Fix bug #79945: Stream wrappers in imagecreatefrompng causes segfault

Closes GH-12696

Author: bukka

ext/gd/tests/bug79945.phpt

@@ -0,0 +1,22 @@
+--TEST--
+Bug #79945 (using php wrappers in imagecreatefrompng causes segmentation fault)
+--EXTENSIONS--
+gd
+--SKIPIF--
+<?php
+set_error_handler(function($errno, $errstr) {
+    if (str_contains($errstr, 'Cannot cast a filtered stream on this system')) {
+        die('skip: fopencookie not support on this system');
+    }
+});
+imagecreatefrompng('php://filter/read=convert.base64-encode/resource=' . __DIR__ . '/test.png');
+restore_error_handler();
+?>
+--FILE--
+<?php
+imagecreatefrompng('php://filter/read=convert.base64-encode/resource=' . __DIR__ . '/test.png');
+?>
+--CLEAN--
+--EXPECTF--
+
+Warning: imagecreatefrompng(): "php://filter/read=convert.base64-encode/resource=%s" is not a valid PNG file in %s on line %d

main/php_streams.h

@@ -211,6 +211,9 @@ struct _php_stream  {
 	 * PHP_STREAM_FCLOSE_XXX as appropriate */
 	uint8_t fclose_stdiocast:2;
 
+	/* whether stdio cast flushing is in progress */
+	int8_t fclose_stdiocast_flush_in_progress:1;
+
 	char mode[16];			/* "rwb" etc. ala stdio */
 
 	uint32_t flags;	/* PHP_STREAM_FLAG_XXX */

main/streams/streams.c

@@ -1294,8 +1294,13 @@ PHPAPI zend_off_t _php_stream_tell(php_stream *stream)
 PHPAPI int _php_stream_seek(php_stream *stream, zend_off_t offset, int whence)
 {
 	if (stream->fclose_stdiocast == PHP_STREAM_FCLOSE_FOPENCOOKIE) {
-		/* flush to commit data written to the fopencookie FILE* */
-		fflush(stream->stdiocast);
+		/* flush can call seek internally so we need to prevent an infinite loop */
+		if (!stream->fclose_stdiocast_flush_in_progress) {
+			stream->fclose_stdiocast_flush_in_progress = 1;
+			/* flush to commit data written to the fopencookie FILE* */
+			fflush(stream->stdiocast);
+			stream->fclose_stdiocast_flush_in_progress = 0;
+		}
 	}
 
 	/* handle the case where we are in the buffer */