refactor: coalesce cold-start GetStatus poll with std::call_once

Author: beekld

libs/server-sdk/src/data_components/big_segments/big_segment_store_wrapper.cpp

@@ -142,8 +142,12 @@ BigSegmentStoreStatus BigSegmentStoreWrapper::GetStatus() {
         }
     }
     // No poll has completed yet; do one now so the caller gets an accurate
-    // staleness reading rather than a default.
-    return PollStoreAndUpdateStatus();
+    // staleness reading rather than a default. call_once coalesces concurrent
+    // first-callers onto a single store query.
+    std::call_once(first_poll_once_,
+                   [this] { PollStoreAndUpdateStatus(); });
+    std::lock_guard lock(status_mutex_);
+    return *last_status_;
 }
 
 BigSegmentStoreStatus BigSegmentStoreWrapper::PollStoreAndUpdateStatus() {

libs/server-sdk/src/data_components/big_segments/big_segment_store_wrapper.hpp

@@ -138,17 +138,22 @@ class BigSegmentStoreWrapper
     // Cancels the pending poll delay on destruction, ending the poll loop.
     async::CancellationSource poll_cancel_;
 
+    // Internally thread-safe.
     MembershipCache cache_;
 
+    // Broadcasts status changes to listeners registered via OnStatusChange.
+    boost::signals2::signal<void(BigSegmentStoreStatus)> status_signal_;
+
+    // Coalesces the cold-start fallback poll in GetStatus().
+    std::once_flag first_poll_once_;
+
     // In-flight store queries keyed by context key. Protected by load_mutex_.
     std::mutex load_mutex_;
     std::unordered_map<std::string, std::shared_ptr<InFlightQuery>> in_flight_;
 
     // Protected by status_mutex_; nullopt until the first poll completes.
     std::mutex status_mutex_;
     std::optional<BigSegmentStoreStatus> last_status_;
-
-    boost::signals2::signal<void(BigSegmentStoreStatus)> status_signal_;
 };
 
 }  // namespace launchdarkly::server_side::data_components