fix: keep Go's preemption signal off the PHP threads

frankenphp.c

@@ -1874,6 +1874,20 @@ static void *php_thread(void *arg) {
    * grace period can wake it from a busy PHP loop or blocking syscall. */
   frankenphp_register_thread_for_kill(thread_index);
 
+#if !defined(PHP_WIN32) && defined(SIGURG)
+  /* That was the first call of this thread into Go, which unblocks, for
+   * good, the signal its runtime preempts goroutines with. The signal is
+   * sent to a thread while it runs Go code, and may only land once the
+   * thread is back in PHP and entering a system call, which then fails with
+   * EINTR: stream_select() returns false, a poll context throws. Blocked
+   * again from here on, the Go code this thread runs is preempted at its
+   * safe points only, which the callbacks of a PHP thread reach quickly. */
+  sigset_t preemption;
+  sigemptyset(&preemption);
+  sigaddset(&preemption, SIGURG);
+  pthread_sigmask(SIG_BLOCK, &preemption, NULL);
+#endif
+
   bool thread_is_healthy = true;
   bool has_attempted_shutdown = false;
 

frankenphp_test.go

@@ -191,6 +191,23 @@ func TestEnvVarsInPhpIni(t *testing.T) {
 	})
 }
 
+// The Go runtime preempts goroutines with SIGURG, which would interrupt the
+// system call a PHP thread sleeps in when it lands there
+func TestPreemptionSignalIsBlocked_module(t *testing.T) { testPreemptionSignalIsBlocked(t, nil) }
+func TestPreemptionSignalIsBlocked_worker(t *testing.T) {
+	testPreemptionSignalIsBlocked(t, &testOptions{workerScript: "signal-mask.php"})
+}
+func testPreemptionSignalIsBlocked(t *testing.T, opts *testOptions) {
+	if runtime.GOOS != "linux" {
+		t.Skip("the signal mask of a thread is read from /proc")
+	}
+
+	runTest(t, func(handler func(http.ResponseWriter, *http.Request), _ *httptest.Server, _ int) {
+		body, _ := testGet("http://example.com/signal-mask.php", handler, t)
+		assert.Equal(t, "blocked", body)
+	}, opts)
+}
+
 func TestFinishRequest_module(t *testing.T) { testFinishRequest(t, &testOptions{}) }
 func TestFinishRequest_worker(t *testing.T) {
 	testFinishRequest(t, &testOptions{workerScript: "finish-request.php"})

testdata/signal-mask.php

@@ -0,0 +1,11 @@
+<?php
+
+require_once __DIR__.'/_executor.php';
+
+// Tells whether the calling thread blocks SIGURG, the signal the Go runtime
+// preempts goroutines with: bit 22 of the mask the kernel reports.
+return function () {
+    preg_match('/^SigBlk:\s*([0-9a-f]+)/m', file_get_contents('/proc/thread-self/status'), $mask);
+
+    echo hexdec($mask[1]) >> 22 & 1 ? 'blocked' : 'not blocked';
+};