Full Light Client WASM support

[thanethomson]

Follows from #463. I've modified this issue to turn it into a tracking issue for WASM support for the Light Client.

At present, the tendermint-light-client-js crate just exposes the Light Client's verify method to JavaScript, but leaves networking and the verification algorithm (e.g. forward bisection, or backwards verification) up to the implementer to implement in JavaScript.

In order to achieve a fully functional, WASM-compatible Light Client, we probably need several issues to be addressed:

• async-ify the light client #953
• WASM-compatible networking layer for the Light Client #957

There may be more issues that pop up in this process.

What's the definition of "done" for this issue?

When we can compile the tendermint-light-client crate to WASM and use all of its primary functionality (including networking, the verification algorithms, fork detection and evidence reporting) from JavaScript.

[shravanshetty1]

I have been trying to get this to work. I Implemented custom IO and EvidenceReporter traits using in memory store, however I am not able to figure out how to make requests in the trait methods.

tendermint-rs/light-client/src/components/io.rs

Line 120 in bbad6fa

fn fetch_light_block(&self, height: AtHeight) -> Result<LightBlock, IoError> {

tendermint-rs/light-client/src/components/io.rs

Line 158 in bbad6fa

let res = block_on(self.timeout, async move {

In the current implementation, when a request has to be made it blocks the thread until the call is complete, however I am not sure how this can be done in wasm since it does not allow any kind of blocking.

Also other than networking I see alot of blocking functionality being used in the crate such as message passing with crossbeam - I am not sure how we can make it wasm compatible.

[thanethomson]

@shravanshetty1 you're right. So right now there are two options here:

1. Use the fetch API to do the low-level networking, and then figure out how to make the async code play nicely with the non-async code.
2. Implement all I/O and the forwards verification algorithm in JavaScript and just make use of the verify method from the tendermint-light-client-js crate. This wraps the verification functionality from the tendermint-light-client crate without any I/O support.

[shravanshetty1]

For now on my fork, I made everything async and removed send+sync conditions, the tests pass. Seems to atleast get initialized but its panicing because the crate is using std::Time. Probably need to port it to https://github.com/chronotope/chrono for wasm support