Merge bitcoin/bitcoin#31841: fuzz: Use immediate task runner to increase fuzz stability

fa4fb6a fuzz: Use serial task runner to increase fuzz stability (MarcoFalke)

Pull request description:

  Leaking a scheduler with a non-empty queue from the fuzz initialization phase into the fuzz target execution phase is problematic, because it messes with coverage data. This in turn is problematic, because it leads to:

  * Decrease in fuzz target execution stability (non-determinism when running the fuzz target).
  * Decrease in fuzz input merge stability (non-determinism when selecting a minimum set of fuzz input to reach maximum coverage), which leads to qa-assets bloat.

  Fix one such issue. Tracking issue: bitcoin/bitcoin#29018

  Can be tested via: `RUST_BACKTRACE=1 cargo run --manifest-path ./contrib/devtools/deterministic-fuzz-coverage/Cargo.toml -- $PWD/bld-cmake $PWD/../b-c-qa-assets/fuzz_corpora/ partially_downloaded_block`.

  The failure is non-deterministic (obviously) and will show coverage in validation signals such as `UpdatedBlockTip` before this change and will have this one fixed after this change.

ACKs for top commit:
  marcofleon:
    ACK fa4fb6a
  dergoegge:
    Code review ACK fa4fb6a

Tree-SHA512: fd1f66562c1d3c21553c7dd324399cdc16faa2fedfdb8e7544ea6a68b8b356e7c81d81815ecf70e0d334307dab6b275c1889b3b889b6f15eec514beee22c95f4

Author: fanquake

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{};