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[2013-10-02 22:09 UTC] askalski at gmail dot com
Description: ------------ php_module_startup() initializes modules first, extensions second. However, zend_shutdown() destroys them in the same order (modules first, extensions second), rather than in stack order as one would expect. Furthermore, it seems (based on reading the zend_startup_extensions() zend_shutdown_extensions() functions) that if multiple zend extensions are loaded, they are destroyed in the wrong order as well. Multiple modules work fine; they are destroyed in stack order. To reproduce the issue, load an extension and module which both override the same Zend structure. For example, loading both OpCache 7.0.2 and APC 3.1.13 will cause a segfault on shutdown because of improper cleanup order of orig_interned_strings_start, old_interned_strings_start, and compiler_globals.interned_strings_start. I'm aware that the example sounds like a bizarre combination of modules and extensions here; I'm reporting the bug because it points at an issue in PHP itself. The specific use case for loading both APC and OpCache is to use OpCache for opcodes and APC with apc.cache_by_default=0 for the apc_store/apc_fetch userland functions. Test script: --------------- zend_extension=/usr/lib64/php/modules/opcache.so extension=apc.so Load both OpCache and APC in mod_php in Apache "prefork" mode. Send SIGTERM to one of the workers (or simply send enough requests to make Apache reap the worker), and watch for the Segmentation fault in Apache's error_log. Expected result: ---------------- No crash. Actual result: -------------- Segmentation fault. PatchesPull RequestsHistoryAllCommentsChangesGit/SVN commits
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Spent some time digging into this. It's not as simple as calling zend_shutdown_extensions before zend_destroy_modules; there are a few issues at play which complicate matters. The two Zend Extensions I investigated (OpCache and XDebug) implement both the zend_extension and zend_module API. Both of these extensions load the zend_module portion by calling zend_startup_module in the zend_extension startup function. Also, it is possible to load modules at runtime using the dl() userland function. Consequently, the shutdown order is not a simple matter of "extensions first, then modules". Because startup order of extensions and modules can be interleaved, this precise order must be recorded during initialization. The implementation might be something as simple as a stack of enumerated values: { MODULE, MODULE, MODULE, MODULE, EXTENSION, MODULE, EXTENSION, MODULE }Thanks; wasn't aware this had been fixed on APC trunk. Just noticed now that APC has been moved from subversion to git - this made my day. Unaware of the already-committed fixes, I had started on a parallel development path of configuration to disable opcode caching and interned strings. One thing I did in my version of the patch which you might consider: In PHP_MINIT_FUNCTION(apc), I have it call zend_get_extension("Zend OPcache") to see if opcache is loaded, and if so, force-disable opcode caching and interned strings. It's a minor convenience, but could help to avoid questions in the future.