # echo # echo 'diff original-merge angel-merge:' # git diff original-merge angel-merge