process.stdin seems to fail to populate the internal buffer

[gh-andre]

Please, note I'm not looking for advice on how to read process.stdin using events.

I'm trying to read from the process.stdin stream using paused mode and no matter what I do, process.stdin.read() always returns null.

Here's a simple example.

// this flips process.stdin.readableFlowing from null to false
process.stdin.pause();

// true
console.log("isPaused(): " + process.stdin.isPaused());
// true
console.log("readable: " + process.stdin.readable);
// false
console.log("readableFlowing: " + process.stdin.readableFlowing);

// this will print stdout
// console.log("fs: " + require('fs').readFileSync(process.stdin.fd).toString());

// this will produce chunk with XYZ
// process.stdin.push(Buffer.from("XYZ"));

var chunk = process.stdin.read();

// chunk is always null
console.log("Chunk? " + (chunk ? ("yes: " + chunk.toString()) : "no"));

I run this on Windows 10, Node v16.13.0, as:

echo "ABC" | node abc.js

Using events works, as does using fs to read directly from the stdin handle. Pushing buffers into process.stdin also works. It's as if process.stdin doesn't do the first read to populate the internal buffer. Thoughts?

[gh-andre]

I did some debugging and turns out that the internal stream read buffer is indeed not populated initially, even if there is data in the input. The implementation is such that the readable event must be used it has nothing to do with the readable state indicator. In fact, the latter just indicates that the stream hasn't gone bad.

The example from the docs is the only way to read from process.stdin via read().

console.log("isPaused(): " + process.stdin.isPaused());
console.log("readable: " + process.stdin.readable);
console.log("readableFlowing: " + process.stdin.readableFlowing);

process.stdin.on("readable", () => {
  let chunk = process.stdin.read();

  if(chunk)
    console.log("chunk: " + chunk.toString(), "readable: " + process.stdin.readable);
  else
    console.log("end; readable: " + process.stdin.readable);
});

, when launched this way.

echo ABC | node abc.js

, and will produce this output:

isPaused(): false
readable: true
readableFlowing: null
chunk: ABC
 readable: true
end; readable: true

The implementation appears fragile and may behave in unpredictable ways. For example, calling process.stdin.pause() and process.stdin.read() outside of the readable event will suppress the event in the first place, but calling either those those alone will not.

The readable indicator evaluates as true at all times and is really a misnomer. It should have been called good or usable or something along these lines. It would be useful if the docs would mention that it doesn't indicate that there's data to read.

If wish process.stdin would provide a simpler implementation for small streams, that would make a lot of utility scripts simpler, like this:

while(process.stdin.really_readabale) {
  if((chunk = process.stdin.read()))
    consume_chunk(chunk);
  else
    process_eof();
}

if(!seen_eof)
  go_async_for_more();

Posting in hopes somebody finds this useful.