Thread reboots leak static TLS when OPcache is loaded but disabled (opcache.enable=0): extensions stop loading #2730

What happened?

Prepared with an AI assistant (Claude), which ran the investigation and the reproduction below on my machine; I asked for this report.

With OPcache loaded but disabled (opcache.enable=0), every reboot of FrankenPHP's PHP threads (a worker restart through the admin API, or a file change with watch) leaks static TLS. After enough reboots, extensions can no longer be loaded at the next PHP startup:

PHP Warning:  PHP Startup: Unable to load dynamic library 'intl' (tried: …/intl (…: cannot open shared object file: No such file or directory), …/no-debug-zts-20240924/intl.so (…/intl.so: cannot allocate memory in static TLS block)) in Unknown on line 0

From then on the extension is missing in every request until the process is restarted (Class "Locale" not found, …). In our app (about 30 extensions, watch in development) this happened after about 95 saved files; with imagick the victim was libgomp.so.1: cannot allocate memory in static TLS block.

Minimal reproduction (official image, only intl)

Dockerfile:

FROM dunglas/frankenphp:1.13.1-php8.4-trixie
RUN install-php-extensions intl

Caddyfile:

{
	frankenphp {
		worker /app/public/worker.php 2
		php_ini opcache.enable 0
	}
}

:80 {
	root * /app/public
	php_server
}

public/worker.php:

<?php
while (frankenphp_handle_request(static function (): void {
    echo 'worker';
})) {
    gc_collect_cycles();
}

public/check.php:

<?php
echo extension_loaded('intl') ? 'intl loaded' : 'intl MISSING';

docker build -t fp-tls .
docker run -d --name fp-tls -v "$PWD/Caddyfile:/etc/frankenphp/Caddyfile:ro" -v "$PWD/public:/app/public:ro" fp-tls
for i in $(seq 1 1000); do
  docker exec fp-tls curl -s -X POST localhost:2019/frankenphp/workers/restart >/dev/null
  docker logs fp-tls 2>&1 | grep -q 'static TLS' && break
done
echo "after $i restarts: $(docker exec fp-tls curl -s localhost/check.php)"

Results (x86_64):

Likely cause

When OPcache is enabled, accel_startup() sets extension->handle = 0 ("Prevent unloading", ext/opcache/ZendAccelerator.c), so opcache.so stays loaded. When it is disabled, accel_startup() returns before that, so opcache.so is unloaded at module shutdown and loaded again by the php_module_startup() that rebootAllThreads() runs (frankenphp_new_main_thread). It looks like glibc does not give back the static TLS of the unloaded ZTS module, so every reboot takes more of the surplus until another extension no longer fits. This is our reading of the source and the measurements; we have not traced glibc's allocator.

Why opcache.enable=0, and workarounds

We disabled OPcache because of the OPcache races under ZTS (php-src GH-8739, #23637, #24319; see also #2714, #2715): in worker mode, restarting workers under load crashed the process with zend_mm_heap corrupted about once in 20 to 50 restarts on PHP 8.4.26.

What worked for us:

• Not loading OPcache instead of disabling it (what we use now).
• Raising glibc's reserve with GLIBC_TUNABLES=glibc.rtld.optional_static_tls=1048576: 400 reboots without failure. Note that glibc ignores GLIBC_TUNABLES for the image's /usr/local/bin/frankenphp, because its cap_net_bind_service file capability puts it in secure-execution mode. A copy of the binary without the capability honoured it.

Perhaps worth a note in the docs (disable OPcache by not loading it, not with opcache.enable=0). Alternatively, keep extensions loaded across a thread reboot, as an enabled OPcache does for itself. You know the trade-offs there better than we do.

Build Type

Docker (Debian Trixie)

Worker Mode

Yes

Operating System

GNU/Linux

CPU Architecture

x86_64

PHP configuration

FrankenPHP v1.13.1 PHP 8.4.26 Caddy v2.11.7 (dunglas/frankenphp:1.13.1-php8.4-trixie)
Thread Safety => enabled
glibc 2.41 (Debian 13), host kernel Linux 7.2 x86_64, Docker
Extensions: the image's defaults + intl (installed with install-php-extensions)
php_ini opcache.enable 0

Relevant log output

(after many "rebooting all PHP threads" / "workers restarted from admin api")
{"level":"warn","ts":1792408364.6600497,"logger":"frankenphp","msg":"PHP Warning:  PHP Startup: Unable to load dynamic library 'intl' (tried: /usr/local/lib/php/extensions/no-debug-zts-20240924/intl (/usr/local/lib/php/extensions/no-debug-zts-20240924/intl: cannot open shared object file: No such file or directory), /usr/local/lib/php/extensions/no-debug-zts-20240924/intl.so (/usr/local/lib/php/extensions/no-debug-zts-20240924/intl.so: cannot allocate memory in static TLS block)) in Unknown on line 0","syslog_level":"warning"}