Add an exception mapper for CompletionExceptions

service/src/main/java/org/whispersystems/textsecuregcm/mappers/CompletionExceptionMapper.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2020 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.mappers;
+
+import java.util.concurrent.CompletionException;
+import javax.ws.rs.core.Context;
+import javax.ws.rs.core.Response;
+import javax.ws.rs.ext.ExceptionMapper;
+import javax.ws.rs.ext.Provider;
+import javax.ws.rs.ext.Providers;
+
+@Provider
+public class CompletionExceptionMapper implements ExceptionMapper<CompletionException> {
+
+  @Context
+  private Providers providers;
+
+  @Override
+  public Response toResponse(final CompletionException exception) {
+    final Throwable cause = exception.getCause();
+
+    if (cause != null) {
+      final Class type = cause.getClass();
+      return providers.getExceptionMapper(type).toResponse(cause);
+    }
+
+    return Response.serverError().build();
+  }
+}