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RFC 7301 mandates that the server SHALL respond with a fatal "no_application_protocol" alert when there is no overlap between the client's supplied list and the server's list of supported protocols. In commit 062178678f5374b09f00d70796f6e692e8775aca we changed from ignoring non-success returns from the supplied alpn_select_cb() to treating such non-success returns as indicative of non-overlap and sending the fatal alert. In effect, this is using the presence of an alpn_select_cb() as a proxy to attempt to determine whether the application has configured a list of supported protocols. However, there may be cases in which an application's architecture leads it to supply an alpn_select_cb() but have that callback be configured to take no action on connections that do not have ALPN configured; returning SSL_TLSEXT_ERR_NOACK from the callback would be the natural way to do so. Unfortunately, the aforementioned behavior change also treated SSL_TLSEXT_ERR_NOACK as indicative of no overlap and terminated the connection; this change supplies special handling for SSL_TLSEXT_ERR_NOACK returns from the callback. In effect, it provides a way for a callback to obtain the behavior that would have occurred if no callback was registered at all, which was not possible prior to this change. Reviewed-by: Matt Caswell <matt@openssl.org> Reviewed-by: Rich Salz <rsalz@openssl.org> (Merged from https://github.com/openssl/openssl/pull/2570)