libdweb/protocol handler: fallback to public gateways if no node available

[lidel]

Summary

There should be an option to disable local IPFS node and run ipfs:// and ipns:// with a public gateway as a backend (#491) (or do that as an automatic fallback if no node is available).

Rationale

Given protocol handler API from libdweb (#533) we are able to separate security perimeter from transport layer.

IPFS is content-addressed, trustless system. It means as long we verify hash of received payload matches we can use public gateways as a fallback transport. Due to this the feature should be enabled only for browsers with native protocol handler.

It would be a huge UX win for end user: ipfs:// and ipns:// would always work, even if the only transport allowed in local network is HTTP or user is concerned with battery/bandwidth use and just want to browse IPFS in light, read-only mode (#507).

When native protocol handler is available, read-only public gateway could be listed as an additional backend (#491).

Additional Resources

• We have a semi-official list of public gateways donated by community:
  https://github.com/ipfs/public-gateway-checker/blob/master/gateways.json
  It could be used in round-robin fashion.

[lidel]

Unable to implement this right now: cross-protocol redirect via webRequest.onBeforeRequest gets blocked.

Upstream issue: mozilla/libdweb#74
There may be some code reuse with #593