Size passed to IStorageServer.allocate_bytes is bigger than actual share size

[itamarst]

immutable/layout.py calculates share size using the _maximum_ possible URI extension size. The actual share has a specific, typically smaller URI extension size (as generated in immutable/encode.py). So IBucketWriter.close() is called after sending fewer bytes than were claimed in allocate_buckets().

For Foolscap this works, however ugly it is. For current HTTP storage protocol, this does not.

Instead of fixing the HTTP protocol to have a notion of "closing early", seems better to just fix the size calculation so it uses the actual size of the URI extension.

Note that there may be additional sources of discrepancy :(

[GitHub <noreply@…>]

In 508dda1/trunk:

Merge pull request #1211 from tahoe-lafs/3915-immutable-size

Match up allocated size of immutable and what actually gets written

Fixes ticket:3915