Add SSL_CTX_get_keylog_callback.

Conscrypt would like to write a CTS test that the callback isn't set
unexpectedly.

Change-Id: I11f987422daf0544e90f5cff4d7aaf557ac1f5a2
Reviewed-on: https://boringssl-review.googlesource.com/11060
Reviewed-by: Kenny Root <kroot@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/ssl.h

@@ -2847,6 +2847,11 @@ OPENSSL_EXPORT void SSL_set_msg_callback_arg(SSL *ssl, void *arg);
 OPENSSL_EXPORT void SSL_CTX_set_keylog_callback(
     SSL_CTX *ctx, void (*cb)(const SSL *ssl, const char *line));
 
+/* SSL_CTX_get_keylog_callback returns the callback configured by
+ * |SSL_CTX_set_keylog_callback|. */
+OPENSSL_EXPORT void (*SSL_CTX_get_keylog_callback(const SSL_CTX *ctx))(
+    const SSL *ssl, const char *line);
+
 /* SSL_CTX_set_current_time_cb configures a callback to retrieve the current
  * time, which should be set in |*out_clock|. This can be used for testing
  * purposes; for example, a callback can be configured that returns a time

ssl/ssl_lib.c

@@ -2524,6 +2524,11 @@ void SSL_CTX_set_keylog_callback(SSL_CTX *ctx,
   ctx->keylog_callback = cb;
 }
 
+void (*SSL_CTX_get_keylog_callback(const SSL_CTX *ctx))(const SSL *ssl,
+                                                        const char *line) {
+  return ctx->keylog_callback;
+}
+
 void SSL_CTX_set_current_time_cb(SSL_CTX *ctx,
                                  void (*cb)(const SSL *ssl,
                                             struct timeval *out_clock)) {