stream_copy_to_stream returns false when data copied is less than $maxlength

Description

The following code:

<?php
$streaminput = popen("my_command", "r");  // Some command which has generated output that sometimes has less than the 10000 read size bytes of data (if you care, I was actually calling postgres's pg_dump)
$streamoutput = popen("another_command", "w"); // I was actually using input pipe (i.e. stdin) of command started with proc_open() here (actually it was gzip, and I used proc_open so that I could read stdout here too), but AFAIK, that's just a stream to stdin as is the case in this example)
stream_set_blocking($streamoutput, false);

$expectedPos = 0;
while (!feof($streaminput)) {
  $copied = stream_copy_to_stream($streaminput, $streamoutput, 10000);
  $expectedPos += $copied;

  // if ($copied === false) { echo "\nCopied is false!";
  // if ($copied === 0) { echo "\nCopied is 0!"; // This message never printed

  if ($copied == 0) {  // Note: I later discovered that the unexpected false return caused the branch to execute because of == instead of ===, even though the stream position had advanced.
   echo ".";
   sleep 1;
   continue;
  else {
    echo "\nCopied $copied, Expected position $expectedPos, actual position " . ftell($streaminput);
  }

}

Resulted in this output:

Copied 10000, Expected position 10000, actual position 10000
Copied 10000, Expected position 20000, actual position 20000
Copied 10000, Expected position 30000, actual position 30000.
Copied 10000, Expected position 40000, actual position 48192
Copied 10000, Expected position 50000, actual position 58192

But I expected this output instead:

Copied 10000, Expected position 10000, actual position 10000
Copied 10000, Expected position 20000, actual position 20000
Copied 10000, Expected position 30000, actual position 30000
Copied 8192, Expected position 38192, actual position 38192
Copied 10000, Expected position 48192, actual position 48192

Notice how, in the real output, when the dot appears after 30000 (suggesting 0 bytes were copied, in fact 8192 bytes appear to have been copied according to the position returned by ftell()) - you can confirm $copied is false by uncommenting the relevant lines.

Each time I got a "." in the output I observed that the actual stream position had advanced, but always by less than the 10000 .... if it advanced by 10000, the value of $copied was 10000 and I got the full message. (edit: I've confirmed this by printing ftell($inputstream) with the "." in my local code, but I've not updated the example code/output above)

Also note that $streaminput never got to an eof state, and appeared most time to have simply stopped adding to the stream, and I end up killing the php script with ^C

PHP Version

PHP 8.1.2-1ubuntu2.13 (cli) (built: Jun 28 2023 14:01:49) (NTS)

Operating System

Ubuntu 22.04.2 LTS \n \l (running on WSL in Windows 11)

Start by reproducing the provided stream_copy_to_stream example with a non-blocking output stream and inspect the stream_copy_to_stream entry point in PHP's source. Compare the returned value with the input stream position when fewer than 10000 bytes are available. Done means partial copies report the number of bytes copied and the loop can reach EOF without hanging.