howto/update-hook-example.txt

From: Junio C Hamano <gitster@pobox.com> and Carl Baldwin <cnb@fc.hp.com>
Subject: control access to branches.
Date: Thu, 17 Nov 2005 23:55:32 -0800
Message-ID: <7vfypumlu3.fsf@assigned-by-dhcp.cox.net>
Abstract: An example hooks/update script is presented to
 implement repository maintenance policies, such as who can push
 into which branch and who can make a tag.
Content-type: text/asciidoc

How to use the update hook
==========================

When your developer runs git-push into the repository,
git-receive-pack is run (either locally or over ssh) as that
developer, so is hooks/update script. Quoting from the relevant
section of the documentation:

 Before each ref is updated, if $GIT_DIR/hooks/update file exists
 and executable, it is called with three parameters:

 $GIT_DIR/hooks/update refname sha1-old sha1-new

 The refname parameter is relative to $GIT_DIR; e.g. for the
 master head this is "refs/heads/master". Two sha1 are the
 object names for the refname before and after the update. Note
 that the hook is called before the refname is updated, so either
 sha1-old is 0{40} (meaning there is no such ref yet), or it
 should match what is recorded in refname.

So if your policy is (1) always require fast-forward push
(i.e. never allow "git-push repo +branch:branch"), (2) you
have a list of users allowed to update each branch, and (3) you
do not let tags to be overwritten, then you can use something
like this as your hooks/update script.

[jc: editorial note. This is a much improved version by Carl
since I posted the original outline]

----------------------------------------------------
#!/bin/bash

umask 002

# If you are having trouble with this access control hook script
# you can try setting this to true. It will tell you exactly
# why a user is being allowed/denied access.

verbose=false

# Default shell globbing messes things up downstream
GLOBIGNORE=*

function grant {
 $verbose && echo >&2 "-Grant-	$1"
 echo grant
 exit 0
}

function deny {
 $verbose && echo >&2 "-Deny-	$1"
 echo deny
 exit 1
}

function info {
 $verbose && echo >&2 "-Info-	$1"
}

# Implement generic branch and tag policies.
# - Tags should not be updated once created.
# - Branches should only be fast-forwarded unless their pattern starts with '+'
case "$1" in
 refs/tags/*)
 git rev-parse --verify -q "$1" &&
 deny >/dev/null "You can't overwrite an existing tag"
 ;;
 refs/heads/*)
 # No rebasing or rewinding
 if expr "$2" : '0*$' >/dev/null; then
 info "The branch '$1' is new..."
 else
 # updating -- make sure it is a fast-forward
 mb=$(git merge-base "$2" "$3")
 case "$mb,$2" in
 "$2,$mb") info "Update is fast-forward" ;;
*) noff=y; info "This is not a fast-forward update.";;
 esac
 fi
 ;;
 *)
 deny >/dev/null \
 "Branch is not under refs/heads or refs/tags. What are you trying to do?"
 ;;
esac

# Implement per-branch controls based on username
allowed_users_file=$GIT_DIR/info/allowed-users
username=$(id -u -n)
info "The user is: '$username'"

if test -f "$allowed_users_file"
then
 rc=$(cat $allowed_users_file | grep -v '^#' | grep -v '^$' |
 while read heads user_patterns
 do
 # does this rule apply to us?
 head_pattern=${heads#+}
 matchlen=$(expr "$1" : "${head_pattern#+}")
 test "$matchlen" = ${#1} || continue

 # if non-ff, $heads must be with the '+' prefix
 test -n "$noff" &&
 test "$head_pattern" = "$heads" && continue

 info "Found matching head pattern: '$head_pattern'"
 for user_pattern in $user_patterns; do
 info "Checking user: '$username' against pattern: '$user_pattern'"
 matchlen=$(expr "$username" : "$user_pattern")
 if test "$matchlen" = "${#username}"
 then
 grant "Allowing user: '$username' with pattern: '$user_pattern'"
 fi
 done
 deny "The user is not in the access list for this branch"
 done
 )
 case "$rc" in
 grant) grant >/dev/null "Granting access based on $allowed_users_file" ;;
 deny) deny >/dev/null "Denying access based on $allowed_users_file" ;;
 *) ;;
 esac
fi

allowed_groups_file=$GIT_DIR/info/allowed-groups
groups=$(id -G -n)
info "The user belongs to the following groups:"
info "'$groups'"

if test -f "$allowed_groups_file"
then
 rc=$(cat $allowed_groups_file | grep -v '^#' | grep -v '^$' |
 while read heads group_patterns
 do
 # does this rule apply to us?
 head_pattern=${heads#+}
 matchlen=$(expr "$1" : "${head_pattern#+}")
 test "$matchlen" = ${#1} || continue

 # if non-ff, $heads must be with the '+' prefix
 test -n "$noff" &&
 test "$head_pattern" = "$heads" && continue

 info "Found matching head pattern: '$head_pattern'"
 for group_pattern in $group_patterns; do
 for groupname in $groups; do
 info "Checking group: '$groupname' against pattern: '$group_pattern'"
 matchlen=$(expr "$groupname" : "$group_pattern")
 if test "$matchlen" = "${#groupname}"
 then
 grant "Allowing group: '$groupname' with pattern: '$group_pattern'"
 fi
 done
 done
 deny "None of the user's groups are in the access list for this branch"
 done
 )
 case "$rc" in
 grant) grant >/dev/null "Granting access based on $allowed_groups_file" ;;
 deny) deny >/dev/null "Denying access based on $allowed_groups_file" ;;
 *) ;;
 esac
fi

deny >/dev/null "There are no more rules to check. Denying access"
----------------------------------------------------

This uses two files, $GIT_DIR/info/allowed-users and
allowed-groups, to describe which heads can be pushed into by
whom. The format of each file would look like this:

 refs/heads/master junio
 +refs/heads/pu junio
 refs/heads/cogito$ pasky
 refs/heads/bw/.* linus
 refs/heads/tmp/.* .*
 refs/tags/v[0-9].* junio

With this, Linus can push or create "bw/penguin" or "bw/zebra"
or "bw/panda" branches, Pasky can do only "cogito", and JC can
do master and pu branches and make versioned tags. And anybody
can do tmp/blah branches. The '+' sign at the pu record means
that JC can make non-fast-forward pushes on it.