Adding a relaxed url currently prevents verifying the content of the object when transferring it between repositories. This risks data corruption going unnoticed. Could the hash be recorded when a relaxed url is downloaded from the web, and then used for verification of other transfers?

This would need a new per-key file in the git-annex branch, that can list one or more hash-based keys. Then implement verification for url keys, that checks if hash-based keys are recorded in the file, and if so uses them to verify the content.

The web special remote can hash the content as it's downloading it from the web, and record the resulting hash-based key.

handling upgrades

A repository that currently contains the content of a relaxed url needs to keep working. So, the object stored in the repository has to be treated as valid, even though it does not correspond to any hash-based keys listed for the url key.

This presents a problem; there is no way to tell the difference between a valid object in such a repository and an object that was downloaded from the web with its hash recorded, but has since gotten corrupted.

Seems that the only possible way to resolve this problem is to change to a new type of url key, that is known to always have its hash recorded on download from the web. (Call this a "dynamic" url key.) And handle all existing relaxed url keys as before.

That would leave it up to the user to migrate their relaxed url keys to dynamic urls keys, if desired. Now that distributed migration is implemented, that seems sufficiently easy.

other special remotes

If the web special remote is what takes care of hashing the content on download and recording the hash-based key, what about other special remotes that claim an url?

This could also be implemented in the bittorrent special remote, but what about external special remotes?

An alternative would be to add a downloadDynamicUrl that is called instead of retrieveKeyFile and returns a hash-based key (allowing hashing the download on the fly). Then git-annex would take care of recording the hash-based key. The external special remote interface could be extended to include that.

hash-based key choice

Should annex.backend gitconfig be used to pick which hash-based key to use? The risk is that config changes and several different hash-based keys get recorded for a dynamic url. Not really a problem, but would increase the size of the git-annex branch unncessarily, and require extra work when verifying the key.)

use for other types of keys

It would also be possible to use these new git-annex branch log files for other types of keys, like WORM, or perhaps SHA1. But, the same upgrade problem would apply. So, there's problably no benefit in doing that, compared with migrating the key to the desired hash backend.

Does seem to be some chance this could help implementing wishlist degraded files.