git-checkout: safety when coming back from the detached HEAD state.

After making commits in the detached HEAD state, if you run "git
checkout" to switch to an existing branch, you will lose your
work.  Make sure the switched-to branch is a fast-forward of the
current HEAD, or require -f when switching.

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

diff --git a/git-checkout.sh b/git-checkout.sh
index 3250f64ccc683606d420a82b3ec95e2f41bb52b3..69d0c1c43a9f78908f13d3e0e9c34c44587367dd 100755 (executable)
--- a/git-checkout.sh
+++ b/git-checkout.sh
@@ -14,6 +14,8 @@ branch=
 newbranch=
 newbranch_log=
 merge=
+LF='
+'
 while [ "$#" != "0" ]; do
     arg="$1"
     shift
@@ -164,6 +166,22 @@ associate a new branch with the wanted checkout.  Example:
   git checkout -b <new_branch_name> $arg
 "
        fi
+elif test -z "$oldbranch" && test -n "$branch"
+then
+       # Coming back...
+       if test -z "$force"
+       then
+               mb=$(git merge-base --all $old $new) &&
+               case "$LF$mb$LF" in
+               *"$LF$old$LF"*) : ;;
+               *)      false ;;
+               esac || {
+                       echo >&2 \
+"You are not on a branch and switching to $new_name branch may lose
+your changes.  Use 'git checkout -f $new_name' if you want to."
+                       exit 1;
+               }
+       fi
 fi
 
 if [ "X$old" = X ]