NBash

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#!/bin/sh
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#
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# Copyright (c) 2006, 2008 Junio C Hamano
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#
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# The "pre-rebase" hook is run just before "git rebase" starts doing
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# its job, and can prevent the command from running by exiting with
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# non-zero status.
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#
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# The hook is called with the following parameters:
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#
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# $1 -- the upstream the series was forked from.
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# $2 -- the branch being rebased (or empty when rebasing the current branch).
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#
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# This sample shows how to prevent topic branches that are already
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# merged to 'next' branch from getting rebased, because allowing it
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# would result in rebasing already published history.
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publish=next
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basebranch="$1"
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if test "$#" = 2
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then
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	topic="refs/heads/$2"
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else
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	topic=`git symbolic-ref HEAD` ||
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	exit 0 ;# we do not interrupt rebasing detached HEAD
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fi
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case "$topic" in
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refs/heads/??/*)
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	;;
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*)
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	exit 0 ;# we do not interrupt others.
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	;;
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esac
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# Now we are dealing with a topic branch being rebased
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# on top of master.  Is it OK to rebase it?
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# Does the topic really exist?
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git show-ref -q "$topic" || {
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	echo >&2 "No such branch $topic"
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	exit 1
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}
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# Is topic fully merged to master?
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not_in_master=`git rev-list --pretty=oneline ^master "$topic"`
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if test -z "$not_in_master"
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then
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	echo >&2 "$topic is fully merged to master; better remove it."
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	exit 1 ;# we could allow it, but there is no point.
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fi
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# Is topic ever merged to next?  If so you should not be rebasing it.
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only_next_1=`git rev-list ^master "^$topic" ${publish} | sort`
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only_next_2=`git rev-list ^master           ${publish} | sort`
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if test "$only_next_1" = "$only_next_2"
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then
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	not_in_topic=`git rev-list "^$topic" master`
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	if test -z "$not_in_topic"
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	then
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		echo >&2 "$topic is already up to date with master"
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		exit 1 ;# we could allow it, but there is no point.
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	else
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		exit 0
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	fi
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else
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	not_in_next=`git rev-list --pretty=oneline ^${publish} "$topic"`
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	perl -e '
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		my $topic = $ARGV[0];
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		my $msg = "* $topic has commits already merged to public branch:\n";
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		my (%not_in_next) = map {
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			/^([0-9a-f]+) /;
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			($1 => 1);
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		} split(/\n/, $ARGV[1]);
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		for my $elem (map {
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				/^([0-9a-f]+) (.*)$/;
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				[$1 => $2];
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			} split(/\n/, $ARGV[2])) {
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			if (!exists $not_in_next{$elem->[0]}) {
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				if ($msg) {
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					print STDERR $msg;
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					undef $msg;
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				}
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				print STDERR " $elem->[1]\n";
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			}
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		}
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	' "$topic" "$not_in_next" "$not_in_master"
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	exit 1
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fi
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<<\DOC_END
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This sample hook safeguards topic branches that have been
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published from being rewound.
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The workflow assumed here is:
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 * Once a topic branch forks from "master", "master" is never
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   merged into it again (either directly or indirectly).
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 * Once a topic branch is fully cooked and merged into "master",
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   it is deleted.  If you need to build on top of it to correct
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   earlier mistakes, a new topic branch is created by forking at
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   the tip of the "master".  This is not strictly necessary, but
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   it makes it easier to keep your history simple.
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 * Whenever you need to test or publish your changes to topic
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   branches, merge them into "next" branch.
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The script, being an example, hardcodes the publish branch name
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to be "next", but it is trivial to make it configurable via
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$GIT_DIR/config mechanism.
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With this workflow, you would want to know:
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(1) ... if a topic branch has ever been merged to "next".  Young
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    topic branches can have stupid mistakes you would rather
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    clean up before publishing, and things that have not been
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    merged into other branches can be easily rebased without
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    affecting other people.  But once it is published, you would
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    not want to rewind it.
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(2) ... if a topic branch has been fully merged to "master".
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    Then you can delete it.  More importantly, you should not
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    build on top of it -- other people may already want to
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    change things related to the topic as patches against your
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    "master", so if you need further changes, it is better to
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    fork the topic (perhaps with the same name) afresh from the
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    tip of "master".
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Let's look at this example:
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		   o---o---o---o---o---o---o---o---o---o "next"
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		  /       /           /           /
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		 /   a---a---b A     /           /
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		/   /               /           /
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	       /   /   c---c---c---c B         /
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	      /   /   /             \         /
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	     /   /   /   b---b C     \       /
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	    /   /   /   /             \     /
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    ---o---o---o---o---o---o---o---o---o---o---o "master"
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A, B and C are topic branches.
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 * A has one fix since it was merged up to "next".
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 * B has finished.  It has been fully merged up to "master" and "next",
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   and is ready to be deleted.
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 * C has not merged to "next" at all.
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We would want to allow C to be rebased, refuse A, and encourage
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B to be deleted.
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To compute (1):
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	git rev-list ^master ^topic next
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	git rev-list ^master        next
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	if these match, topic has not merged in next at all.
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To compute (2):
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	git rev-list master..topic
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	if this is empty, it is fully merged to "master".
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DOC_END
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