Obfuscate the GlobalOpenTelemetry instance. (#2829)

* Obfuscate the GlobalOpenTelemetry instance.
This is to prevent people from casting to the SDK implementation.
Resolves #2788

* Add some more tests for the global obfuscation

api/all/src/main/java/io/opentelemetry/api/GlobalOpenTelemetry.java

@@ -14,6 +14,7 @@ import java.lang.reflect.Method;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
 
 /**
  * A global singleton for the entrypoint to telemetry functionality for tracing, metrics and
@@ -33,7 +34,7 @@ public final class GlobalOpenTelemetry {
 
   private static final Object mutex = new Object();
 
-  @Nullable private static volatile OpenTelemetry globalOpenTelemetry;
+  @Nullable private static volatile ObfuscatedOpenTelemetry globalOpenTelemetry;
 
   @GuardedBy("mutex")
   @Nullable
@@ -86,7 +87,7 @@ public final class GlobalOpenTelemetry {
                 + "instead. Previous invocation set to cause of this exception.",
             setGlobalCaller);
       }
-      globalOpenTelemetry = openTelemetry;
+      globalOpenTelemetry = new ObfuscatedOpenTelemetry(openTelemetry);
       setGlobalCaller = new Throwable();
     }
   }
@@ -166,4 +167,29 @@ public final class GlobalOpenTelemetry {
       return null;
     }
   }
+
+  /**
+   * Static global instances are obfuscated when they are returned from the API to prevent users
+   * from casting them to their SDK-specific implementation. For example, we do not want users to
+   * use patterns like {@code (OpenTelemetrySdk) GlobalOpenTelemetry.get()}.
+   */
+  @ThreadSafe
+  static class ObfuscatedOpenTelemetry implements OpenTelemetry {
+
+    private final OpenTelemetry delegate;
+
+    ObfuscatedOpenTelemetry(OpenTelemetry delegate) {
+      this.delegate = delegate;
+    }
+
+    @Override
+    public TracerProvider getTracerProvider() {
+      return delegate.getTracerProvider();
+    }
+
+    @Override
+    public ContextPropagators getPropagators() {
+      return delegate.getPropagators();
+    }
+  }
 }

sdk/all/src/test/java/io/opentelemetry/sdk/OpenTelemetrySdkTest.java

@@ -6,6 +6,7 @@
 package io.opentelemetry.sdk;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.assertj.core.api.InstanceOfAssertFactories.type;
 import static org.mockito.Mockito.mock;
 
@@ -43,11 +44,29 @@ class OpenTelemetrySdkTest {
 
   @Test
   void testRegisterGlobal() {
-    OpenTelemetrySdk sdk = OpenTelemetrySdk.builder().buildAndRegisterGlobal();
-    assertThat(sdk).isSameAs(GlobalOpenTelemetry.get());
-    assertThat(GlobalOpenTelemetry.get()).isSameAs(sdk);
-    assertThat(((OpenTelemetrySdk) GlobalOpenTelemetry.get()).getSdkTracerProvider().get(""))
-        .isSameAs(GlobalOpenTelemetry.getTracerProvider().get(""));
+    OpenTelemetrySdk sdk =
+        OpenTelemetrySdk.builder().setPropagators(propagators).buildAndRegisterGlobal();
+    assertThat(GlobalOpenTelemetry.get()).extracting("delegate").isSameAs(sdk);
+    assertThat(sdk.getTracerProvider().get(""))
+        .isSameAs(GlobalOpenTelemetry.getTracerProvider().get(""))
+        .isSameAs(GlobalOpenTelemetry.get().getTracer(""));
+
+    assertThat(GlobalOpenTelemetry.getPropagators())
+        .isSameAs(GlobalOpenTelemetry.get().getPropagators())
+        .isSameAs(sdk.getPropagators())
+        .isSameAs(propagators);
+  }
+
+  @Test
+  void castingGlobalToSdkFails() {
+    OpenTelemetrySdk.builder().buildAndRegisterGlobal();
+
+    assertThatThrownBy(
+            () -> {
+              @SuppressWarnings("unused")
+              OpenTelemetrySdk shouldFail = (OpenTelemetrySdk) GlobalOpenTelemetry.get();
+            })
+        .isInstanceOf(ClassCastException.class);
   }
 
   @Test