git script to manually associate files in a conflicting merge when rename detection fails

git-merge-associate

#!/bin/bash
#
# Purpose: manually associate missed renames in merge conflicts
#
# Usage: git merge-associate <our-target> <base> <theirs>
#
# Example: After a failed rename detection A/a -> B/b which results
# in CONFLICT (delete/modify) for A/a and corresponding "deleted by us"
# messages in git status, the following invocation can be used to manually
# establish the link:
#
#     git merge-associate B/b :1:A/a :3:A/a
#
# or using a shell with brace expansion:
#
#     git merge-associate B/b :{1,3}:A/a
#
# This registers 
# - :1:A/a as   :1:B/b
# - HEAD:B/b as :2:B/b
# - :3:A/a as   :3:B/b
# and replaces B/b with a merge-conflict version using  "git checkout -m -- B/b".
# 
# After manual resolution of B/b and "git add B/b", A/a can be resolved by 
# "git rm A/a"
#

set -e
#set -x

get_tree()
{
    SPEC="${!1}"
    TREE=""
    
    if [[ -z "$SPEC" ]]; then
        TREE="EMPTY"
    elif [[ "$SPEC" =~ ^:(.): ]]; then
        TREE="INDEX:${BASH_REMATCH[1]}"
    elif [[ "$SPEC" =~ ^: ]]; then
        TREE="INDEX:0"
    elif [[ "$SPEC" =~ ^([^:]+): ]]; then
        TREE=${BASH_REMATCH[1]}
    else # no colon, considered as HEAD
        eval ${1}="HEAD:${!1}"
        TREE="HEAD"
    fi
    eval ${1}_TREE="$TREE"
    eval ${1}_BASENAME="${SPEC##*:}"
}

get_mode_and_sha()
{
    eval SPEC="\${$1}"
    eval TREE="\${$1_TREE}"
    eval BASENAME="\${$1_BASENAME}"
    RESULT_MODE=""
    RESULT_SHA=""
    case "$TREE" in
        EMPTY)
            RESULT_MODE="100644"
            RESULT_SHA="e69de29bb2d1d6434b8b29ae775ad8c2e48c5391"
        ;;
        INDEX:*)
            [[ "$TREE" =~ :(.) ]]
            REQ_STAGE=${BASH_REMATCH[1]}
            while read MODE SHA STAGE FILEPATH; do
                if ! [ "$STAGE" = "$REQ_STAGE" ]; then
                    continue
                fi
                RESULT_MODE="$MODE"
                RESULT_SHA="$SHA"
                break
            done < <(git ls-files --stage "$BASENAME")
        ;;
        *)
            while read MODE TYPE SHA FILEPATH; do
                RESULT_MODE="$MODE"
                RESULT_SHA="$SHA"
            done < <(git ls-tree "$TREE" "$BASENAME")
        ;;
    esac
    if [ -z "$RESULT_MODE" ] ; then
        echo "Could not determine mode for $SPEC"
        exit 1
    fi
    if [ -z "$RESULT_SHA" ] ; then
        echo "Could not determine SHA1 for $SPEC"
        exit 1
    fi
    eval $1_SHA="$RESULT_SHA"
    eval $1_MODE="$RESULT_MODE"
}

if [ "$#" -ne 3 ]; then
    echo "Usage: git merge-associate <our-target> <base> <theirs>"
    exit 1
fi

TARGET="$1"
BASE="$2"
THEIRS="$3"

for VAR in TARGET BASE THEIRS; do
    get_tree $VAR
    get_mode_and_sha $VAR
done

git update-index --index-info <<EOI
000000 0000000000000000000000000000000000000000 0	$TARGET_BASENAME
$BASE_MODE $BASE_SHA 1	$TARGET_BASENAME
$TARGET_MODE $TARGET_SHA 2	$TARGET_BASENAME
$THEIRS_MODE $THEIRS_SHA 3	$TARGET_BASENAME
EOI

git checkout -m -- $TARGET_BASENAME

It wasn’t awfully obvious to me as I was working out how this script worked, that you can use commit ids as well as branch names. In my case, the file similarity was low enough that there were no useful :1: or :2: references but I was able to patch things up using:

$ git merge-assist target_file_name `git merge-base HEAD other_branch_name`:original_file_name other_branch_name:other_file_name

A little hard to automate since I generally had to manually look up the file names in all of these branches, but still a very workable solution.

This script has saved me a ton of time. Thank you.

Does not work with modern git citing:
fatal: malformed index info 000000 0000000000000000000000000000000000000000 0 XXXX/xxx.file