[PATCH] Add "git-update-ref" to update the HEAD (or other) ref

This is a careful version of the script stuff that currently just
blindly writes HEAD with a new value.

You can use

	git-update-ref HEAD <newhead>

or

	git-update-ref HEAD <newhead> <oldhead>

where the latter version verifies that the old value of HEAD matches
oldhead.

It basically allows a "ref" file to be a symbolic pointer to another ref
file by starting with the four-byte header sequence of "ref:".

More importantly, it allows the update of a ref file to follow these
symbolic pointers, whether they are symlinks or these "regular file
symbolic refs".

NOTE! It follows _real_ symlinks only if they start with "refs/":
otherwise it will just try to read them and update them as a regular file
(ie it will allow the filesystem to follow them, but will overwrite such a
symlink to somewhere else with a regular filename).

In general, using

	git-update-ref HEAD "$head"

should be a _lot_ safer than doing

	echo "$head" > "$GIT_DIR/HEAD"

both from a symlink following standpoint _and_ an error checking
standpoint.  The "refs/" rule for symlinks means that symlinks that point
to "outside" the tree are safe: they'll be followed for reading but not
for writing (so we'll never write through a ref symlink to some other
tree, if you have copied a whole archive by creating a symlink tree).

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 1 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index d18153de5d..afd4b14c41 100644
--- a/Makefile
+++ b/Makefile
@@ -116,6 +116,7 @@ PROGRAMS = \
 	git-ssh-upload git-tar-tree git-unpack-file \
 	git-unpack-objects git-update-index git-update-server-info \
 	git-upload-pack git-verify-pack git-write-tree \
+	git-update-ref \
 	$(SIMPLE_PROGRAMS)
 
 # Backward compatibility -- to be removed in 0.99.8