Make space in the session pickle for chain index

Keeping track of the chain index is a useful thing to do, but is only required
if we've enabled diagnostics. Extend the session pickle format to make a space
for it, so that pickles can be transferred between the logging_enabled branch
and the master branch without loss of information.

Also add some tests for session pickling which explicitly check that we can
unpickle both formats of pickle.

2 files changed, 34 insertions, 6 deletions

diff --git a/src/ratchet.cpp b/src/ratchet.cpp
index dd1d42c..671d260 100644
--- a/src/ratchet.cpp
+++ b/src/ratchet.cpp
@@ -363,6 +363,10 @@ std::size_t olm::pickle_length(
     length += olm::pickle_length(value.sender_chain);
     length += olm::pickle_length(value.receiver_chains);
     length += olm::pickle_length(value.skipped_message_keys);
+
+    // the logging_enabled branch includes a 'chain_index' field
+    length += olm::pickle_length(std::uint32_t(0));
+
     return length;
 }
 
@@ -374,18 +378,30 @@ std::uint8_t * olm::pickle(
     pos = pickle(pos, value.sender_chain);
     pos = pickle(pos, value.receiver_chains);
     pos = pickle(pos, value.skipped_message_keys);
+
+    // the logging_enabled branch includes a 'chain_index' field; for us, it is
+    // empty.
+    pos = pickle(pos, std::uint32_t(0));
+
     return pos;
 }
 
 
 std::uint8_t const * olm::unpickle(
     std::uint8_t const * pos, std::uint8_t const * end,
-    olm::Ratchet & value
+    olm::Ratchet & value,
+    bool includes_chain_index
 ) {
     pos = unpickle(pos, end, value.root_key);
     pos = unpickle(pos, end, value.sender_chain);
     pos = unpickle(pos, end, value.receiver_chains);
     pos = unpickle(pos, end, value.skipped_message_keys);
+
+    // pickle v2 includes a chain index; pickle v1 did not.
+    if (includes_chain_index) {
+        std::uint32_t dummy;
+        pos = unpickle(pos, end, dummy);
+    }
     return pos;
 }
 
diff --git a/src/session.cpp b/src/session.cpp
index c148c97..b76a6b8 100644
--- a/src/session.cpp
+++ b/src/session.cpp
@@ -396,7 +396,7 @@ std::size_t olm::Session::decrypt(
 }
 
 namespace {
-static const std::uint32_t SESSION_PICKLE_VERSION = 1;
+static const std::uint32_t SESSION_PICKLE_VERSION = 2;
 }
 
 std::size_t olm::pickle_length(
@@ -433,14 +433,26 @@ std::uint8_t const * olm::unpickle(
 ) {
     uint32_t pickle_version;
     pos = olm::unpickle(pos, end, pickle_version);
-    if (pickle_version != SESSION_PICKLE_VERSION) {
-        value.last_error = OlmErrorCode::OLM_UNKNOWN_PICKLE_VERSION;
-        return end;
+
+    bool includes_chain_index;
+    switch (pickle_version) {
+        case 1:
+            includes_chain_index = false;
+            break;
+
+        case 2:
+            includes_chain_index = true;
+            break;
+
+        default:
+            value.last_error = OlmErrorCode::OLM_UNKNOWN_PICKLE_VERSION;
+            return end;
     }
+
     pos = olm::unpickle(pos, end, value.received_message);
     pos = olm::unpickle(pos, end, value.alice_identity_key);
     pos = olm::unpickle(pos, end, value.alice_base_key);
     pos = olm::unpickle(pos, end, value.bob_one_time_key);
-    pos = olm::unpickle(pos, end, value.ratchet);
+    pos = olm::unpickle(pos, end, value.ratchet, includes_chain_index);
     return pos;
 }