Add static helper to extract X509 from cert

Summary:
Since callsites wanting an X509 from an AsyncTransportCertificate will have to
perform a dynamic cast to an OpenSSLTransportCertificiate, we might as well
abstract that away into a static helper.

This makes it potentially a bit easier to change implementation details in the
future without having to update a bunch of existing callsites (and it hides the
ugliness of the dynamic cast).

Reviewed By: yfeldblum, mingtaoy

Differential Revision: D31358948

fbshipit-source-id: 9ed2f81db4cf746a83871961895a0c1f1fb5c297

folly/io/async/ssl/OpenSSLTransportCertificate.h

@@ -33,5 +33,11 @@ class OpenSSLTransportCertificate : public AsyncTransportCertificate {
   // TODO: Once all callsites using getX509() perform dynamic casts to this
   // OpenSSLTransportCertificate type, we can move that method to be declared
   // here instead.
+
+  static ssl::X509UniquePtr tryExtractX509(
+      const AsyncTransportCertificate* cert) {
+    auto opensslCert = dynamic_cast<const OpenSSLTransportCertificate*>(cert);
+    return opensslCert ? opensslCert->getX509() : nullptr;
+  }
 };
 } // namespace folly