Better macOS git difftool + FileMerge integration

opendiff-git.sh

#!/bin/bash

# 
# Copyright 2022 David Hogan <david.q.hogan@gmail.com>
# 
# Redistribution and use in source and binary forms, with or without modification,
# are permitted provided that the following conditions are met:
# 
# 1. Redistributions of source code must retain the above copyright notice,
#    this list of conditions and the following disclaimer.
# 
# 2. Redistributions in binary form must reproduce the above copyright notice,
#    this list of conditions and the following disclaimer in the documentation
#    and/or other materials provided with the distribution.
# 
# 3. Neither the name of the copyright holder nor the names of its contributors
#    may be used to endorse or promote products derived from this software without
#    specific prior written permission.
# 
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
# IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
# INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
# LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
# OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
# OF THE POSSIBILITY OF SUCH DAMAGE.
# 

set -o nounset
set -o errexit

#
# FEATURES:
#
# - Present a tree view of (only) new, changed, or deleted files
# - Files edited within FileMerge save to working copy
# - If a file is edited, saved, then closed in FileMerge, then
#   reopening in the same FileMerge session will display the latest
#   changes
# - No unnecessary file copying
#
# INSTALL:
#
# 1. Place a copy of this script somewhere. If you don't put it in
#    the $PATH, then you'll need to put the full path to it in the
#	 'cmd' entry in the config below.
#
# 2. Edit .gitconfig in your home directory to include the following:
#
# [difftool]
#     prompt = false
# [difftool "opendiff-git"]
#     cmd = opendiff-git.sh \"$MERGED\" \"$LOCAL\"
# [diff]
#     tool = opendiff-git
#
# With these steps complete, running 'git difftool' from within
# a git working copy will open FileMerge via this script.
#
# IMPLEMENTATION NOTES:
#
# This command will be launched for each changed file. If we just
# invoke opendiff for each invocation, we slowly end up with
# one FileMerge window for each changed file. What we want is
# a single FileMerge window showing the folder structure
# of changed files, from which the user can choose which
# changes to review / revert etc.
#
# To achieve this, we create a single background process that
# waits for the parent process (git) to terminate, before using
# FileMerge to compare temporary left/right folder structures
# containing hardlinks to the original / changed files. Any changes
# will be saved into the git working copy.
#
# Unfortunately, FileMerge breaks a hardlink when saving. So while
# using hardlinks rather than copies is good for performance, they
# act like copies if you edit a file in FileMerge. We handle this
# by monitoring for changes in the files, and then replacing our
# (now effectively copied) temp version of the file with a hardlink
# to the new saved version. This can take a moment to happen so if
# you very quickly reopen a saved file during a FileMerge session you
# may see an out of date version.
#

GIT_FILE="$1"
ORIGINAL_FILE="$2"

# Start in the root checkout path
cd "$(git rev-parse --show-toplevel)"

# Find the topmost 'git' parent process id
export GIT_PID=$(ps -jc | awk '$10 == "git" { print $2; exit }')

#
# Attempt to create a folder for this overall git diff operation.
# We use the shared git parent process ID for this
#

export WORK_DIR_BASE=".opendiff-git-$GIT_PID"
export WORK_DIR="$(pwd)/$WORK_DIR_BASE"
export ORIGINAL_DIR="$WORK_DIR/original"
export MODIFIED_DIR="$WORK_DIR/modified"
export WATCH_PLIST_FILE="$WORK_DIR/watch.plist"
export ON_FILE_CHANGED_SCRIPT="$WORK_DIR/on_file_changed.sh"
export MERGE_DIR="$(pwd)"

function launch_filemerge {
	set -o errexit
	set -o nounset

	# Wait for the git process to exit. Can't use 'wait' because git is not a child process.
	while kill -0 $GIT_PID 2>/dev/null
	do
		sleep 0.1
	done

	# Finish off the plist file
	cat <<- HEREDOC >> "$WATCH_PLIST_FILE"
		        </array>
		    </dict>
		</plist>
	HEREDOC

	launchctl load "$WATCH_PLIST_FILE"

	# opendiff will block if we pipe its output ...
	opendiff "$ORIGINAL_DIR" "$MODIFIED_DIR" -merge "$MERGE_DIR" | cat > /dev/null

	launchctl unload "$WATCH_PLIST_FILE"

