Pass in a buffer to olm_session_describe

instead of having a static one, as that could end up taking up a
lot of memory if your app keeps olm sessions hanging about.

include/olm/olm.h

@@ -319,12 +319,10 @@ int olm_session_has_received_message(
 );
 
 /**
- * Return a string describing the internal state of an olm session,
- * for debugging and logging purposes. The pointer returned points
- * to an internal buffer and is valid until the next call to
- * olm_session_describe on the same olm session object.
+ * Write a string describing the internal state of an olm session
+ * to the buffer provided for debugging and logging purposes.
  */
-const char * olm_session_describe(OlmSession * session);
+void olm_session_describe(OlmSession * session, char *buf, size_t buflen);
 
 /** Checks if the PRE_KEY message is for this in-bound session. This can happen
  * if multiple messages are sent to this account before this account sends a

include/olm/session.hh

@@ -17,8 +17,6 @@
 
 #include "olm/ratchet.hh"
 
-#define DESCRIBE_BUFFER_LEN 256
-
 namespace olm {
 
 struct Account;
@@ -37,8 +35,6 @@ struct Session {
 
     bool received_message;
 
-    char describe_buffer[DESCRIBE_BUFFER_LEN];
-
     _olm_curve25519_public_key alice_identity_key;
     _olm_curve25519_public_key alice_base_key;
     _olm_curve25519_public_key bob_one_time_key;
@@ -137,12 +133,12 @@ struct Session {
     );
 
     /**
-     * Return a string describing this session and its state (not including the
-     * private key)
-     * The pointer returned refers to an internal buffer which will be valid
-     * up until the next time describe() is called.
+     * Write a string describing this session and its state (not including the
+     * private key) into the buffer provided.
+     *
+     * Takes a buffer to write to and the length of that buffer
      */
-    const char *describe();
+    void describe(char *buf, size_t buflen);
 };
 
 

javascript/olm_post.js

@@ -460,10 +460,16 @@ Session.prototype['decrypt'] = restore_stack(function(
 });
 
 Session.prototype['describe'] = restore_stack(function() {
-    var description_buf = session_method(Module['_olm_session_describe'])(
-        this.ptr
+    var description_buf;
+    try {
+        description_buf = malloc(256);
+        session_method(Module['_olm_session_describe'])(
+            this.ptr, description_buf, 256
         );
         return UTF8ToString(description_buf);
+    } finally {
+        if (description_buf !== undefined) free(description_buf);
+    }
 });
 
 function Utility() {

src/olm.cpp

@@ -535,10 +535,10 @@ int olm_session_has_received_message(
     return from_c(session)->received_message;
 }
 
-const char * olm_session_describe(
-    OlmSession * session
+void olm_session_describe(
+    OlmSession * session, char *buf, size_t buflen
 ) {
-    return from_c(session)->describe();
+    from_c(session)->describe(buf, buflen);
 }
 
 size_t olm_matches_inbound_session(

src/session.cpp

@@ -398,32 +398,32 @@ std::size_t olm::Session::decrypt(
     return result;
 }
 
-const char * olm::Session::describe() {
+void olm::Session::describe(char *describe_buffer, size_t buflen) {
+    if (buflen == 0) return;
+
     describe_buffer[0] = '\0';
     char *buf_pos = describe_buffer;
 
     buf_pos += snprintf(
-        buf_pos, DESCRIBE_BUFFER_LEN - (buf_pos - describe_buffer),
+        buf_pos, buflen - (buf_pos - describe_buffer),
         "sender chain index: %d ", ratchet.sender_chain[0].chain_key.index
     );
 
-    buf_pos += snprintf(buf_pos, DESCRIBE_BUFFER_LEN - (buf_pos - describe_buffer), "receiver chain indcies:");
+    buf_pos += snprintf(buf_pos, buflen - (buf_pos - describe_buffer), "receiver chain indcies:");
     for (size_t i = 0; i < ratchet.receiver_chains.size(); ++i) {
         buf_pos += snprintf(
-            buf_pos, DESCRIBE_BUFFER_LEN - (buf_pos - describe_buffer),
+            buf_pos, buflen - (buf_pos - describe_buffer),
             " %d", ratchet.receiver_chains[i].chain_key.index
         );
     }
 
-    buf_pos += snprintf(buf_pos, DESCRIBE_BUFFER_LEN - (buf_pos - describe_buffer), " skipped message keys:");
+    buf_pos += snprintf(buf_pos, buflen - (buf_pos - describe_buffer), " skipped message keys:");
     for (size_t i = 0; i < ratchet.skipped_message_keys.size(); ++i) {
         buf_pos += snprintf(
-            buf_pos, DESCRIBE_BUFFER_LEN - (buf_pos - describe_buffer),
+            buf_pos, buflen - (buf_pos - describe_buffer),
             " %d", ratchet.skipped_message_keys[i].message_key.index
         );
     }
-
-    return describe_buffer;
 }
 
 namespace {