Build in checkout

The only differences in behavior should be:

 - git checkout -m with non-trivial merging won't print out
   merge-recursive messages (see the change in t7201-co.sh)

 - git checkout -- paths... will give a sensible error message if
   HEAD is invalid as a commit.

 - some intermediate states which were written to disk in the shell
   version (in particular, index states) are only kept in memory in
   this version, and therefore these can no longer be revealed by
   later write operations becoming impossible.

 - when we change branches, we discard MERGE_MSG, SQUASH_MSG, and
   rr-cache/MERGE_RR, like reset always has.

I'm not 100% sure I got the merge recursive setup exactly right; the
base for a non-trivial merge in the shell code doesn't seem
theoretically justified to me, but I tried to match it anyway, and the
tests all pass this way.

Other than these items, the results should be identical to the shell
version, so far as I can tell.

[jc: squashed lock-file fix from Dscho in]

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 298 insertions, 0 deletions

diff --git a/contrib/examples/git-checkout.sh b/contrib/examples/git-checkout.sh
new file mode 100755
index 000000000..5621c69d8
--- /dev/null
+++ b/contrib/examples/git-checkout.sh
@@ -0,0 +1,298 @@
+#!/bin/sh
+
+OPTIONS_KEEPDASHDASH=t
+OPTIONS_SPEC="\
+git-checkout [options] [<branch>] [<paths>...]
+--
+b=          create a new branch started at <branch>
+l           create the new branch's reflog
+track       arrange that the new branch tracks the remote branch
+f           proceed even if the index or working tree is not HEAD
+m           merge local modifications into the new branch
+q,quiet     be quiet
+"
+SUBDIRECTORY_OK=Sometimes
+. git-sh-setup
+require_work_tree
+
+old_name=HEAD
+old=$(git rev-parse --verify $old_name 2>/dev/null)
+oldbranch=$(git symbolic-ref $old_name 2>/dev/null)
+new=
+new_name=
+force=
+branch=
+track=
+newbranch=
+newbranch_log=
+merge=
+quiet=
+v=-v
+LF='
+'
+
+while test $# != 0; do
+	case "$1" in
+	-b)
+		shift
+		newbranch="$1"
+		[ -z "$newbranch" ] &&
+			die "git checkout: -b needs a branch name"
+		git show-ref --verify --quiet -- "refs/heads/$newbranch" &&
+			die "git checkout: branch $newbranch already exists"
+		git check-ref-format "heads/$newbranch" ||
+			die "git checkout: we do not like '$newbranch' as a branch name."
+		;;
+	-l)
+		newbranch_log=-l
+		;;
+	--track|--no-track)
+		track="$1"
+		;;
+	-f)
+		force=1
+		;;
+	-m)
+		merge=1
+		;;
+	-q|--quiet)
+		quiet=1
+		v=
+		;;
+	--)
+		shift
+		break
+		;;
+	*)
+		usage
+		;;
+	esac
+	shift
+done
+
+arg="$1"
+if rev=$(git rev-parse --verify "$arg^0" 2>/dev/null)
+then
+	[ -z "$rev" ] && die "unknown flag $arg"
+	new_name="$arg"
+	if git show-ref --verify --quiet -- "refs/heads/$arg"
+	then
+		rev=$(git rev-parse --verify "refs/heads/$arg^0")
+		branch="$arg"
+	fi
+	new="$rev"
+	shift
+elif rev=$(git rev-parse --verify "$arg^{tree}" 2>/dev/null)
+then
+	# checking out selected paths from a tree-ish.
+	new="$rev"
+	new_name="$arg^{tree}"
+	shift
+fi
+[ "$1" = "--" ] && shift
+
+case "$newbranch,$track" in
+,--*)
+	die "git checkout: --track and --no-track require -b"
+esac
+
+case "$force$merge" in
+11)
+	die "git checkout: -f and -m are incompatible"
+esac
+
+# The behaviour of the command with and without explicit path
+# parameters is quite different.
+#
+# Without paths, we are checking out everything in the work tree,
+# possibly switching branches.  This is the traditional behaviour.
+#
+# With paths, we are _never_ switching branch, but checking out
+# the named paths from either index (when no rev is given),
+# or the named tree-ish (when rev is given).
+
+if test "$#" -ge 1
+then
+	hint=
+	if test "$#" -eq 1
+	then
+		hint="
+Did you intend to checkout '$@' which can not be resolved as commit?"
+	fi
+	if test '' != "$newbranch$force$merge"
+	then
+		die "git checkout: updating paths is incompatible with switching branches/forcing$hint"
+	fi
+	if test '' != "$new"
+	then
+		# from a specific tree-ish; note that this is for
+		# rescuing paths and is never meant to remove what
+		# is not in the named tree-ish.
+		git ls-tree --full-name -r "$new" "$@" |
+		git update-index --index-info || exit $?
+	fi
+
+	# Make sure the request is about existing paths.
+	git ls-files --full-name --error-unmatch -- "$@" >/dev/null || exit
+	git ls-files --full-name -- "$@" |
+		(cd_to_toplevel && git checkout-index -f -u --stdin)
+
+	# Run a post-checkout hook -- the HEAD does not change so the
