git-checkout: handle local changes sanely when detaching HEAD

When switching branches, we usually first try read-tree to make
sure that we do not lose the local changes and then updated the
HEAD using update-ref.  However, we detached and updated HEAD
before these checks, which was quite bad in a repository with
local changes.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff --git a/git-checkout.sh b/git-checkout.sh
index dcf6ddbdf16c5c2c666aa01026912d9c425c55f2..8f4356d49a4935ff4a238cad3d0f90760d3e1afd 100755 (executable)
--- a/git-checkout.sh
+++ b/git-checkout.sh
@@ -150,21 +150,18 @@ fi
 # correct, it feels somewhat funny.  More importantly, we do not
 # want "git checkout" nor "git checkout -f" to detach HEAD.
 
+detached=
+detach_warn=
+
 if test -z "$branch$newbranch" && test "$new" != "$old"
 then
-       # NEEDSWORK: we would want to have a command here
-       # that allows us to detach the HEAD atomically.  Perhaps
-       # something like "git update-ref --detach HEAD $new"
-       echo "$new" >"$GIT_DIR/HEAD.new" &&
-       mv "$GIT_DIR/HEAD.new" "$GIT_DIR/HEAD" || die "Cannot detach HEAD"
-
+       detached="$new"
        if test -n "$oldbranch"
        then
-               echo >&2 "warning: you are not on ANY branch anymore.
+               detach_warn="warning: you are not on ANY branch anymore.
 If you meant to create a new branch from the commit, you need -b to
 associate a new branch with the wanted checkout.  Example:
-  git checkout -b <new_branch_name> $arg
-"
+  git checkout -b <new_branch_name> $arg"
        fi
 elif test -z "$oldbranch" && test -n "$branch"
 then
@@ -258,8 +255,25 @@ if [ "$?" -eq 0 ]; then
                git-update-ref -m "checkout: Created from $new_name" "refs/heads/$newbranch" $new || exit
                branch="$newbranch"
        fi
-       [ "$branch" ] &&
-       GIT_DIR="$GIT_DIR" git-symbolic-ref HEAD "refs/heads/$branch"
+       if test -n "$branch"
+       then
+               GIT_DIR="$GIT_DIR" git-symbolic-ref HEAD "refs/heads/$branch"
+       elif test -n "$detached"
+       then
+               # NEEDSWORK: we would want a command to detach the HEAD
+               # atomically, instead of this handcrafted command sequence.
+               # Perhaps:
+               #       git update-ref --detach HEAD $new
+               # or something like that...
+               #
+               echo "$detached" >"$GIT_DIR/HEAD.new" &&
+               mv "$GIT_DIR/HEAD.new" "$GIT_DIR/HEAD" ||
+                       die "Cannot detach HEAD"
+               if test -n "$detach_warn"
+               then
+                       echo >&2 "$detach_warn"
+               fi
+       fi
        rm -f "$GIT_DIR/MERGE_HEAD"
 else
        exit 1