Add ability to hint AsyncTransport to drop certificate

Summary:
Many protocols often only require certificate information at the beginning of
the connection (for authentication and authorization). Often, the
protocol implementation will itself store its own representation of a transport
identity.

Certain certificates, such as X509 certificates, may be large and unnecessary
to hold onto during the entirety of a connection past the initial connection
establishment / handshaking phase. It is desirable to hint to transport
implementations when certificate information is no longer needed.

Reviewed By: AjanthanAsogamoorthy

Differential Revision: D26031748

fbshipit-source-id: de5164acfc141755debc0aa4f36474b1c7fd3109

folly/io/async/AsyncSocket.h

@@ -900,11 +900,15 @@ class AsyncSocket : public AsyncTransport {
     return peerCertData_.get();
   }
 
+  void dropPeerCertificate() noexcept override { peerCertData_.reset(); }
+
   void setSelfCertificate(
       std::unique_ptr<const AsyncTransportCertificate> cert) {
     selfCertData_ = std::move(cert);
   }
 
+  void dropSelfCertificate() noexcept override { selfCertData_.reset(); }
+
   const AsyncTransportCertificate* getSelfCertificate() const override {
     return selfCertData_.get();
   }

folly/io/async/AsyncTransport.h

@@ -634,6 +634,26 @@ class AsyncTransport : public DelayedDestruction,
     return nullptr;
   }
 
+  /**
+   * Hints to transport implementations that the associated certificate is no
+   * longer required by the application. The transport implementation may
+   * choose to free up resources associated with the peer certificate.
+   *
+   * After this call, `getPeerCertificate()` may return nullptr, even if it
+   * previously returned non-null
+   */
+  virtual void dropPeerCertificate() noexcept {}
+
+  /**
+   * Hints to transport implementations that the associated certificate is no
+   * longer required by the application. The transport implementation may
+   * choose to free up resources associated with the self certificate.
+   *
+   * After this call, `getPeerCertificate()` may return nullptr, even if it
+   * previously returned non-null
+   */
+  virtual void dropSelfCertificate() noexcept {}
+
   /**
    * Get the certificate information of this transport, if any
    */

folly/io/async/DecoratedAsyncTransportWrapper.h

@@ -165,10 +165,18 @@ class DecoratedAsyncTransportWrapper : public folly::AsyncTransport {
     return transport_->getPeerCertificate();
   }
 
+  void dropPeerCertificate() noexcept override {
+    transport_->dropPeerCertificate();
+  }
+
   const AsyncTransportCertificate* getSelfCertificate() const override {
     return transport_->getSelfCertificate();
   }
 
+  void dropSelfCertificate() noexcept override {
+    transport_->dropSelfCertificate();
+  }
+
   bool setZeroCopy(bool enable) override {
     return transport_->setZeroCopy(enable);
   }