+	# current HEAD is passed in for both args
+	if test -x "$GIT_DIR"/hooks/post-checkout; then
+	    "$GIT_DIR"/hooks/post-checkout $old $old 0
+	fi
+
+	exit $?
+else
+	# Make sure we did not fall back on $arg^{tree} codepath
+	# since we are not checking out from an arbitrary tree-ish,
+	# but switching branches.
+	if test '' != "$new"
+	then
+		git rev-parse --verify "$new^{commit}" >/dev/null 2>&1 ||
+		die "Cannot switch branch to a non-commit."
+	fi
+fi
+
+# We are switching branches and checking out trees, so
+# we *NEED* to be at the toplevel.
+cd_to_toplevel
+
+[ -z "$new" ] && new=$old && new_name="$old_name"
+
+# If we don't have an existing branch that we're switching to,
+# and we don't have a new branch name for the target we
+# are switching to, then we are detaching our HEAD from any
+# branch.  However, if "git checkout HEAD" detaches the HEAD
+# from the current branch, even though that may be logically
+# correct, it feels somewhat funny.  More importantly, we do not
+# want "git checkout" nor "git checkout -f" to detach HEAD.
+
+detached=
+detach_warn=
+
+describe_detached_head () {
+	test -n "$quiet" || {
+		printf >&2 "$1 "
+		GIT_PAGER= git log >&2 -1 --pretty=oneline --abbrev-commit "$2" --
+	}
+}
+
+if test -z "$branch$newbranch" && test "$new_name" != "$old_name"
+then
+	detached="$new"
+	if test -n "$oldbranch" && test -z "$quiet"
+	then
+		detach_warn="Note: moving to \"$new_name\" which isn't a local branch
+If you want to create a new branch from this checkout, you may do so
+(now or later) by using -b with the checkout command again. Example:
+  git checkout -b <new_branch_name>"
+	fi
+elif test -z "$oldbranch" && test "$new" != "$old"
+then
+	describe_detached_head 'Previous HEAD position was' "$old"
+fi
+
+if [ "X$old" = X ]
+then
+	if test -z "$quiet"
+	then
+		echo >&2 "warning: You appear to be on a branch yet to be born."
+		echo >&2 "warning: Forcing checkout of $new_name."
+	fi
+	force=1
+fi
+
+if [ "$force" ]
+then
+    git read-tree $v --reset -u $new
+else
+    git update-index --refresh >/dev/null
+    merge_error=$(git read-tree -m -u --exclude-per-directory=.gitignore $old $new 2>&1) || (
+	case "$merge" in
+	'')
+		echo >&2 "$merge_error"
+		exit 1 ;;
+	esac
+
+	# Match the index to the working tree, and do a three-way.
+	git diff-files --name-only | git update-index --remove --stdin &&
+	work=`git write-tree` &&
+	git read-tree $v --reset -u $new || exit
+
+	eval GITHEAD_$new='${new_name:-${branch:-$new}}' &&
+	eval GITHEAD_$work=local &&
+	export GITHEAD_$new GITHEAD_$work &&
+	git merge-recursive $old -- $new $work
+
+	# Do not register the cleanly merged paths in the index yet.
+	# this is not a real merge before committing, but just carrying
+	# the working tree changes along.
+	unmerged=`git ls-files -u`
+	git read-tree $v --reset $new
+	case "$unmerged" in
+	'')	;;
+	*)
+		(
+			z40=0000000000000000000000000000000000000000
+			echo "$unmerged" |
+			sed -e 's/^[0-7]* [0-9a-f]* /'"0 $z40 /"
+			echo "$unmerged"
+		) | git update-index --index-info
+		;;
+	esac
+	exit 0
+    )
+    saved_err=$?
+    if test "$saved_err" = 0 && test -z "$quiet"
+    then
+	git diff-index --name-status "$new"
+    fi
+    (exit $saved_err)
+fi
+
+#
+# Switch the HEAD pointer to the new branch if we
+# checked out a branch head, and remove any potential
+# old MERGE_HEAD's (subsequent commits will clearly not
+# be based on them, since we re-set the index)
+#
+if [ "$?" -eq 0 ]; then
+	if [ "$newbranch" ]; then
+		git branch $track $newbranch_log "$newbranch" "$new_name" || exit
+		branch="$newbranch"
+	fi
+	if test -n "$branch"
+	then
+		old_branch_name=`expr "z$oldbranch" : 'zrefs/heads/\(.*\)'`
+		GIT_DIR="$GIT_DIR" git symbolic-ref -m "checkout: moving from ${old_branch_name:-$old} to $branch" HEAD "refs/heads/$branch"
+		if test -n "$quiet"
+		then
+			true	# nothing
+		elif test "refs/heads/$branch" = "$oldbranch"
+		then
+			echo >&2 "Already on branch \"$branch\""
+		else
+			echo >&2 "Switched to${newbranch:+ a new} branch \"$branch\""
+		fi
+	elif test -n "$detached"
+	then
+		old_branch_name=`expr "z$oldbranch" : 'zrefs/heads/\(.*\)'`
+		git update-ref --no-deref -m "checkout: moving from ${old_branch_name:-$old} to $arg" HEAD "$detached" ||
+			die "Cannot detach HEAD"
+		if test -n "$detach_warn"
+		then
+			echo >&2 "$detach_warn"
+		fi
+		describe_detached_head 'HEAD is now at' HEAD
+	fi
+	rm -f "$GIT_DIR/MERGE_HEAD"
+else
+	exit 1
+fi
+
+# Run a post-checkout hook
+if test -x "$GIT_DIR"/hooks/post-checkout; then
+	"$GIT_DIR"/hooks/post-checkout $old $new 1
+fi