add get_available_space() to NativeStorageServer
It uses a new 'available-space' key in the server's v1 version dict, or falls
back to 'maximum-immutable-share-size' (which presently always has the same
value but could have a different meaning in the future).
This is a squash merge of 9773555bb87fab71145ad7a0e84785a4e92d11f7
This introduces new client and server halves to the Introducer (renaming the
old one with a _V1 suffix). Both have fallbacks to accomodate talking to a
different version: the publishing client switches on whether the server's
.get_version() advertises V2 support, the server switches on which
subscription method was invoked by the subscribing client.
The V2 protocol sends a three-tuple of (serialized announcement dictionary,
signature, pubkey) for each announcement. The V2 server dispatches messages
to subscribers according to the service-name, and throws errors for invalid
signatures, but does not otherwise examine the messages. The V2 receiver's
subscription callback will receive a (serverid, ann_dict) pair. The
'serverid' will be equal to the pubkey if all of the following are true:
the originating client is V2, and was told a privkey to use
the announcement went through a V2 server
the signature is valid
If not, 'serverid' will be equal to the tubid portion of the announced FURL,
as was the case for V1 receivers.
Servers will create a keypair if one does not exist yet, stored in
private/server.privkey .
The signed announcement dictionary puts the server FURL in a key named
"anonymous-storage-FURL", which anticipates upcoming Accounting-related
changes in the server advertisements. It also provides a key named
"permutation-seed-base32" to tell clients what permutation seed to use. This
is computed at startup, using tubid if there are existing shares, otherwise
the pubkey, to retain share-order compatibility for existing servers.
