rebase -i: cross-validate rebase--helper's functionality

Inspired by GitHub's "Scientist" library (and by this developer's
validation of the builtin replacement for RCS merge, back in the days),
we run things *twice*: once with the original shell script code, and
once with the new rebase--helper method.

Note that this will take a toll on users for now: merge conflicts and
rewords have to be addressed twice, too, and care needs to be taken that
exec commands (and copying commit notes) can be performed twice; If not,
the environment variable GIT_USE_REBASE_HELPER=true needs to be set (or,
in case of breakage, it needs to be set to "false" before rerunning the
rebase).

Please note that for the same reason, some tests fail with merge
conflicts, or specifically test amending commits via "reword" or "edit"
commands. Naturally, these tests cannot be cross-validated easily,
because the same "interactive" commands would have to be run again. So
let's just run the rebase--helper method to verify that these tests
still pass.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

git-rebase--interactive.sh

@@ -1,3 +1,6 @@
+test ! -z "$GIT_USE_REBASE_HELPER" ||
+GIT_USE_REBASE_HELPER=cross-validate
+
 # This shell script fragment is sourced by git-rebase to implement
 # its interactive mode.  "git rebase --interactive" makes it easy
 # to fix up commits in the middle of a series and rearrange commits.
@@ -676,6 +679,30 @@ do_next () {
 	fi &&
 	warn "Successfully rebased and updated $head_name."
 
+	# Run the same shebang using rebase--helper
+	if test cross-validate = "$GIT_USE_REBASE_HELPER" &&
+		test -d "$GIT_DIR"/saved-rebase-merge
+	then
+		rm -rf "$GIT_DIR"/shell-rebase-merge
+
+		echo "Cross-validating with rebase--helper" >&2
+		mv "$GIT_DIR"/rebase-merge "$GIT_DIR"/shell-rebase-merge &&
+		cp -R "$GIT_DIR"/saved-rebase-merge "$GIT_DIR"/rebase-merge &&
+		echo 1 >"$GIT_DIR"/saved-rebase-merge/cross-validating &&
+		git rev-parse HEAD >"$GIT_DIR"/saved-rebase-merge/shell-head &&
+		output git checkout \
+			$(cat "$GIT_DIR"/saved-rebase-merge/start-head) &&
+		case "$(cat "$GIT_DIR"/rebase-merge/head-name)" in
+		refs/*) git update-ref -m "re-run rebase" \
+				$(cat "$GIT_DIR"/rebase-merge/head-name) \
+				$(cat "$GIT_DIR"/rebase-merge/orig-head)
+			;;
+		esac &&
+		git rebase--helper ${force_rebase:+--no-ff} --continue ||
+		die "Builtin rebase--helper failed"
+		check_rebase__helper
+	fi
+
 	return 1 # not failure; just to break the do_rest loop
 }
 
@@ -743,11 +770,27 @@ transform_todo_ids () {
 }
 
 expand_todo_ids() {
+	cp "$todo" "$todo".transform
+	transform_todo_ids
+	mv "$todo" "$todo".transformed
+	cp "$todo".transform "$todo"
 	git rebase--helper --expand-sha1s
+	if ! git diff --no-index -w -- "$todo" "$todo.transformed"
+	then
+		die "rebase--helper failed to expand todo IDs"
+	fi
 }
 
 collapse_todo_ids() {
+	cp "$todo" "$todo".transform
+	transform_todo_ids --short
+	mv "$todo" "$todo".transformed
+	cp "$todo".transform "$todo"
 	git rebase--helper --shorten-sha1s
+	if ! git diff --no-index -w -- "$todo" "$todo.transformed"
+	then
+		die "rebase--helper failed to collapse todo IDs"
+	fi
 }
 
 # Rearrange the todo list that has both "pick sha1 msg" and
@@ -1032,6 +1075,23 @@ check_todo_list () {
 	fi
 }
 
+check_rebase__helper () {
+	echo "Cross-validating rebase--helper's result" >&2
+	test -f "$GIT_DIR"/saved-rebase-merge/cross-validating ||
+	return 0
+	rm -rf "$GIT_DIR"/before-rebase-merge
+	mv "$GIT_DIR"/saved-rebase-merge "$GIT_DIR"/before-rebase-merge
+	start_head=$(cat "$GIT_DIR"/before-rebase-merge/start-head)
+	shell_head=$(cat "$GIT_DIR"/before-rebase-merge/shell-head)
+	git rev-parse HEAD >"$GIT_DIR"/before-rebase-merge/builtin-head
+	git log --raw $start_head..$shell_head |
+	sed "s/^commit .*$/commit/" >"$GIT_DIR"/before-rebase-merge/shell_log
+	git log --raw $start_head.. |
+	sed "s/^commit .*$/commit/" >"$GIT_DIR"/before-rebase-merge/builtin_log
+	cmp "$GIT_DIR"/before-rebase-merge/shell_log "$GIT_DIR"/before-rebase-merge/builtin_log ||
+	die "Builtin rebase--helper produced different result"
+}
+
 # The whole contents of this file is run by dot-sourcing it from
 # inside a shell function.  It used to be that "return"s we see
 # below were not inside any function, and expected to return
@@ -1045,10 +1105,21 @@ git_rebase__interactive () {
 
