Cache SSL_CREDENTIAL selected during handshake so getSelectedCredential works post-handshake

SSL_get0_selected_credential reads the handshake state, which BoringSSL frees
before SSL_do_handshake returns, so SSL.getSelectedCredential always returned 0
once the handshake completed.

Sample the credential in ssl_info_callback on SSL_CB_HANDSHAKE_DONE, hold a ref in
tcn_ssl_state_t and release it in free_ssl_state. On renegotiation the previous
ref is released and replaced, so the cache reflects the most recent completed
handshake. getSelectedCredential tries the live lookup first (non-NULL only while
the handshake state exists) and falls back to the cached credential.

Author: Alex Choulos

openssl-classes/src/main/java/io/netty/internal/tcnative/SSL.java

@@ -997,7 +997,10 @@ public static AsyncTask getAsyncTask(long ssl) {
      * <p>This is a BoringSSL-specific feature. See 
      * <a href="https://commondatastorage.googleapis.com/chromium-boringssl-docs/ssl.h.html#SSL_get0_selected_credential">SSL_get0_selected_credential</a>
      * for detailed documentation.</p>
-     * 
+     *
+     * <p>Once a handshake has completed this returns the credential selected during the most recent completed
+     * handshake. The returned pointer is borrowed and stays valid until {@link #freeSSL(long)}; do not free it.</p>
+     *
      * @param ssl the SSL instance (SSL *)
      * @return the selected credential (SSL_CREDENTIAL *) or {@code 0} if none
      * @throws Exception if an error occurred

openssl-dynamic/src/main/c/ssl.c

@@ -933,6 +933,13 @@ static void free_ssl_state(JNIEnv* e, tcn_ssl_state_t* state) {
     tcn_ssl_task_free(e, state->ssl_task);
     state->ssl_task = NULL;
 
+#ifdef OPENSSL_IS_BORINGSSL
+    if (state->selected_credential != NULL) {
+        SSL_CREDENTIAL_free((SSL_CREDENTIAL*) state->selected_credential);
+        state->selected_credential = NULL;
+    }
+#endif
+
     // Free the tcn_ssl_state_t itself as it was allocated via OPENSSL_malloc(...) before
     //
     // https://github.com/netty/netty-tcnative/issues/532
@@ -2763,6 +2770,13 @@ TCN_IMPLEMENT_CALL(jlong, SSL, getSelectedCredential)(TCN_STDARGS, jlong ssl) {
     SSL *ssl_ = J2P(ssl, SSL *);
     TCN_CHECK_NULL(ssl_, ssl, 0);
     const SSL_CREDENTIAL* credential = SSL_get0_selected_credential(ssl_);
+    if (credential == NULL) {
+        // Handshake state is gone once the handshake completed, use the credential cached by ssl_info_callback.
+        tcn_ssl_state_t* state = tcn_SSL_get_app_state(ssl_);
+        if (state != NULL) {
+            credential = state->selected_credential;
+        }
+    }
     if (credential == NULL) {
         return 0;
     }

openssl-dynamic/src/main/c/ssl_private.h

@@ -411,6 +411,10 @@ struct tcn_ssl_state_t {
     // len < sk_CRYPTO_BUFFER_num(chain) check (both locals are 0/NULL there).
     int task_array_len;
     int task_chain_num;
+#ifdef OPENSSL_IS_BORINGSSL
+    // Ref-counted credential sampled in SSL_CB_HANDSHAKE_DONE, as the selection is gone once the handshake completes.
+    const SSL_CREDENTIAL *selected_credential;
+#endif
 };
 
 #define TCN_GET_SSL_CTX(ssl, C)                             \

openssl-dynamic/src/main/c/sslcontext.c

@@ -189,6 +189,22 @@ static void ssl_info_callback(const SSL *ssl, int where, int ret) {
             state->handshakeCount++;
         }
     }
+#ifdef OPENSSL_IS_BORINGSSL
+    if (0 != (where & SSL_CB_HANDSHAKE_DONE)) {
+        // The selection lives in the handshake state, which BoringSSL frees before SSL_do_handshake returns, so it
+        // must be sampled here. On renegotiation the previous ref is replaced by the most recent selection.
+        if ((state = tcn_SSL_get_app_state(ssl)) != NULL) {
+            const SSL_CREDENTIAL* credential = SSL_get0_selected_credential(ssl);
+            if (credential != NULL) {
+                SSL_CREDENTIAL_up_ref((SSL_CREDENTIAL*) credential);
+                if (state->selected_credential != NULL) {
+                    SSL_CREDENTIAL_free((SSL_CREDENTIAL*) state->selected_credential);
+                }
+                state->selected_credential = credential;
+            }
+        }
+    }
+#endif
 }
 
 /* Initialize server context */