howto/update-hook-example.html

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 735 <body class="article">
 736 <div id="header">
 737 <h1>How to use the update hook</h1>
 738 <span id="revdate">2023-07-25</span>
 739 </div>
 740 <div id="content">
 741 <div id="preamble">
 742 <div class="sectionbody">
 743 <div class="paragraph"><p>When your developer runs git-push into the repository,
 744 git-receive-pack is run (either locally or over ssh) as that
 745 developer, so is hooks/update script.  Quoting from the relevant
 746 section of the documentation:</p></div>
 747 <div class="literalblock">
 748 <div class="content">
 749 <pre><code>Before each ref is updated, if $GIT_DIR/hooks/update file exists
 750 and executable, it is called with three parameters:</code></pre>
 751 </div></div>
 752 <div class="literalblock">
 753 <div class="content">
 754 <pre><code>$GIT_DIR/hooks/update refname sha1-old sha1-new</code></pre>
 755 </div></div>
 756 <div class="literalblock">
 757 <div class="content">
 758 <pre><code>The refname parameter is relative to $GIT_DIR; e.g. for the
 759 master head this is "refs/heads/master".  Two sha1 are the
 760 object names for the refname before and after the update.  Note
 761 that the hook is called before the refname is updated, so either
 762 sha1-old is 0{40} (meaning there is no such ref yet), or it
 763 should match what is recorded in refname.</code></pre>
 764 </div></div>
 765 <div class="paragraph"><p>So if your policy is (1) always require fast-forward push
 766 (i.e. never allow "git-push repo +branch:branch"), (2) you
 767 have a list of users allowed to update each branch, and (3) you
 768 do not let tags to be overwritten, then you can use something
 769 like this as your hooks/update script.</p></div>
 770 <div class="paragraph"><p>[jc: editorial note.  This is a much improved version by Carl
 771 since I posted the original outline]</p></div>
 772 <div class="listingblock">
 773 <div class="content">
 774 <pre><code>#!/bin/bash
 775
 776 umask 002
 777
 778 # If you are having trouble with this access control hook script
 779 # you can try setting this to true.  It will tell you exactly
 780 # why a user is being allowed/denied access.
 781
 782 verbose=false
 783
 784 # Default shell globbing messes things up downstream
 785 GLOBIGNORE=*
 786
 787 function grant {
 788   $verbose &amp;&amp; echo &gt;&amp;2 "-Grant-         $1"
 789   echo grant
 790   exit 0
 791 }
 792
 793 function deny {
 794   $verbose &amp;&amp; echo &gt;&amp;2 "-Deny-          $1"
 795   echo deny
 796   exit 1
 797 }
 798
 799 function info {
 800   $verbose &amp;&amp; echo &gt;&amp;2 "-Info-          $1"
 801 }
 802
 803 # Implement generic branch and tag policies.
 804 # - Tags should not be updated once created.
 805 # - Branches should only be fast-forwarded unless their pattern starts with '+'
 806 case "$1" in
 807   refs/tags/*)
 808     git rev-parse --verify -q "$1" &amp;&amp;
 809     deny &gt;/dev/null "You can't overwrite an existing tag"
 810     ;;
 811   refs/heads/*)
 812     # No rebasing or rewinding
 813     if expr "$2" : '0*$' &gt;/dev/null; then
 814       info "The branch '$1' is new..."
 815     else
 816       # updating -- make sure it is a fast-forward
 817       mb=$(git merge-base "$2" "$3")
 818       case "$mb,$2" in
 819         "$2,$mb") info "Update is fast-forward" ;;
 820         *)        noff=y; info "This is not a fast-forward update.";;
 821       esac
 822     fi
 823     ;;
 824   *)
 825     deny &gt;/dev/null \
 826     "Branch is not under refs/heads or refs/tags.  What are you trying to do?"
 827     ;;
 828 esac
 829
 830 # Implement per-branch controls based on username
 831 allowed_users_file=$GIT_DIR/info/allowed-users
 832 username=$(id -u -n)
 833 info "The user is: '$username'"
 834
 835 if test -f "$allowed_users_file"
 836 then
 837   rc=$(cat $allowed_users_file | grep -v '^#' | grep -v '^$' |
 838     while read heads user_patterns
 839     do
 840       # does this rule apply to us?
