fs: extracts syscallfs.ReaderAtOffset for WASI and gojs

[codefromthecrypt]

This extracts a utility syscallfs.ReaderAtOffset() to allow WASI and gojs to re-use the same logic to implement syscall.Pread.

What's different than before is that if WASI passes multiple iovecs an emulated ReaderAt will seek to the read position on each call to Read vs once per loop. This was a design decision to keep the call sites compatible between files that implement ReaderAt and those that emulate them with Seeker (e.g. avoid the need for a read-scoped closer/ defer function). The main use case for emulation is embed.file, whose seek function is cheap, so there's little performance impact to this.

goos: darwin
goarch: amd64
pkg: github.com/tetratelabs/wazero/internal/syscallfs
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
BenchmarkReaderAtOffset
BenchmarkReaderAtOffset/os.DirFS_io.File
BenchmarkReaderAtOffset/os.DirFS_io.File-16         	  549439	      2196 ns/op
BenchmarkReaderAtOffset/os.DirFS_readerAtOffset
BenchmarkReaderAtOffset/os.DirFS_readerAtOffset-16  	  612918	      1985 ns/op
BenchmarkReaderAtOffset/embed.FS_embed.file
BenchmarkReaderAtOffset/embed.FS_embed.file-16      	10059037	       126.1 ns/op
BenchmarkReaderAtOffset/embed.FS_seekToOffsetReader
BenchmarkReaderAtOffset/embed.FS_seekToOffsetReader-16         	 7055109	       179.0 ns/op

[codefromthecrypt]

cc @ncruces as this moves around the stuff you made, and thanks for that!

[ncruces]

I'll try to have a stab at convincing the Go folks to add ReaderAt to embed. There's no good reason to for it not to. But that's always at least a year off.

[codefromthecrypt]

I'll try to have a stab at convincing the Go folks to add ReaderAt to embed. There's no good reason to for it not too. But that's always at least a year off.

Thanks, I scratched my head looking at it, too, as it seems as easy as mapfs. You are right though, there is a socialization, review and landing concern. Thanks for offering to take that on which is much more effort than the code and tests!

[ncruces]

Got it merged, should be in 1.21:
golang/go#57803 (comment)