wasi: return NotPermitted for stream open permission failures

[SebTardif]

Summary

read_via_stream, write_via_stream, and append_via_stream returned ErrorCode::BadDescriptor when the descriptor lacked the required read/write permission. The non-stream read / write paths in the same module already return NotPermitted for the same condition.

Why this change is needed

Permission denial should use the permission-denied error code. BadDescriptor maps to a different condition (invalid/wrong kind of FD). Guests that branch on not-permitted vs bad-descriptor currently get the wrong signal when opening a stream view on a file opened without the needed access.

Change

Three sites in crates/wasi/src/p2/host/filesystem.rs:

- Err(types::ErrorCode::BadDescriptor)?;
+ Err(types::ErrorCode::NotPermitted)?;

aligned with:

// async fn read / write
if !f.perms.contains(FilePerms::READ/WRITE) {
    return Err(ErrorCode::NotPermitted.into());
}

Testing

• cargo test -p wasmtime-wasi --test all --features p2 p2_file_stream_not_permitted passes (sync + async harness).
• Guest program p2_file_stream_not_permitted covers:
  • read-only preopen (FilePerms::READ): write_via_stream / append_via_stream return ErrorCode::NotPermitted; read_via_stream still works.
  • write-only preopen (FilePerms::WRITE): read_via_stream returns ErrorCode::NotPermitted; write_via_stream still works.

[alexcrichton]

Thanks! Could you add a test for this as well?

[SebTardif]

@alexcrichton Added a focused test in cbf77768a5:

• Guest: crates/test-programs/src/bin/p2_file_stream_not_permitted.rs
• Harness: p2_file_stream_not_permitted in both crates/wasi/tests/all/p2/sync.rs and async_.rs

It asserts ErrorCode::NotPermitted for write_via_stream / append_via_stream under a read-only preopen and for read_via_stream under a write-only preopen (while the allowed stream path still succeeds).

Verified locally with:

cargo test -p wasmtime-wasi --test all --features p2 p2_file_stream_not_permitted
# 2 passed (sync + async)