	# Cleanup
	rm -rf "$WORK_DIR"
}
export -f launch_filemerge

link_original_file() {
	set -o errexit
	set -o nounset

	original_file="$1"
	git_file="$2"

	mkdir -p "$ORIGINAL_DIR/$(dirname $git_file)"
	ln "$ORIGINAL_FILE" "$ORIGINAL_DIR/$git_file"
}

link_modified_file() {
	set -o errexit
	set -o nounset

	git_file="$1"

	mkdir -p "$MODIFIED_DIR/$(dirname $git_file)"
	ln "$MERGE_DIR/$git_file" "$MODIFIED_DIR/$git_file"
	echo "                <string>$MERGE_DIR/$git_file</string>" >> "$WATCH_PLIST_FILE"
}

if mkdir "$WORK_DIR" 2>/dev/null
then
	#
	# We are the first of possibly many diff-cmd invocations.
	# Launch a process that waits for GIT_PID and then opens FileMerge.
	#

	mkdir "$ORIGINAL_DIR"
	mkdir "$MODIFIED_DIR"

	#
	# To get around FileMerge breaking hardlinks on save,
	# we monitor for changes in merged paths and re-hardlink
	# them into our modified temp directory. This allows us
	# to see the changes if we reopen the diff.
	#
	# It's a bit convoluted .. FileMerge saves into the git
	# working copy. We then look for files that are no longer
	# hardlinks in our temp 'modified' folder, and replace them
	# with hardlinks back to the git working copy. It could be
	# a bit simpler but defensively I didn't want to have this
	# script modify files in the working copy.
	#
	# If we used the git repo as the right hand side of
	# FileMerge we'd have to look at tons of irrelevant files.
	# 'added to right'.
	#

	cat <<- HEREDOC > "$ON_FILE_CHANGED_SCRIPT"
		#!/bin/bash
		set -o errexit
		set -o nounset

		#
		# Each file in 'modified' that is no longer hardlinked
		# is an old copy of something that was saved in FileMerge.
		# This script replaces that with a new hardlink back to
		# the working copy.
		#

		cd "$MODIFIED_DIR"
		for changed_file in \$(find . -type f -links 1)
		do
		    if [ -f "$MERGE_DIR/\$changed_file" ]
		    then
		        rm "./\$changed_file"
		        ln "$MERGE_DIR/\$changed_file" "./\$changed_file"
		    fi
		done
	HEREDOC
	chmod +x "$ON_FILE_CHANGED_SCRIPT"

	# Begin creating the path watching launchctl plist file
	cat <<- HEREDOC > "$WATCH_PLIST_FILE"
		<?xml version=“1.0” encoding=“UTF-8”?>
		<!DOCTYPE plist PUBLIC “-//Apple//DTD PLIST 1.0//EN” “http://www.apple.com/DTDs/PropertyList-1.0.dtd”>
		<plist version=“1.0”>
		    <dict>
		        <key>Label</key>
		            <string>au.id.dqh.opendiff-git-$GIT_PID</string>
		        <key>ProgramArguments</key>
		        <array>
		            <string>$ON_FILE_CHANGED_SCRIPT</string>
		        </array>
		        <key>WatchPaths</key>
		        <array>
	HEREDOC

	#
	# Since git difftool won't invoke this script for new files, we
	# cheekily add them in ourselves.
	#

	for untracked_file in $(git ls-files --others --exclude-standard)
	do
		echo $untracked_file
		link_modified_file "$untracked_file"
	done

	nohup bash -c launch_filemerge </dev/null >/dev/null 2>&1 &
fi

echo $GIT_FILE

GIT_STATUS=$(git -c color.status=false status -s "$GIT_FILE" | awk '{ print $1 }')

# Merge cases with dupicate implementations
case "$GIT_STATUS" in
MM)
	GIT_STATUS=M
	;;
esac

case "$GIT_STATUS" in
A)
	# New file
	link_modified_file "$GIT_FILE"
	;;
M)
	# Modified file
	link_original_file "$ORIGINAL_FILE" "$GIT_FILE"
	link_modified_file "$GIT_FILE"
	;;
D)
	# Deleted file
	link_original_file "$ORIGINAL_FILE" "$GIT_FILE"
	;;
*)
	echo "Warning: unhandled git status -s '$GIT_STATUS' for '$GIT_FILE'"
	;;
esac