Fix sync Nigori cache GUID left empty upon mismatch

In the error-handling codepath where a mismatch is found on the cache
GUID, when sync is starting for Nigori, the new cache GUID should be
stored in the ModelTypeState. Otherwise, it would remain empty, which
means the next sync start the mismatch detection doesn't take place.

This can explain cache-GUID mismatches being ignored, and bad
consequences such as violating the sync protocol by sending a non-empty
progress marker despite the sync birthday being empty.

Bug: 1063021
Change-Id: I05c1c000c17b1f10ff37984d1b8b9f812f02a7f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111301Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752085}

components/sync/nigori/nigori_model_type_processor.cc

@@ -390,6 +390,11 @@ bool NigoriModelTypeProcessor::IsConnectedForTest() const {
   return IsConnected();
 }
 
+const sync_pb::ModelTypeState&
+NigoriModelTypeProcessor::GetModelTypeStateForTest() {
+  return model_type_state_;
+}
+
 bool NigoriModelTypeProcessor::IsTrackingMetadata() {
   return model_type_state_.initial_sync_done();
 }
@@ -412,13 +417,14 @@ void NigoriModelTypeProcessor::ConnectIfReady() {
     return;
   }
 
-  if (!model_type_state_.has_cache_guid()) {
-    model_type_state_.set_cache_guid(activation_request_.cache_guid);
-  } else if (model_type_state_.cache_guid() != activation_request_.cache_guid) {
+  if (model_type_state_.initial_sync_done() &&
+      model_type_state_.cache_guid() != activation_request_.cache_guid) {
     ClearMetadataAndReset();
     DCHECK(model_ready_to_sync_);
   }
 
+  model_type_state_.set_cache_guid(activation_request_.cache_guid);
+
   // Cache GUID verification earlier above guarantees the user is the same.
   model_type_state_.set_authenticated_account_id(
       activation_request_.authenticated_account_id.ToString());

components/sync/nigori/nigori_model_type_processor.h

@@ -59,6 +59,7 @@ class NigoriModelTypeProcessor : public ModelTypeProcessor,
   bool IsTrackingMetadata() override;
 
   bool IsConnectedForTest() const;
+  const sync_pb::ModelTypeState& GetModelTypeStateForTest();
 
  private:
   // Returns true if the handshake with sync thread is complete.

components/sync/nigori/nigori_model_type_processor_unittest.cc

@@ -527,6 +527,8 @@ TEST_F(NigoriModelTypeProcessorTest, ShouldResetDataOnCacheGuidMismatch) {
   processor()->OnSyncStarting(request, base::DoNothing());
 
   EXPECT_FALSE(processor()->IsTrackingMetadata());
+  EXPECT_EQ(processor()->GetModelTypeStateForTest().cache_guid(),
+            kOtherCacheGuid);
 
   EXPECT_FALSE(ProcessorHasEntity());