C binding: wasi std out to string

[ShuP1]

Feature

Rust library can bind WritePipe to WasiCtx stdout which can be far simpler and effective than OS files went dealing with multiple short-lived instances.

It is not currently possible with C/C++ bindings

Benefit

• Avoid useless error-prone and slow interactions by filesystem
• Allow simpler integration in freestanding targets

Implementation

Leverage existing wasi_common::pipe::WritePipe and ReadPipe on the rust side to implemented a binding in c-api crate.

Alternatives

Export some basic filesystem operations in c binding

[gzurl]

+1 for this feature.

When embedding Wasmtime in C (or C++), and enabling WASI extensions for a Wasm module, there is no way to capture stdout into a string. The only alternative seems to use [wasi.h]wasi_config_set_stdout_file(), which brings many other drawbacks (ie: no buffering, slow down, etc.)

[guregu]

Is this the blocker issue for bytecodealliance/wasmtime-go#34 and bytecodealliance/wasmtime-py#34? (cc @alexcrichton)

I would love to switch to wasmtime for my projects but this is preventing me from doing so.
We also need the ability to set a string for stdin as well. I'm not sure if there is a separate issue for that.

[alexcrichton]

Yes this is required for those issues.

[ShuP1]

The binding for stdout/stderr is a bit more complicated

First this api needs to return a new opaque struct for retrieve output after store drop.

Furthermore there is 2 properties of WritePipe than should probably be accessible:

• bounded or not:
  • provide a &mut [u8] for preallocated bounded size
  • use an internal Vec<u8> for unpredictable unbounded size
• shared: (note: internally buffer is always in Arc<RwLock<T>>)
  • WritePipe::new_in_memory can only be read after store drop
  • WritePipe::from_shared allows to read between function calls

[ShuP1]

Side question: is stdin streaming a desired feature ?

wasi_read_pipe_t in;
wasi_config_set_stdin_pipe(wasi_config, &in);
...
wasi_read_pipe_write(&in, &binary);
wasmtime_func_call(...); // consume stdin
wasi_read_pipe_write(&in, &binary);
wasmtime_func_call(...);
wasi_read_pipe_delete(&in);