Fix merge of encrypted files with conflicts

Fix transcrypt's handling of merges where encrypted
files have conflicting changes, a situation which
would lead to Git producing "merged" files with
conflict markers around partially- or fully-
encrypted content that cannot be sensibly merged
by a person.

The root problem is that git does not run the
smudge/textconv filter on all BASE, LOCAL, REMOTE
conflicting version files before attempting a
three-way merge.

This change adds:
- a merge driver script to pre-decrypt
  conflicting BASE, LOCAL, and REMOTE file
  versions then run git's internal `merge-file`
  command to merge the decrypted versions
- git repo settings to configure the merge driver
- recommendation to add the extra "merge=crypt"
  setting to .gitattribute definitions.

Author: jmurty

transcrypt

@@ -316,8 +316,18 @@ save_helper_scripts() {
 		fi
 	EOF
 
+	cat <<-'EOF' >"${GIT_DIR}/crypt/merge"
+		#!/usr/bin/env bash
+		# Decrypt BASE, LOCAL, and REMOTE versions of file being merged
+		echo "$(./.git/crypt/textconv $1)" > $1
+		echo "$(./.git/crypt/textconv $2)" > $2
+		echo "$(./.git/crypt/textconv $3)" > $3
+		# Use git's internal merge-file to merge the decrypted files
+		git merge-file --marker-size=$4 -L local -L base -L remote $2 $1 $3
+	EOF
+
 	# make scripts executable
-	for script in {clean,smudge,textconv}; do
+	for script in {clean,smudge,textconv,merge}; do
 		chmod 0755 "${GIT_DIR}/crypt/${script}"
 	done
 }
@@ -338,15 +348,18 @@ save_configuration() {
 		git config filter.crypt.clean '"$(git rev-parse --git-common-dir)"/crypt/clean %f'
 		git config filter.crypt.smudge '"$(git rev-parse --git-common-dir)"/crypt/smudge'
 		git config diff.crypt.textconv '"$(git rev-parse --git-common-dir)"/crypt/textconv'
+		git config merge.crypt.driver '"$(git rev-parse --git-common-dir)"/crypt/merge %O %A %B %L %P'
 	else
 		git config filter.crypt.clean '"$(git rev-parse --git-dir)"/crypt/clean %f'
 		git config filter.crypt.smudge '"$(git rev-parse --git-dir)"/crypt/smudge'
 		git config diff.crypt.textconv '"$(git rev-parse --git-dir)"/crypt/textconv'
+		git config merge.crypt.driver '"$(git rev-parse --git-dir)"/crypt/merge %O %A %B %L %P'
 	fi
 	git config filter.crypt.required 'true'
 	git config diff.crypt.cachetextconv 'true'
 	git config diff.crypt.binary 'true'
 	git config merge.renormalize 'true'
+	git config merge.crypt.name 'Merge transcrypt secret files'
 
 	# add a git alias for listing encrypted files
 	git config alias.ls-crypt "!git ls-files | git check-attr --stdin filter | awk 'BEGIN { FS = \":\" }; /crypt$/{ print \$1 }'"
@@ -374,6 +387,7 @@ clean_gitconfig() {
 	git config --remove-section transcrypt 2>/dev/null || true
 	git config --remove-section filter.crypt 2>/dev/null || true
 	git config --remove-section diff.crypt 2>/dev/null || true
+	git config --remove-section merge.crypt 2>/dev/null || true
 	git config --unset merge.renormalize
 
 	# remove the merge section if it's now empty
@@ -451,7 +465,7 @@ uninstall_transcrypt() {
 		clean_gitconfig
 
 		# remove helper scripts
-		for script in {clean,smudge,textconv}; do
+		for script in {clean,smudge,textconv,merge}; do
 			[[ ! -f "${GIT_DIR}/crypt/${script}" ]] || rm "${GIT_DIR}/crypt/${script}"
 		done
 		[[ ! -d "${GIT_DIR}/crypt" ]] || rmdir "${GIT_DIR}/crypt"
@@ -475,10 +489,10 @@ uninstall_transcrypt() {
 		# remove any defined crypt patterns in gitattributes
 		case $OSTYPE in
 		darwin*)
-			/usr/bin/sed -i '' '/filter=crypt diff=crypt[ \t]*$/d' "$GIT_ATTRIBUTES"
+			/usr/bin/sed -i '' '/filter=crypt diff=crypt merge=crypt[ \t]*$/d' "$GIT_ATTRIBUTES"
 			;;
 		linux*)
-			sed -i '/filter=crypt diff=crypt[ \t]*$/d' "$GIT_ATTRIBUTES"
+			sed -i '/filter=crypt diff=crypt merge=crypt[ \t]*$/d' "$GIT_ATTRIBUTES"
 			;;
 		esac
 
@@ -665,7 +679,7 @@ help() {
 		     a file in your repository, the file  will  be  transparently  encrypted
 		     once you stage and commit it:
 
-		         $ echo 'sensitive_file  filter=crypt diff=crypt' >> .gitattributes
+		         $ echo 'sensitive_file  filter=crypt diff=crypt merge=crypt' >> .gitattributes
 		         $ git add .gitattributes sensitive_file
 		         $ git commit -m 'Add encrypted version of a sensitive file'
 
@@ -851,7 +865,7 @@ fi
 # ensure the git attributes file exists
 if [[ ! -f $GIT_ATTRIBUTES ]]; then
 	mkdir -p "${GIT_ATTRIBUTES%/*}"
-	printf '#pattern  filter=crypt diff=crypt\n' >"$GIT_ATTRIBUTES"
+	printf '#pattern  filter=crypt diff=crypt merge=crypt\n' >"$GIT_ATTRIBUTES"
 fi
 
 printf 'The repository has been successfully configured by transcrypt.\n'