[php-src] Issue #7846: curl and other native libraries block pcntl async signal propagation

Issue: https://github.com/php/php-src/issues/7846 Author: olsavmic ### Description The following code: ```php <?php pcntl_async_signals(true); $handler = function() { echo 'HANDLER CALLED' . PHP_EOL; pcntl_alarm(1); }; pcntl_signal(SIGALRM, $handler); pcntl_alarm(1); while (true) { $ch = curl_init(); //curl_setopt($ch, CURLOPT_NOPROGRESS, false); //curl_setopt($ch, CURLOPT_PROGRESSFUNCTION, function() {}); curl_setopt($ch, CURLOPT_URL, 'http://localhost/timeout.php'); curl_exec($ch); curl_close($ch); } ``` Gives following output: ``` Request started HANDLER CALLED Request done ``` But I expected this output instead: ``` Request started HANDLER CALLED HANDLER CALLED HANDLER CALLED HANDLER CALLED HANDLER CALLED HANDLER CALLED HANDLER CALLED HANDLER CALLED HANDLER CALLED Request done ``` PHP main thread is blocked during the entire request, the handler is called only after that. This behaviour renders pcntl_async_signals unusable for most workloads (eg. sending heartbeats to keep connections open during long running operations). The issue is not restricted to SIGALRM only, it applies to all signals. Such behaviour is not documented anywhere (including the RFC https://wiki.php.net/rfc/async_signals) yet the same problem actually applies to other native libraries like PDO/mysqli,... (see already reported issue for mysqli https://bugs.php.net/bug.php?id=81515) If this is expected behaviour, it must be documented as right now it seems like the only RELIABLE way to implement specific features but it behaves unpredictably with the use of 3rd party libraries and it's very hard to debug. Adding both curl_setopt($ch, CURLOPT_NOPROGRESS, false); and curl_setopt($ch, CURLOPT_PROGRESSFUNCTION, function() {}); fixes this particular issue but it's an undocumented required configuration and most likely just a side-effect. -------------------- Example of PDO query blocking: ```php <?php pcntl_async_signals(true); $handler = function () { echo 'Handler called'; pcntl_alarm(1); }; pcntl_signal(SIGALRM, $handler); pcntl_alarm(1); $conn = new PDO('mysql:host=localhost;dbname=test', 'root', 'admin'); $conn->query("SELECT SLEEP(10)"); ``` ### PHP Version 8.1, 8.0 ### Operating System Debian Buster, MacOS