 841       head_pattern=${heads#+}
 842       matchlen=$(expr "$1" : "${head_pattern#+}")
 843       test "$matchlen" = ${#1} || continue
 844
 845       # if non-ff, $heads must be with the '+' prefix
 846       test -n "$noff" &amp;&amp;
 847       test "$head_pattern" = "$heads" &amp;&amp; continue
 848
 849       info "Found matching head pattern: '$head_pattern'"
 850       for user_pattern in $user_patterns; do
 851         info "Checking user: '$username' against pattern: '$user_pattern'"
 852         matchlen=$(expr "$username" : "$user_pattern")
 853         if test "$matchlen" = "${#username}"
 854         then
 855           grant "Allowing user: '$username' with pattern: '$user_pattern'"
 856         fi
 857       done
 858       deny "The user is not in the access list for this branch"
 859     done
 860   )
 861   case "$rc" in
 862     grant) grant &gt;/dev/null "Granting access based on $allowed_users_file" ;;
 863     deny)  deny  &gt;/dev/null "Denying  access based on $allowed_users_file" ;;
 864     *) ;;
 865   esac
 866 fi
 867
 868 allowed_groups_file=$GIT_DIR/info/allowed-groups
 869 groups=$(id -G -n)
 870 info "The user belongs to the following groups:"
 871 info "'$groups'"
 872
 873 if test -f "$allowed_groups_file"
 874 then
 875   rc=$(cat $allowed_groups_file | grep -v '^#' | grep -v '^$' |
 876     while read heads group_patterns
 877     do
 878       # does this rule apply to us?
 879       head_pattern=${heads#+}
 880       matchlen=$(expr "$1" : "${head_pattern#+}")
 881       test "$matchlen" = ${#1} || continue
 882
 883       # if non-ff, $heads must be with the '+' prefix
 884       test -n "$noff" &amp;&amp;
 885       test "$head_pattern" = "$heads" &amp;&amp; continue
 886
 887       info "Found matching head pattern: '$head_pattern'"
 888       for group_pattern in $group_patterns; do
 889         for groupname in $groups; do
 890           info "Checking group: '$groupname' against pattern: '$group_pattern'"
 891           matchlen=$(expr "$groupname" : "$group_pattern")
 892           if test "$matchlen" = "${#groupname}"
 893           then
 894             grant "Allowing group: '$groupname' with pattern: '$group_pattern'"
 895           fi
 896         done
 897       done
 898       deny "None of the user's groups are in the access list for this branch"
 899     done
 900   )
 901   case "$rc" in
 902     grant) grant &gt;/dev/null "Granting access based on $allowed_groups_file" ;;
 903     deny)  deny  &gt;/dev/null "Denying  access based on $allowed_groups_file" ;;
 904     *) ;;
 905   esac
 906 fi
 907
 908 deny &gt;/dev/null "There are no more rules to check.  Denying access"</code></pre>
 909 </div></div>
 910 <div class="paragraph"><p>This uses two files, $GIT_DIR/info/allowed-users and
 911 allowed-groups, to describe which heads can be pushed into by
 912 whom.  The format of each file would look like this:</p></div>
 913 <div class="literalblock">
 914 <div class="content">
 915 <pre><code>refs/heads/master   junio
 916 +refs/heads/seen    junio
 917 refs/heads/cogito$  pasky
 918 refs/heads/bw/.*    linus
 919 refs/heads/tmp/.*   .*
 920 refs/tags/v[0-9].*  junio</code></pre>
 921 </div></div>
 922 <div class="paragraph"><p>With this, Linus can push or create "bw/penguin" or "bw/zebra"
 923 or "bw/panda" branches, Pasky can do only "cogito", and JC can
 924 do master and "seen" branches and make versioned tags.  And anybody
 925 can do tmp/blah branches. The <em>+</em> sign at the "seen" record means
 926 that JC can make non-fast-forward pushes on it.</p></div>
 927 </div>
 928 </div>
 929 </div>
 930 <div id="footnotes"><hr /></div>
 931 <div id="footer">
 932 <div id="footer-text">
 933 Last updated
 934  2023-07-25 13:20:33 PDT
 935 </div>
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