Fix fdPread: shouldn't move file offset

[ncruces]

Fixes #958.

This is breaking a zig test, I still haven't been able to figure out why.

[ncruces]

It seems, at the root of it, we have that embed.openfile does not implement io.ReaderAt. 😔

[ncruces]

I could Seek, Read and then Seek back. But if any operation fails, I'm breaking the invariant of pread: “The file offset is not changed.”

[codefromthecrypt]

@ncruces is it required for embed to implement this, or is it just due to our tests using embed?

[ncruces]

No, not required. It's just due to tests using embed.

But it also means anyone using embed will start seeing pread failures. And it seems zig uses pread for most (all?) reads.

I came up with an alternative implementation that tries io.ReaderAt and falls back to io.Seeker doing multiple seeks. All tests pass. ncruces@e12b34e

There's some prior to doing this. For instance, see Go's implementation of pread under Windows:
https://github.com/golang/go/blob/e870de9936a7efa42ac1915ff4ffb16017dbc819/src/internal/poll/fd_windows.go#L538-L546

[codefromthecrypt]

thanks for the notes and the research leading to them!

[codefromthecrypt]

If you want to go with an alternative fix, works for me, @achille-roussel will be starting some more abstraction work soon, and the important part is to have tests in to make sure this doesn't re-break later.

[ncruces]

I'll go with this since at least it means no regression for zig folks.

The big issue is this leaving the file offset in an inconsistent state if the initial seeks succeed but the final one fails. It's still an improvement over the current situation (final offset is always wrong).

Other considerations don't really apply (pread is supposed to be thread-safe, but there's no concurrency in WASM). I can add some comments pointing this out.

I'll work on tests now.

[codefromthecrypt]

@ncruces sounds good. we're going to cut the release in a couple days, and this is great to go in. You can add notes in RATIONALE.md and then code comment to there on the tradeoffs. you'll notice there are others.

[codefromthecrypt]

work in progress looks excellent! I'll leave you to continue with stdlib tests unless you've run out of steam.

[codefromthecrypt]

I drifted you. for names, use export (capitalize first letter) and dot import like so . "github.com/tetratelabs/wazero/internal/wasi_snapshot_preview1"

[codefromthecrypt]

great and professional work!

if you like I can fix the drift I caused.

[codefromthecrypt]

thanks for hanging in there @ncruces!

[ncruces]

No problem.
Thank you for wazero!