[MERGE][#248153] Add autopack RPC

[Andrew Bennetts]

Robert Collins wrote:
> On Thu, 2008-10-30 at 16:24 -0500, John Arbash Meinel wrote:
> > True, but as long as it is just a "key, value" pair in the index, it
> > seems to work just fine. Looking at Andrew's code he doesn't do any
> > direct decoding of what is in the index, just packs it into a tuple and
> > ships it to the other side.
> > 
> > Another possibility would be to ship the bytes of 'pack-names' back
> > directly, if you think it is an issue. It does seem like we are wasting
> > a return value if we don't pass something back from the "autopack" RPC.
> 
> I think it makes too many assumptions about repository internals.
> Assuming that there is a names file to pass back is just one such thing.
> 
> If we were using streaming fetch we wouldn't have this data; and we
> shouldn't (except with lightweight checkouts) be querying the repository
> after the autopack anyway.

Personally, I'm inclined to agree with Robert.  It only saves a single
round trip at best, and it does depend a bit more on internals than I
feel totally happy with.

If for now we *don't* return the pack-names data from the RPC it doesn't
stop us from extending the return value to include it in a later
version.

-Andrew.