From 75d807add9e33c52d419b63e2c9e4e546fa5319b Mon Sep 17 00:00:00 2001
From: Trask Stalnaker <trask.stalnaker@gmail.com>
Date: Thu, 13 Feb 2020 12:46:49 -0800
Subject: [PATCH] Fix race condition on loading SDK (#165)

This broke during last merge from DataDog repo because context class
loader is no longer set to the agent's class loader.
---
 .../auto/tooling/AgentInstaller.java          | 21 ++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/agent-tooling/src/main/java/io/opentelemetry/auto/tooling/AgentInstaller.java b/agent-tooling/src/main/java/io/opentelemetry/auto/tooling/AgentInstaller.java
index b5c45cedc5..17fe0d0390 100644
--- a/agent-tooling/src/main/java/io/opentelemetry/auto/tooling/AgentInstaller.java
+++ b/agent-tooling/src/main/java/io/opentelemetry/auto/tooling/AgentInstaller.java
@@ -54,11 +54,22 @@ public class AgentInstaller {
   public static ResettableClassFileTransformer installBytebuddyAgent(
       final Instrumentation inst, final AgentBuilder.Listener... listeners) {
 
-    // need to trigger loading of OpenTelemetry SDK before instrumentation can possibly cause
-    // io.opentelemetry.OpenTelemetry to be loaded, since as soon as io.opentelemetry.OpenTelemetry,
-    // it looks up implementation via SPI, and if it doesn't find one, it loads the No-op
-    // implementation and it cannot be replaced later
-    OpenTelemetry.getTracerFactory();
+    final ClassLoader savedContextClassLoader = Thread.currentThread().getContextClassLoader();
+    try {
+      // calling (shaded) OpenTelemetry.getTracerFactory() with context class loader set to the
+      // agent class loader, so that SPI finds the agent's (isolated) SDK, and (shaded)
+      // OpenTelemetry registers it, and then when instrumentation calls (shaded)
+      // OpenTelemetry.getTracerFactory() later, they get back the agent's (isolated) SDK
+      //
+      // but if we don't trigger this early registration, then if instrumentation is the first to
+      // call (shaded) OpenTelemetry.getTracerFactory(), then SPI can't see the agent class loader,
+      // and so (shaded) OpenTelemetry registers the no-op TracerFactory, and it cannot be replaced
+      // later
+      Thread.currentThread().setContextClassLoader(AgentInstaller.class.getClassLoader());
+      OpenTelemetry.getTracerFactory();
+    } finally {
+      Thread.currentThread().setContextClassLoader(savedContextClassLoader);
+    }
 
     INSTRUMENTATION = inst;