post-receive-email example hook: detect rewind-only updates and output sensible message

Sometimes a non-fast-forward update doesn't add new commits, it merely
removes old commits.  This patch adds support for detecting that and
outputting a more correct message.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff --git a/contrib/hooks/post-receive-email b/contrib/hooks/post-receive-email
index edb30f6f0f4300f3e8bdd8115e3b2536d54f3ae0..e175b426a406b39f36ecd11c0ff96adb41b263be 100644 (file)
--- a/contrib/hooks/post-receive-email
+++ b/contrib/hooks/post-receive-email
@@ -327,36 +327,67 @@ generate_update_branch_email()
        if [ -z "$fastforward" ]; then
                echo "      from  $oldrev ($oldrev_type)"
        else
+               #  1. Existing revisions were removed.  In this case newrev is a
+               #     subset of oldrev - this is the reverse of a fast-forward,
+               #     a rewind
+               #  2. New revisions were added on top of an old revision, this is
+               #     a rewind and addition.
+
+               # (1) certainly happened, (2) possibly.  When (2) hasn't happened,
+               # we set a flag to indicate that no log printout is required.
+
                echo ""
-               echo "This update added new revisions after undoing old revisions.  That is to"
-               echo "say, the old revision is not a strict subset of the new revision.  This"
-               echo "situation occurs when you --force push a change and generate a"
-               echo "repository containing something like this:"
-               echo ""
-               echo " * -- * -- B -- O -- O -- O ($oldrev)"
-               echo "            \\"
-               echo "             N -- N -- N ($newrev)"
-               echo ""
-               echo "When this happens we assume that you've already had alert emails for all"
-               echo "of the O revisions, and so we here report only the revisions in the N"
-               echo "branch from the common base, B."
+
+               # Find the common ancestor of the old and new revisions and compare
+               # it with newrev
+               baserev=$(git merge-base $oldrev $newrev)
+               rewind_only=""
+               if [ "$baserev" = "$newrev" ]; then
+                       echo "This update discarded existing revisions and left the branch pointing at"
+                       echo "a previous point in the repository history."
+                       echo ""
+                       echo " * -- * -- N ($newrev)"
+                       echo "            \\"
+                       echo "             O -- O -- O ($oldrev)"
+                       echo ""
+                       echo "The removed revisions are not necessarilly gone - if another reference"
+                       echo "still refers to them they will stay in the repository."
+                       rewind_only=1
+               else
+                       echo "This update added new revisions after undoing existing revisions.  That is"
+                       echo "to say, the old revision is not a strict subset of the new revision.  This"
+                       echo "situation occurs when you --force push a change and generate a repository"
+                       echo "containing something like this:"
+                       echo ""
+                       echo " * -- * -- B -- O -- O -- O ($oldrev)"
+                       echo "            \\"
+                       echo "             N -- N -- N ($newrev)"
+                       echo ""
+                       echo "When this happens we assume that you've already had alert emails for all"
+                       echo "of the O revisions, and so we here report only the revisions in the N"
+                       echo "branch from the common base, B."
+               fi
        fi
 
        echo ""
-       echo "Those revisions listed above that are new to this repository have"
-       echo "not appeared on any other notification email; so we list those"
-       echo "revisions in full, below."
+       if [ -z "$rewind_only" ]; then
+               echo "Those revisions listed above that are new to this repository have"
+               echo "not appeared on any other notification email; so we list those"
+               echo "revisions in full, below."
 
-       echo ""
-       echo $LOGBEGIN
-       git rev-parse --not --branches | grep -v $(git rev-parse $refname) |
-       git rev-list --pretty --stdin $oldrev..$newrev
+               echo ""
+               echo $LOGBEGIN
+               git rev-parse --not --branches | grep -v $(git rev-parse $refname) |
+               git rev-list --pretty --stdin $oldrev..$newrev
 
-       # XXX: Need a way of detecting whether git rev-list actually outputted
-       # anything, so that we can issue a "no new revisions added by this
-       # update" message
+               # XXX: Need a way of detecting whether git rev-list actually outputted
+               # anything, so that we can issue a "no new revisions added by this
+               # update" message
 
-       echo $LOGEND
+               echo $LOGEND
+       else
+               echo "No new revisions were added by this update."
+       fi
 
        # The diffstat is shown from the old revision to the new revision.  This
        # is to show the truth of what happened in this change.  There's no point