Using aHash 0.5 with wasm

[emilk]

Hi!

I found a small issue when using aHash with --target wasm32-unknown-unknown. Starting in aHash version 0.5 I get this:

error: target is not supported, for more information see: https://docs.rs/getrandom/#unsupported-targets
   --> /Users/emil.ernerfeldt/.cargo/registry/src/github.com-1ecc6299db9ec823/getrandom-0.2.0/src/lib.rs:224:9
    |
224 | /         compile_error!("target is not supported, for more information see: \
225 | |                         https://docs.rs/getrandom/#unsupported-targets");
    | |_________________________________________________________________________^

error[E0433]: failed to resolve: use of undeclared type or module `imp`
   --> /Users/emil.ernerfeldt/.cargo/registry/src/github.com-1ecc6299db9ec823/getrandom-0.2.0/src/lib.rs:246:5
    |
246 |     imp::getrandom_inner(dest)
    |     ^^^ use of undeclared type or module `imp`

There is a simple workaround:

ahash = { version = "0.5", features = ["std"], default-features = false }
getrandom = { version = "0.2", features = ["js"] } # used by ahash

I was wondering if aHash could have some helpful feature flag for this (features = ["std", "wasm"]), or maybe even detect the wasm32 target? I'm not great with how Cargo.toml dependencies work :)

PS: I really love using aHash as a faster drop-in replacement for HashMap and HashSet.

[tkaitchuck]

Per the doc here it seems like this occurs because its not clear which interface it should use on wasm32-unknown-unknown. So if aHash were to set the "js" flag on getrandom it would be wrong if javascript were not available. So I don't know if there is anything I can directly do. There is some discussion of this issue here.

The only direct workaround I see is to do what you have outlined above or to depend on either wasm-bindgen-getrandom or stdweb-getrandom.

[emilk]

Ah, yeah that makes sense.

What is aHash using randomness for? Is it to defeat DOS attacks? Can it be made an opt-in?

[emilk]

I just realized I need my project to compile and work where there is no (easy) source of randomness (inside of wasmtime), so I think I'm forced to revert to aHash version 0.4 until there is a way to opt-out of the random source in aHash.

[tkaitchuck]

What is aHash using randomness for? Is it to defeat DOS attacks? Can it be made an opt-in?

Yes, and it is. See https://github.com/tkaitchuck/aHash/blob/master/compare/readme.md#dos-resistance

[tkaitchuck]

I just realized I need my project to compile and work where there is no (easy) source of randomness (inside of wasmtime), so I think I'm forced to revert to aHash version 0.4 until there is a way to opt-out of the random source in aHash.

No need to do that, just disable the std feature and enable compile-time-rng instead.

[emilk]

No need to do that, just disable the std feature and enable compile-time-rng instead.

If I do that I don't have access to AHashMap, since that requires the std feature to be enabled.

[tkaitchuck]

This may be orthogonal, but isn't wasmtime wasm32‑wasi rather than the more generic wasm32-unknown-unknown?
There is also some discussion here worth reading: rust-lang/futures-rs#1804

[emilk]

@tkaitchuck wasi is pretty new and experimental, and I would like my library to also work without it.