Do not delete main chunk in zend_gc

Closes GH-18756.

Co-authored-by: Arnaud Le Blanc <arnaud.lb@gmail.com>

Author: 2 people

NEWS

@@ -5,6 +5,7 @@ PHP                                                                        NEWS
 - Core:
   . Fixed GH-18695 (zend_ast_export() - float number is not preserved).
     (Oleg Efimov)
+  . Do not delete main chunk in zend_gc. (danog, Arnaud)
 
 - Curl:
   . Fix memory leak when setting a list via curl_setopt fails. (nielsdos)

Zend/tests/gh18756.phpt

@@ -0,0 +1,13 @@
+--TEST--
+Bug GH-18756: Zend MM may delete the main chunk
+--EXTENSIONS--
+zend_test
+--FILE--
+<?php
+
+zend_test_gh18756();
+
+?>
+==DONE==
+--EXPECT--
+==DONE==

Zend/zend_alloc.c

@@ -2047,7 +2047,7 @@ ZEND_API size_t zend_mm_gc(zend_mm_heap *heap)
 				i++;
 			}
 		}
-		if (chunk->free_pages == ZEND_MM_PAGES - ZEND_MM_FIRST_PAGE) {
+		if (chunk->free_pages == ZEND_MM_PAGES - ZEND_MM_FIRST_PAGE && chunk != heap->main_chunk) {
 			zend_mm_chunk *next_chunk = chunk->next;
 
 			zend_mm_delete_chunk(heap, chunk);

ext/zend_test/test.c

@@ -1516,3 +1516,13 @@ static PHP_FUNCTION(zend_test_create_throwing_resource)
 	zend_resource *res = zend_register_resource(NULL, le_throwing_resource);
 	ZVAL_RES(return_value, res);
 }
+
+static PHP_FUNCTION(zend_test_gh18756)
+{
+	ZEND_PARSE_PARAMETERS_NONE();
+
+	zend_mm_heap *heap = zend_mm_startup();
+	zend_mm_gc(heap);
+	zend_mm_gc(heap);
+	zend_mm_shutdown(heap, true, false);
+}

ext/zend_test/test.stub.php

@@ -262,6 +262,8 @@ function zend_test_cast_fread($stream): void {}
     function zend_test_is_zend_ptr(int $addr): bool {}
 
     function zend_test_log_err_debug(string $str): void {}
+
+    function zend_test_gh18756(): void {}
 }
 
 namespace ZendTestNS {