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'.
uses keepalive module from urlgrabber package. tested against "hg serve",
cgi server, and through http proxy. used ethereal to verify that only
one tcp connection used during entire "hg pull" sequence.
if server supports keepalive, this makes latency of "hg pull" much lower.
only "hg serve" affected yet. http server running cgi script will not
use persistent connections. support for fastcgi will help that.
clients that support keepalive can use one tcp connection for all
commands during clone and pull. this makes latency of binary search
during pull much lower over wan.
if server does not know content-length, it will force connection to
close at end. right fix is to use chunked transfer-encoding but this is
easier and does not hurt performance. only command that is affected is
"changegroup" which is always last command during a pull.
Use case: If a remote repo has two heads and I _want_ to merge them, I merge
and push. Meanwhile someone else pushed on top of one of the heads. He won't
get a warning, because he doesn't create a new head, I won't notice that I
don't close a head, because I don't get a message telling me.
Because older servers don't return any output for unknown commands,
it's tricky to add new commands. The approach is this: we add a
"hello" command that reports any interesting capabilities (and other
things that might be of interest in the future). To detect whether
this new command is supported, we issue both it and our startup
detection command ("between") at the beginning of a connection.
