fix(otel): do not wait for exit handlers to send responses

app/app.php

@@ -43,4 +43,21 @@
 \app\security\oauth\modules\OAuthHandlerModule::register(new \app\security\oauth\modules\GoogleOAuthHandler());
 \app\security\oauth\modules\OAuthHandlerModule::register(new \app\security\oauth\modules\SlackOAuthHandler());
 
-\vhs\web\HttpContext::Server()->handle();
+try {
+    \vhs\web\HttpContext::Server()->handle();
+} catch (\vhs\RequestFinished $e) {
+} finally {
+    /*
+     * This is here to fix a problem where exit-handlers need to finish dealing
+     * with a request e.g. by sending traces to an OpenTelemetry service, and
+     * that may take between a few ms and a while, which we should never make
+     * the client wait for.
+     *
+     * https://stackoverflow.com/questions/15273570/continue-processing-php-after-sending-http-response
+     */
+    session_write_close();
+    fastcgi_finish_request();
+    // Ensure that the root span of the server is always closed cleanly.
+    \vhs\web\HttpContext::Server()->endRootSpan();
+    exit();
+}

vhs/RequestFinished.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace vhs;
+
+/**
+ * Finishes processing the request and stops the HTTP server cleanly.
+ */
+class RequestFinished extends \Exception
+{
+}

vhs/web/HttpServer.php

@@ -81,8 +81,10 @@ private function beginRootSpan(): void {
      * Ends the root span for a request. Expected to be called unconditionally
      * at the end of a request's lifecycle (if not, the trace will most likely
      * be shown as missing a root span).
+     *
+     * Called by the RequestFinished exception handler.
      */
-    private function endRootSpan(): void {
+    public function endRootSpan(): void {
         if ($this->root_span) {
             $this->root_span->end();
         }
@@ -167,8 +169,7 @@ private function endResponse() {
             }
         }
 
-        $this->endRootSpan();
-        exit();
+        throw new \vhs\RequestFinished();
     }
 
     public function clear() {
@@ -217,7 +218,7 @@ public function sendOnlyHeaders() {
 
         $self = $this;
         array_push($this->headerBuffer, function() use ($self) {
-            exit();
+            throw new \vhs\RequestFinished();
         });
     }