extend network protocol to stop clients from locking servers now all repositories have capabilities slot, tuple with list of names. if 'unbundle' capability present, repo supports push where client does not need to lock server. repository classes that have unbundle capability also have unbundle method. implemented for ssh now, will be base for push over http. unbundle protocol acts this way. server tells client what heads it has during normal negotiate step. client starts unbundle by repeat server's heads back to it. if server has new heads, abort immediately. otherwise, transfer changes to server. once data transferred, server locks and checks heads again. if heads same, changes can be added. else someone else added heads, and server aborts. if client wants to force server to add heads, sends special heads list of 'force'.

#!/bin/sh
#
# Corrupt an hg repo with two pulls.
#

# create one repo with a long history
hg init source1
cd source1
touch foo
hg add foo
for i in 1 2 3 4 5 6 7 8 9 10; do
    echo $i >> foo
    hg ci -m $i
done
cd ..

# create a third repo to pull both other repos into it
hg init version2
hg -R version2 pull source1 &
sleep 1

hg clone --pull -U version2 corrupted
hg -R corrupted verify
hg -R version2 verify