Description:
------------
CURLOPT_READFUNCTION waits $fp readable only once.
Is this a bug? Or by design?



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

/*

mkfifo /path/to/fifo

*/


$fp = fopen('/path/to/fifo', 'rb');

$ch = curl_init();
curl_setopt_array($ch, [
    CURLOPT_URL => 'example.com',
    CURLOPT_PUT => true,
    CURLOPT_INFILE => $fp,
    CURLOPT_INFILESIZE => 5,
    CURLOPT_READFUNCTION => function ($ch, $fp, $max) {
        $buf = fread($fp, 1);
        var_dump($buf);
        return $buf;
    },
]);
curl_exec($ch);

/*

We can verify that CURLOPT_READFUNCTION invocation is blocked.
However, after we executed the following command...

printf "a" > /path/to/fifo

*/

?>

Expected result:
----------------
string(1) "a"

...then invocation should be blocked again

Actual result:
--------------
string(1) "a"
string(0) ""

...wtf!?