Preserve changed-flag for concurrent config updates

This should avoid the following race condition between two config updates
arriving within a very short interval:

 1. Config generation _n-1_ contains changes to payload
 2. Subscriber thread does not apply config _n-1_ internally before
    generation _n_ arrives
 3. Config generation _n_ does not contain changes to payload. Overwrites
    change flag of generation _n-1_
 4. Change status lost, subscriber does not update internal config

1 files changed, 4 insertions, 0 deletions

diff --git a/config/src/vespa/config/subscription/configsubscription.cpp b/config/src/vespa/config/subscription/configsubscription.cpp
index 4aaa30323c7..379687e1fd8 100644
--- a/config/src/vespa/config/subscription/configsubscription.cpp
+++ b/config/src/vespa/config/subscription/configsubscription.cpp
@@ -31,7 +31,11 @@ ConfigSubscription::nextUpdate(int64_t generation, std::chrono::milliseconds tim
     if (_closed || !_holder->poll()) {
         return false;
     }
+    auto old = std::move(_next);
     _next = _holder->provide();
+    if (old) {
+        _next->merge(*old);
+    }
     if (isGenerationNewer(_next->getGeneration(), generation)) {
         return true;
     }