add serdes API

[tniessen]

This implements the first suggestion from #69 plus a few utilities:

• functions uvwasi_serdes_{read,write}_X,
• constants UVWASI_SERDES_SIZE_X,
• macros UVWASI_SERDES_CHECK_BOUNDS(offset, size, X) and UVWASI_SERDES_CHECK_ARRAY_BOUNDS(offset, size, X, count).

Where X is any of fdstat_t, filestat_t, prestat_t, event_t, subscription_t, uint8_t, uint16_t, uint32_t, uint64_t.

The read/write functions take a const-correct void* base pointer and a size_t offset to remove the need for pointer casting and arithmetic on the embedder's size.

TODO:

• Add expected serialized data to the test case

Fixes: #69

[tniessen]

Thanks Anna, fixed! :)

[cjihrig]

LGTM, and thank you very much for picking this up!

[tniessen]

@cjihrig Is this what you had in mind for the scalar types? (I intentionally left out uvwasi_filedelta_t because it makes it unnecessarily complicated, and should never be written to or read from WebAssembly memory.)

[cjihrig]

Yes, thank you. LGTM.

[bnoordhuis]

UVWASI_SERDES_SIZE_prestat_t doesn't look correct to me. I see that you're serializing u.dir.pr_name_len as a uint32_t but its type is size_t (i.e., 64 bits on 64 bits architectures.)

I suppose that realistically it always fits in a uint32_t but I'd still add some kind of range check.

[tniessen]

I am confused about this part myself. WASI requires it to be a size_t, but I am not sure if that's a host type size_t or a WASM type size_t. Node.js currently uses 32 bits.

[cjihrig]

I went with 32 bits because that's what the wasi-libc did here.

Also, according to this ABI doc, size_t is defined as unsigned long, which has a sizeof four, and is represented as a wasm i32.

[tniessen]

@bnoordhuis If I am not mistaken, these sizes and types are mandated by the WASI C ABI, but @cjihrig will know for sure. UVWASI_SERDES_SIZE_event_t etc. don't actually represent the size required to serialize the data, but to properly align them in an array (e.g., event_t[n] should be laid out using n * UVWASI_SERDES_SIZE_event_t bytes by the C compiler).

Edit: Didn't see #94 (comment).

[cjihrig]

@tniessen just checking in - is this just waiting on Add expected serialized data to the test case? No rush, I just wanted to make sure you weren't blocked or anything.

[tniessen]

Yes, I would like to check the serialized outputs against known correct serializations. I just don't have that data at hand, I guess I'll have to compile some WASI code to WebAssembly and then extract the data from memory.

[tniessen]

@cjihrig I lost access to the machine that I worked on this PR on :( I am switching this to "ready for review" so you can merge it if you feel like it (or if anyone else wants to improve the unit tests). I don't know when I will find time to re-do this myself.

[cjihrig]

I'll look into using this with the current Node.js integration. If that works, then I'll land this.

[cjihrig]

Squashed and landed in 306e6fa. Also added support for the newly added uvwasi_size_t in 209edc1, and fixed up some typos in 7c9a5dd.

I integrated this with Node.js locally, and everything seemed to work. I'm going to leave #69 open though, as there are a couple missing pieces left to address.

Thanks for the PR Tobias, and thanks for the reviews Anna and Ben.