Description:
------------
This is happening with stream filters which create more data then they consume. For example, base64_encode, zlib.inflate.

The issue is happening because stream_get_line internally calls php_stream_get_record function, which repeatedly calls php_stream_fill_read_buffer until enough data is "accumulated" in the buffer. Problem with that is repeated calls to populate read buffer force reset state of stream->readpos and stream->writepos in case filters exist on the stream.
In other words, if on second call to get_record function, buffer holds some data, but does not have delimiter in it and length of data is less then required, then function attempts to put more data into buffer, which is in effect, resets buffer and content is lost.



Test script:
---------------
<?php

$h = fopen('php://temp', 'r+b');
$len = floor(1.5 * 8 * 1024); // 8*1024 == stream->chunk_size. We need a bit more.
$data = str_repeat('1', $len);
fwrite($h, $data);
rewind($h);
stream_filter_append($h, 'convert.base64-encode');

$out = '';
while (!feof($h)) {
    $out .= stream_get_line($h, 1024);
}

fclose($h);
echo $len, ' and ', strlen(base64_decode($out)), PHP_EOL;
assert($data == base64_decode($out));

Expected result:
----------------
12288 and 12288

Actual result:
--------------
12288 and 11520

Warning: assert(): assert($data == base64_decode($out)) failed in /usr/src/app/t3.php on line 17