Missing ability to pause/resume the parser to prevent race condition in async event handlers & manage backpressure

[pqnet]

The event based parser lacks the ability to pause and resume parsing.
This is a problem because if processing of an event takes some time (for example, it requires doing some asynchronous i/o or network request) the parser will continue to parse the document and emit more events.
This has two unfortunate effects:

• it will quickly fill up memory if the input file is significantly large
• event handler will be executed in a non predictable order, causing race conditions.

This completely negate the main advantage of having an event based parser, that is the ability to deal with large files in a streaming fashion.

Example:

const parser = new SAXParser();
const writer = await Deno.open("data.jsonl", { write: true });
const encoder = new TextEncoder()
parser.on("end_element", async(e) => {
    const row = JSON.stringify(Object.fromEntries(e.attributes.map(a => [a.qName, a.value])) + "\n";
    let buffer = encoder.encode(row);
    while (buffer.length > 0) {
        const written = await writer.write(buffer);
        buffer = buffer.slice(written);
    }
})

const reader = await Deno.open("bigfile.xml");
await parser.parse(reader, "utf-8");

In this case, if the element being parsed is large, writer.write(buffer) won't be able to write the entire buffer in a single call, and other elements may begin being written on the file before the previous elements have finished writing.

A normal way to handle this is to pause parsing when an event handler starts, resuming it when the event handler ends.