Fix megolm decryption of UTF-8

Repeat the fix from b10f90d for megolm messages.

It turns out that the 'length' argument to 'Pointer_stringify' doesn't work if
the input includes characters >= 128.

Rather than try to figure out which methods can return UTF-8, and which always
return plain ascii, replace all uses of Pointer_stringify with a 'length'
argument with the version that expects a NULL-terminated input, and extend the
buffer by a byte to allow space for a null-terminator.

In the case of decrypt, we need to add the null ourself.

Fixes https://github.com/vector-im/vector-web/issues/2078.

javascript/olm_inbound_group_session.js

@@ -1,3 +1,9 @@
+/* The 'length' argument to Pointer_stringify doesn't work if the input includes
+ * characters >= 128; we therefore need to add a NULL character to all of our
+ * strings. This acts as a symbolic constant to help show what we're doing.
+ */
+var NULL_BYTE_PADDING_LENGTH = 1;
+
 function InboundGroupSession() {
     var size = Module['_olm_inbound_group_session_size']();
     this.buf = malloc(size);
@@ -28,11 +34,11 @@ InboundGroupSession.prototype['pickle'] = restore_stack(function(key) {
         Module['_olm_pickle_inbound_group_session_length']
     )(this.ptr);
     var key_buffer = stack(key_array);
-    var pickle_buffer = stack(pickle_length);
+    var pickle_buffer = stack(pickle_length + NULL_BYTE_PADDING_LENGTH);
     inbound_group_session_method(Module['_olm_pickle_inbound_group_session'])(
         this.ptr, key_buffer, key_array.length, pickle_buffer, pickle_length
     );
-    return Pointer_stringify(pickle_buffer, pickle_length);
+    return Pointer_stringify(pickle_buffer);
 });
 
 InboundGroupSession.prototype['unpickle'] = restore_stack(function(key, pickle) {
@@ -66,13 +72,21 @@ InboundGroupSession.prototype['decrypt'] = restore_stack(function(
     // caculating the length destroys the input buffer.
     // So we copy the array to a new buffer
     var message_buffer = stack(message_array);
-    var plaintext_buffer = stack(max_plaintext_length);
+    var plaintext_buffer = stack(max_plaintext_length + NULL_BYTE_PADDING_LENGTH);
     var plaintext_length = session_method(Module["_olm_group_decrypt"])(
         this.ptr,
         message_buffer, message_array.length,
         plaintext_buffer, max_plaintext_length
     );
-    return Pointer_stringify(plaintext_buffer, plaintext_length);
+
+    // Pointer_stringify requires a null-terminated argument (the optional
+    // 'len' argument doesn't work for UTF-8 data).
+    Module['setValue'](
+        plaintext_buffer+plaintext_length,
+        0, "i8"
+    );
+
+    return Pointer_stringify(plaintext_buffer);
 });
 
 olm_exports['InboundGroupSession'] = InboundGroupSession;

javascript/olm_outbound_group_session.js

@@ -1,3 +1,9 @@
+/* The 'length' argument to Pointer_stringify doesn't work if the input includes
+ * characters >= 128; we therefore need to add a NULL character to all of our
+ * strings. This acts as a symbolic constant to help show what we're doing.
+ */
+var NULL_BYTE_PADDING_LENGTH = 1;
+
 
 function OutboundGroupSession() {
     var size = Module['_olm_outbound_group_session_size']();
@@ -29,11 +35,11 @@ OutboundGroupSession.prototype['pickle'] = restore_stack(function(key) {
         Module['_olm_pickle_outbound_group_session_length']
     )(this.ptr);
     var key_buffer = stack(key_array);
-    var pickle_buffer = stack(pickle_length);
+    var pickle_buffer = stack(pickle_length + NULL_BYTE_PADDING_LENGTH);
     outbound_group_session_method(Module['_olm_pickle_outbound_group_session'])(
         this.ptr, key_buffer, key_array.length, pickle_buffer, pickle_length
     );
-    return Pointer_stringify(pickle_buffer, pickle_length);
+    return Pointer_stringify(pickle_buffer);
 });
 
 OutboundGroupSession.prototype['unpickle'] = restore_stack(function(key, pickle) {
@@ -63,35 +69,35 @@ OutboundGroupSession.prototype['encrypt'] = restore_stack(function(plaintext) {
         Module['_olm_group_encrypt_message_length']
     )(this.ptr, plaintext_array.length);
     var plaintext_buffer = stack(plaintext_array);
-    var message_buffer = stack(message_length);
+    var message_buffer = stack(message_length + NULL_BYTE_PADDING_LENGTH);
     outbound_group_session_method(Module['_olm_group_encrypt'])(
         this.ptr,
         plaintext_buffer, plaintext_array.length,
         message_buffer, message_length
     );
-    return Pointer_stringify(message_buffer, message_length);
+    return Pointer_stringify(message_buffer);
 });
 
 OutboundGroupSession.prototype['session_id'] = restore_stack(function(key) {
     var length = outbound_group_session_method(
         Module['_olm_outbound_group_session_id_length']
     )(this.ptr);
-    var session_id = stack(length);
+    var session_id = stack(length + NULL_BYTE_PADDING_LENGTH);
     outbound_group_session_method(Module['_olm_outbound_group_session_id'])(
         this.ptr, session_id, length
     );
-    return Pointer_stringify(session_id, length);
+    return Pointer_stringify(session_id);
 });
 
 OutboundGroupSession.prototype['session_key'] = restore_stack(function(key) {
     var key_length = outbound_group_session_method(
         Module['_olm_outbound_group_session_key_length']
     )(this.ptr);
-    var key = stack(key_length);
+    var key = stack(key_length + NULL_BYTE_PADDING_LENGTH);
     outbound_group_session_method(Module['_olm_outbound_group_session_key'])(
         this.ptr, key, key_length
     );
-    return Pointer_stringify(key, key_length);
+    return Pointer_stringify(key);
 });
 
 OutboundGroupSession.prototype['message_index'] = function() {

javascript/olm_post.js

@@ -5,7 +5,7 @@ var Pointer_stringify = Module['Pointer_stringify'];
 var OLM_ERROR = Module['_olm_error']();
 
 /* The 'length' argument to Pointer_stringify doesn't work if the input includes
- * cahracters >= 128; we therefore need to add a NULL character to all of our
+ * characters >= 128; we therefore need to add a NULL character to all of our
  * strings. This acts as a symbolic constant to help show what we're doing.
  */
 var NULL_BYTE_PADDING_LENGTH = 1;