Allow filtered streams to be casted as fd for select

This removes the artificial limitation that is not necessary. The fact
that some streams can have some data buffered is not a problem because
the similar situation is already present for OpenSSL streams where
OpenSSL can internally buffer data for the unprocessed part of the
record.

Author: bukka

ext/standard/tests/streams/stream_select_filtered_base64_basic.phpt

@@ -0,0 +1,32 @@
+--TEST--
+stream_select() base64-decode filter basic usage
+--FILE--
+<?php
+$file = fopen('php://temp', 'r+');
+
+// Write complete base64 "Hello World"
+fwrite($file, 'SGVsbG8gV29ybGQ=');
+
+stream_filter_append($file, 'convert.base64-decode');
+rewind($file);
+
+$read = [$file];
+$write = null;
+$except = null;
+
+$result = stream_select($read, $write, $except, 0);
+
+if ($result !== false) {
+    echo "stream_select succeeded\n";
+    if (count($read) > 0) {
+        echo fread($file, 1024) . "\n";
+    }
+} else {
+    echo "stream_select failed\n";
+}
+
+fclose($file);
+?>
+--EXPECT--
+stream_select succeeded
+Hello World

ext/standard/tests/streams/stream_select_filtered_base64_buffered.phpt

@@ -0,0 +1,59 @@
+--TEST--
+stream_select() base64-decode filter buffered data usage
+--FILE--
+<?php
+$domain = (strtoupper(substr(PHP_OS, 0, 3) == 'WIN') ? STREAM_PF_INET : STREAM_PF_UNIX);
+$pipes = stream_socket_pair($domain, STREAM_SOCK_STREAM, 0);
+if ($pipes === false) {
+    die("Failed to create socket pair");
+}
+
+list($read_pipe, $write_pipe) = $pipes;
+
+stream_set_blocking($read_pipe, false);
+stream_filter_append($read_pipe, 'convert.base64-decode');
+
+// Write incomplete base64 data: "SGVs" decodes to "Hel" but "SGVs" is incomplete
+fwrite($write_pipe, 'SGVs');
+fflush($write_pipe);
+
+$read = [$read_pipe];
+$write = null;
+$except = null;
+
+$result = fread($read_pipe, 1024);
+echo "Decoded content (before select): " . $result . "\n";
+
+// Should succeed but stream should NOT be readable yet (data buffered in filter)
+$result = stream_select($read, $write, $except, 0, 1000);
+echo "After incomplete data - select result: " . $result . "\n";
+echo "After incomplete data - readable streams: " . count($read) . "\n";
+
+// Now complete the base64 sequence: "SGVsbG8=" decodes to "Hello"
+fwrite($write_pipe, 'bG8=');
+fflush($write_pipe);
+
+$read = [$read_pipe];
+$write = null;
+$except = null;
+
+// Now stream should be readable
+$result = stream_select($read, $write, $except, 0, 1000);
+echo "After complete data - select result: " . $result . "\n";
+echo "After complete data - readable streams: " . count($read) . "\n";
+
+if (count($read) > 0) {
+    $content = fread($read_pipe, 1024);
+    echo "Decoded content: " . $content . "\n";
+}
+
+fclose($read_pipe);
+fclose($write_pipe);
+?>
+--EXPECT--
+Decoded content (before select): Hel
+After incomplete data - select result: 0
+After incomplete data - readable streams: 0
+After complete data - select result: 1
+After complete data - readable streams: 1
+Decoded content: lo

main/streams/cast.c

@@ -297,7 +297,7 @@ PHPAPI zend_result _php_stream_cast(php_stream *stream, int castas, void **ret,
 		}
 	}
 
-	if (php_stream_is_filtered(stream)) {
+	if (php_stream_is_filtered(stream) && castas != PHP_STREAM_AS_FD_FOR_SELECT) {
 		if (show_err) {
 			php_error_docref(NULL, E_WARNING, "Cannot cast a filtered stream on this system");
 		}