fix: refuse to start on a ZTS build with Zend signals

FrankenPHP has its own SAPI and never calls zend_signal_startup(), which
the cli and embed SAPIs of php-src do, so zend_signal_globals_id stays 0.
Registering the ini entries of the signal globals then resolves their
address through ts_resource(0), which in ZTS returns the tsrm_tls_entry
of the calling thread itself, so the write lands on the thread id stored
there. TSRM stops recognizing the thread from then on: every lookup
appends a new set of globals and tail-recurses, copying the constant,
function and class tables each time. The server never finishes booting
and its memory grows by hundreds of megabytes per second until it is
killed, with no error and no log line.

docs/compile.md and the Dockerfiles pass --disable-zend-signals, but PHP
enables Zend signals by default, so building PHP by hand without that
flag is enough to hit this. Check the flag PHP already reports through
frankenphp_get_config() and fail with an actionable error instead.

Non-ZTS builds are unaffected: their ini entries address the globals
directly, TSRM is not involved.

Author: nicolas-grekas

docs/compile.md

@@ -44,6 +44,7 @@ cd php-*/
 
 Then, run the `configure` script with the options needed for your platform.
 The following `./configure` flags are mandatory, but you can add others, for example, to compile extensions or additional features.
+`--disable-zend-signals` is one of them: FrankenPHP does not start Zend signals, and refuses to run on a ZTS build that enables them.
 
 #### Linux and FreeBSD
 

frankenphp.go

@@ -46,6 +46,7 @@ var (
 	ErrInvalidRequest     = errors.New("not a FrankenPHP request")
 	ErrAlreadyStarted     = errors.New("FrankenPHP is already started")
 	ErrInvalidPHPVersion  = errors.New("FrankenPHP is only compatible with PHP 8.2+")
+	ErrZendSignals        = errors.New(`PHP was built with Zend signals but FrankenPHP does not start them, which corrupts the state of PHP threads, recompile PHP with the "--disable-zend-signals" configuration option to fix this issue`)
 	ErrMainThreadCreation = errors.New("error creating the main thread")
 	ErrScriptExecution    = errors.New("error during PHP script execution")
 	ErrNotRunning         = errors.New("server is not registered, you must first call frankenphp.Init() with the WithServer() option")
@@ -156,6 +157,26 @@ func Config() PHPConfig {
 	}
 }
 
+// checkPHPConfig rejects the PHP builds FrankenPHP cannot run on
+func checkPHPConfig(config PHPConfig) error {
+	if config.Version.MajorVersion < 8 || (config.Version.MajorVersion == 8 && config.Version.MinorVersion < 2) {
+		return ErrInvalidPHPVersion
+	}
+
+	// FrankenPHP has its own SAPI and never calls zend_signal_startup(), so
+	// zend_signal_globals_id stays 0. In ZTS, the ini entries of the signal
+	// globals then resolve their address through ts_resource(0), which
+	// returns the TSRM entry of the thread itself, and writing a value there
+	// overwrites the thread id it holds. TSRM stops recognizing the thread
+	// and allocates a new set of globals on every lookup, so PHP never
+	// finishes booting and memory grows without bound
+	if config.ZTS && config.ZendSignals {
+		return ErrZendSignals
+	}
+
+	return nil
+}
+
 func calculateMaxThreads(opt *opt) (numWorkers int, _ error) {
 	maxProcs := runtime.GOMAXPROCS(0) * 2
 	maxThreadsFromWorkers := 0
@@ -294,9 +315,10 @@ func Init(options ...Option) error {
 
 	config := Config()
 
-	if config.Version.MajorVersion < 8 || (config.Version.MajorVersion == 8 && config.Version.MinorVersion < 2) {
+	if err := checkPHPConfig(config); err != nil {
 		shutdown()
-		return ErrInvalidPHPVersion
+
+		return err
 	}
 
 	if config.ZTS {

phpconfig_test.go

@@ -0,0 +1,37 @@
+package frankenphp
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCheckPHPConfig(t *testing.T) {
+	supported := PHPConfig{Version: PHPVersion{MajorVersion: 8, MinorVersion: 2}, ZTS: true}
+
+	t.Run("a supported build is accepted", func(t *testing.T) {
+		require.NoError(t, checkPHPConfig(supported))
+	})
+
+	t.Run("PHP older than 8.2 is rejected", func(t *testing.T) {
+		old := supported
+		old.Version.MinorVersion = 1
+		assert.ErrorIs(t, checkPHPConfig(old), ErrInvalidPHPVersion)
+	})
+
+	t.Run("Zend signals are rejected in ZTS", func(t *testing.T) {
+		signals := supported
+		signals.ZendSignals = true
+		assert.ErrorIs(t, checkPHPConfig(signals), ErrZendSignals)
+	})
+
+	t.Run("Zend signals are allowed without ZTS", func(t *testing.T) {
+		// the ini entries of the signal globals are addressed directly
+		// there, TSRM is not involved
+		signals := supported
+		signals.ZTS = false
+		signals.ZendSignals = true
+		require.NoError(t, checkPHPConfig(signals))
+	})
+}