contrib git-resurrect: find traces of a branch name and resurrect it

Add a tool 'git-resurrect.sh <branch>' that tries to find traces of
the <branch> in the HEAD reflog and, optionally, all merge commits in
the repository.  It can then resurrect the branch, pointing it at the
most recent of all candidate commits found.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/contrib/git-resurrect.sh b/contrib/git-resurrect.sh
new file mode 100755 (executable)
index 0000000..c364dda
--- /dev/null
+++ b/contrib/git-resurrect.sh
@@ -0,0 +1,180 @@
+#!/bin/sh
+
+USAGE="[-a] [-r] [-m] [-t] [-n] [-b <newname>] <name>"
+LONG_USAGE="git-resurrect attempts to find traces of a branch tip
+called <name>, and tries to resurrect it.  Currently, the reflog is
+searched for checkout messages, and with -r also merge messages.  With
+-m and -t, the history of all refs is scanned for Merge <name> into
+other/Merge <other> into <name> (respectively) commit subjects, which
+is rather slow but allows you to resurrect other people's topic
+branches."
+
+OPTIONS_SPEC="\
+git resurrect $USAGE
+--
+b,branch=            save branch as <newname> instead of <name>
+a,all                same as -l -r -m -t
+k,keep-going         full rev-list scan (instead of first match)
+l,reflog             scan reflog for checkouts (enabled by default)
+r,reflog-merges      scan for merges recorded in reflog
+m,merges             scan for merges into other branches (slow)
+t,merge-targets      scan for merges of other branches into <name>
+n,dry-run            don't recreate the branch"
+
+. git-sh-setup
+
+search_reflog () {
+        sed -ne 's~^\([^ ]*\) .*\tcheckout: moving from '"$1"' .*~\1~p' \
+                < "$GIT_DIR"/logs/HEAD
+}
+
+search_reflog_merges () {
+       git rev-parse $(
+               sed -ne 's~^[^ ]* \([^ ]*\) .*\tmerge '"$1"':.*~\1^2~p' \
+                       < "$GIT_DIR"/logs/HEAD
+       )
+}
+
+_x40="[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]"
+_x40="$_x40$_x40$_x40$_x40$_x40$_x40$_x40$_x40"
+
+search_merges () {
+        git rev-list --all --grep="Merge branch '$1'" \
+                --pretty=tformat:"%P %s" |
+        sed -ne "/^$_x40 \($_x40\) Merge .*/ {s//\1/p;$early_exit}"
+}
+
+search_merge_targets () {
+       git rev-list --all --grep="Merge branch '[^']*' into $branch\$" \
+               --pretty=tformat:"%H %s" --all |
+       sed -ne "/^\($_x40\) Merge .*/ {s//\1/p;$early_exit} "
+}
+
+dry_run=
+early_exit=q
+scan_reflog=t
+scan_reflog_merges=
+scan_merges=
+scan_merge_targets=
+new_name=
+
+while test "$#" != 0; do
+       case "$1" in
+           -b|--branch)
+               shift
+               new_name="$1"
+               ;;
+           -n|--dry-run)
+               dry_run=t
+               ;;
+           --no-dry-run)
+               dry_run=
+               ;;
+           -k|--keep-going)
+               early_exit=
+               ;;
+           --no-keep-going)
+               early_exit=q
+               ;;
+           -m|--merges)
+               scan_merges=t
+               ;;
+           --no-merges)
+               scan_merges=
+               ;;
+           -l|--reflog)
+               scan_reflog=t
+               ;;
+           --no-reflog)
+               scan_reflog=
+               ;;
+           -r|--reflog_merges)
+               scan_reflog_merges=t
+               ;;
+           --no-reflog_merges)
+               scan_reflog_merges=
+               ;;
+           -t|--merge-targets)
+               scan_merge_targets=t
+               ;;
+           --no-merge-targets)
+               scan_merge_targets=
+               ;;
+           -a|--all)
+               scan_reflog=t
+               scan_reflog_merges=t
+               scan_merges=t
+               scan_merge_targets=t
+               ;;
+           --)
+               shift
+               break
+               ;;
+           *)
+               usage
+               ;;
+       esac
+       shift
+done
+
+test "$#" = 1 || usage
+
+all_strategies="$scan_reflog$scan_reflog_merges$scan_merges$scan_merge_targets"
+if test -z "$all_strategies"; then
+       die "must enable at least one of -lrmt"
+fi
+
+branch="$1"
+test -z "$new_name" && new_name="$branch"
+
+if test ! -z "$scan_reflog"; then
+       if test -r "$GIT_DIR"/logs/HEAD; then
+               candidates="$(search_reflog $branch)"
+       else
+               die 'reflog scanning requested, but' \
+                       '$GIT_DIR/logs/HEAD not readable'
+       fi
+fi
+if test ! -z "$scan_reflog_merges"; then
+       if test -r "$GIT_DIR"/logs/HEAD; then
+               candidates="$candidates $(search_reflog_merges $branch)"
+       else
+               die 'reflog scanning requested, but' \
+                       '$GIT_DIR/logs/HEAD not readable'
+       fi
+fi
+if test ! -z "$scan_merges"; then
+       candidates="$candidates $(search_merges $branch)"
+fi
+if test ! -z "$scan_merge_targets"; then
+       candidates="$candidates $(search_merge_targets $branch)"
+fi
+
+candidates="$(git rev-parse $candidates | sort -u)"
+
+if test -z "$candidates"; then
+       hint=
+       test "z$all_strategies" != "ztttt" \
+               && hint=" (maybe try again with -a)"
+       die "no candidates for $branch found$hint"
+fi
+
+echo "** Candidates for $branch **"
+for cmt in $candidates; do
+       git --no-pager log --pretty=tformat:"%ct:%h [%cr] %s" --abbrev-commit -1 $cmt
+done \
+| sort -n | cut -d: -f2-
+
+newest="$(git rev-list -1 $candidates)"
+if test ! -z "$dry_run"; then
+       printf "** Most recent: "
+       git --no-pager log -1 --pretty=tformat:"%h %s" $newest
+elif ! git rev-parse --verify --quiet $new_name >/dev/null; then
+       printf "** Restoring $new_name to "
+       git --no-pager log -1 --pretty=tformat:"%h %s" $newest
+       git branch $new_name $newest
+else
+       printf "Most recent: "
+       git --no-pager log -1 --pretty=tformat:"%h %s" $newest
+       echo "** $new_name already exists, doing nothing"
+fi