fuzz: Use serial task runner to increase fuzz stability

Author: MarcoFalke

src/test/util/setup_common.cpp

@@ -49,6 +49,7 @@
 #include <util/rbf.h>
 #include <util/strencodings.h>
 #include <util/string.h>
+#include <util/task_runner.h>
 #include <util/thread.h>
 #include <util/threadnames.h>
 #include <util/time.h>
@@ -220,12 +221,15 @@ ChainTestingSetup::ChainTestingSetup(const ChainType chainType, TestOpts opts)
 {
     const CChainParams& chainparams = Params();
 
-    // We have to run a scheduler thread to prevent ActivateBestChain
+    // A task runner is required to prevent ActivateBestChain
     // from blocking due to queue overrun.
     if (opts.setup_validation_interface) {
         m_node.scheduler = std::make_unique<CScheduler>();
         m_node.scheduler->m_service_thread = std::thread(util::TraceThread, "scheduler", [&] { m_node.scheduler->serviceQueue(); });
-        m_node.validation_signals = std::make_unique<ValidationSignals>(std::make_unique<SerialTaskRunner>(*m_node.scheduler));
+        m_node.validation_signals =
+            // Use synchronous task runner while fuzzing to avoid non-determinism
+            G_FUZZING ? std::make_unique<ValidationSignals>(std::make_unique<util::ImmediateTaskRunner>()) :
+                        std::make_unique<ValidationSignals>(std::make_unique<SerialTaskRunner>(*m_node.scheduler));
     }
 
     bilingual_str error{};