Consider dropping Send requirement on generic Read param to FastxReader

[jchorl]

Howdy!

I was playing around with some wasm stuff using needletail. One of the main challenges is that most js_sys js objects are not Send.

Consider this example:

use needletail::parse_fastx_reader;
use wasm_bindgen::prelude::*;
use web_sys::FileReaderSync;

struct MyReader {
    reader: FileReaderSync,
}

impl std::io::Read for MyReader {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> { Ok(0) }
}

#[wasm_bindgen]
pub fn some_func(reader: FileReaderSync) {
    parse_fastx_reader(MyReader{ reader });
}

Running wasm-pack build --target web:

error[E0277]: `*mut u8` cannot be sent between threads safely                  
    --> src/lib.rs:17:24                                                                                                                                       
     |                                                                         
17   |     parse_fastx_reader(MyReader{ reader });   
     |     ------------------ ^^^^^^^^^^^^^^^^^^ `*mut u8` cannot be sent between threads safely                                                               
     |     |                                                                   
     |     required by a bound introduced by this call                         
     |                                 
     = help: within `MyReader`, the trait `Send` is not implemented for `*mut u8`, which is required by `MyReader: Send`

...

note: required by a bound in `parse_fastx_reader`
    --> /home/j/third-party/needletail/src/parser/mod.rs:85:50
     |
85   | pub fn parse_fastx_reader<'a, R: 'a + io::Read + Send>(
     |                                                  ^^^^ required by this bound in `parse_fastx_reader`

However, it seems Send isn't strictly required. Dropping Send from the FastxReader here:

needletail/src/parser/utils.rs

Line 119 in ba898bd

pub trait FastxReader: Send {

And dropping it from both impls, the fastq and fasta reader, fixes compilation.

I'm sure I could wrap the reader in a mutex to make it Send but hammering a mutex (for lots of reads) introduces a lot of overhead.

From reading, it seems like Send is a nicety but even without the supertrait, if a caller passes in a Read that is also Send, the returned FastxReader would still be Send (but it's not always guaranteed to be). I'm not positive I have this correct.

So anyway, I wanted to see if you'd be open to removing Send from the fastx reader, and if you know of anything that could break in that case.

Thanks!

[luizirber]

I got needletail to work in wasm with an, err, cheat:
https://github.com/sourmash-bio/sourmash/blob/af534a9a04c4712e5d727832036dac73757b55ce/src/core/src/wasm.rs#L231-L233

I think this should be OK because using FileSyncReader implies it is running inside a web worker, and that is "single-threaded" and won't be send across "thread" boundaries. I might be wrong, but for now this has been working in the web component in https://branchwater.sourmash.bio and https://chill-filter.sourmash.bio without issues