Support for OpenSslEngine with no finalizer

[johnou]

Motivation:

Support for Netty SslProvider.OPENSSL_REFCNT (OpenSSL-based implementation which does not have finalizers and instead implements ReferenceCounted).

Modification:

Add destroy method to SslEngineFactory to allow cleaning up reference counted SslContext.

Result:

Users can opt-in to a finalizer free OpenSslEngine and OpenSslContext.

[johnou]

Currently enabling openssl_refcnt using a custom DefaultSslEngineFactory..

public static class ReferenceCountedOpenSslEngineFactory extends DefaultSslEngineFactory {

  private SslProvider provider;

  @Override
  public void init(AsyncHttpClientConfig config) throws SSLException {
    this.provider = config.isUseOpenSsl() ? SslProvider.OPENSSL_REFCNT : SslProvider.JDK;
    super.init(config);
  }

  @Override
  protected SslContextBuilder configureSslContextBuilder(SslContextBuilder builder) {
    return builder.sslProvider(provider);
  }
}

[johnou]

As a side note I ran the ATC testsuite with -Dio.netty.leakDetectionLevel=paranoid which passed without any leak reports.

[slandelle]

Thanks!

[slandelle]

As a side issue, it would be nice to be able to enable from config instead of having to go with a custom SslEngineFactory.

[johnou]

@slandelle I did consider that, however, OPENSSL_REFCNT is currently marked UnstableApi and wanted to avoid dependency issues with Netty.

[slandelle]

Do you mean you're not sure OPENSSL_REFCNT is production ready yet and you want to experiment first?

[johnou]

I believe it is production ready, however it is still marked "unstable" in Netty here https://github.com/netty/netty/blob/4.1/handler/src/main/java/io/netty/handler/ssl/SslProvider.java#L37

[slandelle]

OK, let's leave as is for now.
Thanks!