Resolve go.mod, go.sum conflicts with mergetool

go_mod_sum_merge.md

About

Conflicts in go.mod, go.sum conflicts can be tedious to fix, the steps below help setup a custom git mergtool which helps fix those conflicts.

ref: golang/go#32485

Caveats: this is by no means an exhaustive solution.

Steps

1. Create an executable Git mergetool script in $PATH, /usr/local/sbin/gomod-mergetool

#!/bin/bash
set -e
fileMerge=$4 ## The file containing the conflict markers

echo "$fileMerge: purging conflict markers.."
awk '!/======/' $fileMerge | awk '!/>>>>>>/' | awk '!/<<<<<<</' > ${fileMerge}_union
mv ${fileMerge}_union ${fileMerge}
exit 0

2. Declare mergetool in ~/.gitconfig as gomod (or whatever else, make sure to reference it with the same name in step 4.)

[mergetool "gomod"]
    cmd = /usr/local/sbin/gomod-mergetool "$BASE" "$LOCAL" "$REMOTE" "$MERGED"
    trustExitCode = true

3. Now attempt the rebase (or another merge command) which introduces the conflicts in go.mod, go.sum

❯ git rebase -i main
hint: Waiting for your editor to close the file... vim-latex-live-preview: python required
Press ENTER or type command to continue
Auto-merging go.mod
CONFLICT (content): Merge conflict in go.mod
Auto-merging go.sum
CONFLICT (content): Merge conflict in go.sum
error: could not apply 61713a0... go: update deps
hint: Resolve all conflicts manually, mark them as resolved with
hint: "git add/rm <conflicted_files>", then run "git rebase --continue".
hint: You can instead skip this commit: run "git rebase --skip".
hint: To abort and get back to the state before "git rebase", run "git rebase --abort".
Could not apply 61713a0... go: update deps

4. Finally, run the gomod mergetool with go mod tidy

❯ git mergetool -t gomod go.mod go.sum && go mod tidy

5. If the above command returned with no errors, you can continue with the rebase/merge

❯ git status

...

Unmerged paths:
  (use "git restore --staged <file>..." to unstage)
  (use "git add <file>..." to mark resolution)
        both modified:   go.mod
        both modified:   go.sum

❯ git add go.mod go.sum && \ 
   git commit -m "fixes merge conflicts in go.mod, go.sum" && \
   git rebase --continue

Do you have an example of an actual tool that does this?

edit: if you mean a Go tool that would handle this, nope I don't - it'd be pretty useful if there was.

Do you have an example of an actual tool that does this?

you can find a bunch here https://github.com/git/git/tree/master/mergetools

Using https://git-scm.com/docs/gitattributes#_defining_a_custom_merge_driver might allow you to automate more of this. Instead of having to run git mergetool:

1. You can set this command as a custom merge driver in .git/config: https://git-scm.com/docs/gitattributes#_defining_a_custom_merge_driver
2. Create a .gitattributes for go.mod and go.sum that sets this driver to be used on conflicts: https://git-scm.com/docs/gitattributes#Documentation/gitattributes.txt-String-1

I'd probably have the driver call go mod tidy itself, and use the exit code to determine if the merge was successful or not.

@dnephin running go mod tidy in the script wont work as you need both files to be conflict-free before running it and the script executes on a single file. It would work if you run the script manually instead of using the merge tool.