Fetch directory

Tag 5: the server asks the client for a directory the client mounts by identity and the server does not hold; the client answers with one channel response per file or per piece of a file; the server verifies the whole against the identity before the deploy.

The server opens this channel, before it deploys, for each entry of identity_directory_mounts whose identity it does not hold. Every such channel is opened at once, one per identity.

• The request. The tag byte 5 followed by the identity as JSON. Request states its form.
• The response. One channel response per file, or per piece of a file, and the channel response finish. Response states the sequence.
• Verification. The server stores the directory under the identity only when the manifest of the files received, as the volumes::stat response defines it, has the hash of the identity and the files’ total length is its size. A set that does not match, a stream that ends short, and a finish with no channel response are the run’s error.