 case "$action" in
 continue)
-	if test ! -d "$rewritten"
+	if test -f "$GIT_DIR"/saved-rebase-merge/cross-validating
+	then
+		git rebase--helper ${force_rebase:+--no-ff} --continue &&
+		if test ! -f "$todo"
+		then
+			check_rebase__helper
+		fi
+		return
+	fi
+
+	if test -f "$GIT_DIR"/rebase-merge/use-builtin
 	then
 		exec git rebase--helper ${force_rebase:+--no-ff} --continue
 	fi
+
 	# do we have anything to commit?
 	if git diff-index --cached --quiet HEAD --
 	then
@@ -1106,10 +1177,21 @@ first and then run 'git rebase --continue' again."
 skip)
 	git rerere clear
 
-	if test ! -d "$rewritten"
+	if test -f "$GIT_DIR"/saved-rebase-merge/cross-validating
+	then
+		git rebase--helper ${force_rebase:+--no-ff} --continue &&
+		if test ! -f "$todo"
+		then
+			check_rebase__helper
+		fi
+		return
+	fi
+
+	if test -f "$GIT_DIR"/rebase-merge/use-builtin
 	then
 		exec git rebase--helper ${force_rebase:+--no-ff} --continue
 	fi
+
 	do_rest
 	return 0
 	;;
@@ -1188,27 +1270,25 @@ else
 	revisions=$onto...$orig_head
 	shortrevisions=$shorthead
 fi
-if test t != "$preserve_merges"
-then
-	git rebase--helper --make-script ${keep_empty:+--keep-empty} \
-		$revisions ${restrict_revision+^$restrict_revision} > "$todo"
-else
-	format=$(git config --get rebase.instructionFormat)
-	# the 'rev-list .. | sed' requires %m to parse; the instruction requires %H to parse
-	git rev-list $merges_option --format="%m%H ${format:-%s}" \
-		--reverse --left-right --topo-order \
-		$revisions ${restrict_revision+^$restrict_revision} | \
-		sed -n "s/^>//p" |
-	while read -r sha1 rest
-	do
-
-		if test -z "$keep_empty" && is_empty_commit $sha1 && ! is_merge_commit $sha1
-		then
-			comment_out="$comment_char "
-		else
-			comment_out=
-		fi
+format=$(git config --get rebase.instructionFormat)
+# the 'rev-list .. | sed' requires %m to parse; the instruction requires %H to parse
+git rev-list $merges_option --format="%m%H ${format:-%s}" \
+	--reverse --left-right --topo-order \
+	$revisions ${restrict_revision+^$restrict_revision} | \
+	sed -n "s/^>//p" |
+while read -r sha1 rest
+do
+	if test -z "$keep_empty" && is_empty_commit $sha1 && ! is_merge_commit $sha1
+	then
+		comment_out="$comment_char "
+	else
+		comment_out=
+	fi
 
+	if test t != "$preserve_merges"
+	then
+		printf '%s\n' "${comment_out}pick $sha1 $rest" >>"$todo"
+	else
 		if test -z "$rebase_root"
 		then
 			preserve=t
@@ -1227,7 +1307,23 @@ else
 			touch "$rewritten"/$sha1
 			printf '%s\n' "${comment_out}pick $sha1 $rest" >>"$todo"
 		fi
-	done
+	fi
+done
+if test -z "$rebase_root" && test ! -d "$rewritten"
+then
+	git rebase--helper --make-script  ${keep_empty:+--keep-empty} \
+		$revisions ${restrict_revision+^$restrict_revision} \
+		>"$todo".helped
+	if test ! -s "$todo".helped
+	then
+		test ! -f "$todo" || die "$todo.helped should be empty"
+	elif ! cmp "$todo" "$todo".helped
+	then
+		echo git rebase--helper --make-script \
+			${keep_empty:+--keep-empty} $revisions \
+			${restrict_revision+^$restrict_revision} >"$todo".gen
+		die "make-script generated something incompatible"
+	fi
 fi
 
 # Watch for commits that been dropped by --cherry-pick
@@ -1299,10 +1395,20 @@ expand_todo_ids
 test -d "$rewritten" || test -n "$force_rebase" || skip_unnecessary_picks
 
 checkout_onto
-if test -z "$rebase_root" && test ! -d "$rewritten"
+if test true = "$GIT_USE_REBASE_HELPER" && test -z "$rebase_root" &&
+	test ! -d "$rewritten"
 then
 	require_clean_work_tree "rebase"
+	echo 1 >"$GIT_DIR"/rebase-merge/use-builtin
 	exec git rebase--helper ${force_rebase:+--no-ff} --continue
+elif test cross-validate = "$GIT_USE_REBASE_HELPER"
+then
+	rm -rf "$GIT_DIR"/saved-rebase-merge
+	if test -z "$rebase_root" && test ! -d "$rewritten"
+	then
+		cp -R "$GIT_DIR"/rebase-merge "$GIT_DIR"/saved-rebase-merge
+		git rev-parse HEAD >"$GIT_DIR"/saved-rebase-merge/start-head
+	fi
 fi
 do_rest