Description:
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This was detected in the phar extension, but can be reproduced with regular php code.

Because _php_stream_read in main/streams/streams.c automatically truncates output from a streams filter to the maximum requested amount, this does not allow the possibility of specifying pre-filter amount of bytes to read.

"new b" is 5 bytes, but zlib deflates it to a 7-byte length.    Because writepos is > toread, it is cut to 5 bytes.

A new flag is needed to specify that we want to read a maximum bytes, not write a maximum bytes.

Although I put "int(7)" in the expected output, I don't expect this output unless a flag is passed that specifically asks to do this.

Reproduce code:
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<?php
file_put_contents('test1.txt', 'new b/next thing');
$a = fopen('test1.txt', 'rb');
$newa = fopen('test2.txt', 'wb');
stream_filter_append($a, 'zlib.deflate');
$b = stream_copy_to_stream($a, $newa, 5);
var_dump($b, ftell($newa));

Expected result:
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int(5)
int(7)

Actual result:
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int(5)
int(5)