final boolean tryComplete(@Nullable Throwable cause) {
        if (isComplete) {
            return false;
        }
        isComplete = true;
        if (cause == null) {
            completionFuture.complete(null);
        } else {
            completionFuture.completeExceptionally(cause);
        }

        return true;
    }
Returns whether a response has been finished.
core/src/main/java/com/linecorp/armeria/server/AbstractHttpResponseHandler.java