add function to forget the old fallback key

include/olm/account.hh

@@ -169,6 +169,9 @@ struct Account {
         std::uint8_t * fallback_json, std::size_t fallback_json_length
     );
 
+    /** Forget about the old fallback key */
+    void forget_old_fallback_key();
+
     /** Lookup a one time key with the given public key */
     OneTimeKey const * lookup_key(
         _olm_curve25519_public_key const & public_key

include/olm/olm.h

@@ -310,6 +310,15 @@ OLM_EXPORT size_t olm_account_unpublished_fallback_key(
     void * fallback_key, size_t fallback_key_size
 );
 
+/** Forget about the old fallback key.  This should be called once you are
+ * reasonably certain that you will not receive any more messages that use
+ * the old fallback key (e.g. 5 minutes after the new fallback key has been
+ * published).
+ */
+OLM_EXPORT void olm_account_forget_old_fallback_key(
+    OlmAccount * account
+);
+
 
 /** The number of random bytes needed to create an outbound session */
 OLM_EXPORT size_t olm_create_outbound_session_random_length(

javascript/olm_post.js

@@ -194,8 +194,8 @@ Account.prototype['unpublished_fallback_key'] = restore_stack(function() {
     return UTF8ToString(keys, keys_length);
 });
 
-Account.prototype['forget_previous_fallback'] = restore_stack(function() {
+Account.prototype['forget_old_fallback'] = restore_stack(function() {
-    account_method(Module['_olm_account_forget_previous_fallback_key'])(
+    account_method(Module['_olm_account_forget_old_fallback_key'])(
         this.ptr
     );
 });

src/account.cpp

@@ -398,6 +398,13 @@ std::size_t olm::Account::get_unpublished_fallback_key_json(
     return pos - fallback_json;
 }
 
+void olm::Account::forget_old_fallback_key(
+) {
+    if (num_fallback_keys >= 2) {
+        num_fallback_keys = 1;
+    }
+}
+
 namespace olm {
 
 static std::size_t pickle_length(

src/olm.cpp

@@ -512,6 +512,13 @@ size_t olm_account_unpublished_fallback_key(
 }
 
 
+void olm_account_forget_old_fallback_key(
+    OlmAccount * account
+) {
+    return from_c(account)->forget_old_fallback_key();
+}
+
+
 size_t olm_create_outbound_session_random_length(
     OlmSession const * session
 ) {