Fix read() on a directory fd returning EBADF instead of EISDIR

[SebTardif]

When read() is used on a directory file descriptor, POSIX requires EISDIR. On wasip2 the host read-via-stream call fails with bad-descriptor, and translate_error maps that to EBADF. On wasip3 the same host call traps (ErrorCode::BadDescriptor) because read-via-stream returns a stream, not a result.

That is the mismatch rust-lang/rust#160359 and uutils/coreutils#13625 are already working around in userland.

Change

• wasip2: after read-via-stream fails with bad-descriptor, get_type and return EISDIR if the fd is a directory.
• wasip3: descriptor_stat before opening the stream. Checking after the fact is not possible; the import traps.
• wasip1: after fd_read fails with BADF/ENOTCAPABLE, fd_fdstat_get and return EISDIR for a directory.
• Genuine EBADF (closed or invalid fd) is unchanged. The wasip2 success path does not add a stat.

Test

test/src/read-dir-eisdir.c: open a directory, read it, expect EISDIR. Invalid fd still EBADF. Regular file still reads.

CI on the first push: wasip1/p2 passed; wasip3 trapped in read-via-stream (the p3 pre-check is the follow-up on this branch).

References

• Issue: wasip2: read() returns EBADF instead of EISDIR when reading a directory fd #865
• POSIX read(): https://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html
• wasm32-wasi: opening/reading a directory as a file surfaces EBADF, not IsADirectory rust-lang/rust#160359
• feat(wasi): normalize wasi error codes and exit-code propagation uutils/coreutils#13625
• Introduced with the wasip2 read-via-stream path in wasip2: Use virtual dispatch in the descriptor table #662

Closes #865

[alexcrichton]

Thanks! I don't think though that this is something that's best to fix here, however, but rather inside of Wasmtime itself. Needing to perform extra syscalls just to map an error code feels a bit excessive when I think the fix in Wasmtime would probably be just a line or two

[SebTardif]

@alexcrichton

Thanks! I don't think though that this is something that's best to fix here, however, but rather inside of Wasmtime itself. Needing to perform extra syscalls just to map an error code feels a bit excessive when I think the fix in Wasmtime would probably be just a line or two

Agreed. I opened bytecodealliance/wasmtime#14135 so Descriptor::file() returns is-directory (and wasip3 read-via-stream reports that on the result future instead of trapping).

Once that lands, this libc extra get_type/fd_fdstat_get path is not needed for wasip2/wasip3. wasip1 still returns EBADF in Wasmtime (preview1 match and adapter), so I left that path alone here for now.