Tracer references tests (#759)

* adding tests for tracer provider behaviour
a recent bug highlighted that if there is no reference to a tracer provider, then the shared state will shutdown, even if that shared state is being used by active tracers. Adding a phpdoc comment explaining this behavior, and some tests to demonstrate it

* moving tests into integration

* removing accidental covers annotation

Author: brettmc

examples/traces/demo/src/index.php

@@ -70,12 +70,12 @@
 
 //middleware starts root span based on route pattern, sets status from http code
 $app->add(function (Request $request, RequestHandler $handler) use ($tracer) {
-    $carrier = TraceContextPropagator::getInstance()->extract($request->getHeaders());
+    $parent = TraceContextPropagator::getInstance()->extract($request->getHeaders());
     $routeContext = RouteContext::fromRequest($request);
     $route = $routeContext->getRoute();
     $root = $tracer->spanBuilder($route->getPattern())
         ->setStartTimestamp((int) ($request->getServerParams()['REQUEST_TIME_FLOAT'] * 1e9))
-        ->setParent($carrier)
+        ->setParent($parent)
         ->setSpanKind(SpanKind::KIND_SERVER)
         ->startSpan();
     $root->activate();

src/SDK/Trace/TracerProvider.php

@@ -63,7 +63,11 @@ public function forceFlush(): bool
         return $this->tracerSharedState->getSpanProcessor()->forceFlush();
     }
 
-    /** @inheritDoc */
+    /**
+     * @inheritDoc
+     * @note Getting a tracer without keeping a strong reference to the TracerProvider will cause the TracerProvider to
+     *       immediately shut itself down including its shared state, ie don't do this: $tracer = (new TracerProvider())->getTracer('foo')
+     */
     public function getTracer(
         string $name,
         ?string $version = null,

tests/Integration/SDK/TracerProviderTest.php

@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OpenTelemetry\Tests\Integration\SDK;
+
+use OpenTelemetry\SDK\Trace\SpanProcessorInterface;
+use OpenTelemetry\SDK\Trace\TracerProvider;
+use PHPUnit\Framework\TestCase;
+use Prophecy\Prophet;
+
+/**
+ * @coversNothing
+ */
+class TracerProviderTest extends TestCase
+{
+    /**
+     * @doesNotPerformAssertions
+     */
+    public function test_tracer_shuts_down_immediately_when_out_of_scope(): void
+    {
+        $prophet = new Prophet();
+        $spanProcessor = $prophet->prophesize(SpanProcessorInterface::class);
+        // @phpstan-ignore-next-line
+        $spanProcessor->shutdown()->shouldBeCalledTimes(1);
+
+        /* Because no reference is kept to the TracerProvider, it will immediately __destruct and shutdown,
+        which will also shut down span processors in shared state. */
+        $tracer = (new TracerProvider($spanProcessor->reveal()))->getTracer('test');
+
+        $spanProcessor->checkProphecyMethodsPredictions();
+    }
+
+    /**
+     * @doesNotPerformAssertions
+     */
+    public function test_tracer_remains_in_scope(): void
+    {
+        $prophet = new Prophet();
+        $spanProcessor = $prophet->prophesize(SpanProcessorInterface::class);
+        // @phpstan-ignore-next-line
+        $spanProcessor->shutdown()->shouldBeCalledTimes(0);
+
+        $tracerProvider = new TracerProvider($spanProcessor->reveal());
+        $tracer = $tracerProvider->getTracer('test');
+
+        $spanProcessor->checkProphecyMethodsPredictions();
+    }
+}