Avoid starting up storage expiration thread with zero backends

src/storage/Manager.cc

@@ -22,7 +22,7 @@ void detail::ExpirationTimer::Dispatch(double t, bool is_expire) {
     // start another one running. If this causes us to skip a cycle, that's not
     // a big deal as the next cycle will catch anything that should be expired
     // in the interim.
-    if ( ! expire_running.test_and_set() ) {
+    if ( ! expire_running.test_and_set() && storage_mgr->BackendCount() > 0 ) {
         DBG_LOG(DBG_STORAGE, "Starting new expiration thread");
         storage_mgr->expiration_thread = zeek::jthread([t]() { storage_mgr->Expire(t); });
     }
@@ -146,4 +146,11 @@ void Manager::RegisterBackend(BackendPtr backend) {
     DBG_LOG(DBG_STORAGE, "Registered backends: %zu", backends.size());
 }
 
+size_t Manager::BackendCount() {
+    // Expiration runs on a separate thread and loops over the vector of backends. The mutex
+    // here ensures exclusive access.
+    std::unique_lock<std::mutex> lk(backends_mtx);
+    return backends.size();
+}
+
 } // namespace zeek::storage

src/storage/Manager.h

@@ -113,6 +113,7 @@ protected:
     friend class storage::detail::ExpirationTimer;
     void RunExpireThread();
     void StartExpirationTimer();
+    size_t BackendCount();
     zeek::jthread expiration_thread;
 
     friend class storage::OpenResultCallback;

testing/btest/scripts/base/frameworks/storage/redis-expiration.zeek

@@ -4,7 +4,7 @@
 # @TEST-PORT: REDIS_PORT
 
 # @TEST-EXEC: btest-bg-run redis-server run-redis-server ${REDIS_PORT%/tcp}
-# @TEST-EXEC: zcat <$TRACES/echo-connections.pcap.gz | zeek -B storage -b -r - %INPUT > out
+# @TEST-EXEC: zeek -B storage -b %INPUT > out
 # @TEST-EXEC: btest-bg-wait -k 1
 
 # @TEST-EXEC: btest-diff out