POSIX `readv()`/`preadv()`/`writev()`/`pwritev()` do not use all buffers under preview 2 (as found by Python)

[brettcannon]

Test Case

TODO: upload Wasm file here

Steps to Reproduce

• https://github.com/python/cpython/blob/07236f5b39a2e534cf190cd4f7c73300d209520b/Lib/test/test_posix.py#L520-L525
• https://github.com/python/cpython/blob/07236f5b39a2e534cf190cd4f7c73300d209520b/Lib/test/test_posix.py#L307-L311
• https://github.com/python/cpython/blob/07236f5b39a2e534cf190cd4f7c73300d209520b/Lib/test/test_posix.py#L487-L490
• https://github.com/python/cpython/blob/07236f5b39a2e534cf190cd4f7c73300d209520b/Lib/test/test_posix.py#L364-L369

Expected Results

To be told 10 bytes were read/written.

Actual Results

5 bytes, which matches the first buffer, not the total bytes written across all buffers.

Versions and Environment

Wasmtime version or commit: 17.0.0

Operating system: Linux/WSL

Architecture: x64

Extra Info

Success logs when testing readv():
success-readv.txt

Failuse logs:
failure-readv.txt

[brettcannon]

If I'm looking this up right,

wasmtime/crates/wasi/src/preview2/preview1.rs

Line 1309 in 5c0027b

async fn fd_read<'a>(

seems to ignore all but the first buffer.

wasmtime/crates/wasi-preview1-component-adapter/src/lib.rs

Line 989 in 5c0027b

pub unsafe extern "C" fn fd_read(

examines all of them.

[bjorn3]

The preview2 based preview1 implementation which is now used by default indeed only looks at the first buffer. See also #7757. wasi-preview1-component-adapter also only looks at the first buffer as far as I can see.

[alexcrichton]

Yes I can confirm that this is expected right now. It's expected that this is still posix-conformant in that posix doesn't guarantee everything is written. Vectored reads/writes aren't present in preview2 so they're polyfilled from preview1 by looking at the first buffer.

[brettcannon]

Yes I can confirm that this is expected right now.

As in this isn't expected to change? I think I can mark those POSIX functions as unsupported in https://github.com/python/cpython/blob/main/Tools/wasm/config.site-wasm32-wasi so the tests pass if you plan to keep the current semantics.

[alexcrichton]

Correct yeah, that's not expected to change.

[brettcannon]

I've tweaked the title and closing this as it's working as intended. I'll patch CPython to consider these functions as unavailable.