Merge bitcoin/bitcoin#36046: fuzz: Use ImmediateBackgroundTaskRunner in process_messages

fae6665f01 fuzz: Use ImmediateBackgroundTaskRunner in process_messages (MarcoFalke)

Pull request description:

  The `process_messages` target may complain about false-positive debug lock-order issues:

  ```
  echo 'Gv8uXPBdXV0QEP//dHVhxyoVKP////8A/0BrLmNrAEEAIP+MXHR0OQAAAAD+///txgIUADBgAAEC
  fgAAAK0ArQEAAAD/AFwAQf9cdHf5XGhlYWRlcltbyzHIw8RcX2Jsb2NrAAAAAGNtcAAAADAftOvd
  D0sFqXEx6US5VIknlsOJqZ5goMwtmwZBPdCxQZbatBsWPOR3FcUvSLLsKwcgKT8XdmjvDgskH5pK
  iAUI/uVJTf//fyAAAAAAAQIAAAAAAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAP//
  //8DAskA/v///wIA+QKVAAAAAAFRAAAAAAAAAAAmaiSqIant4vYcP3HR3v0/qZnfo2lTdVxcaQaJ
  eZlitIvr2DaXToz5ASAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAMgAAAD/XPB0eAD/
  AgAAAAD9ABZqCwAAAAAEAAAAXPBhYVtbW1tbW1tbW1tbW1tbW1tbW1tbW1tbW1tbW1tbW1tbW1tb
  W1tb//9bW1tbW1tbW1sAAAAxNgAAADc5MzU5NDk2ODEwNzg3NAAAAAICAgL9a4jAhyQCAgICAQAA
  AAAABSpvdGhlcir/8wICAgICAAAAeAL0AAAAAAAAaW52O///BAAAAAAAtbW1tWFbW2FhtbVhYWFh
  YSkpW1tbW2QAJwAAAAAAAAAAMTYAAAA3OTM1OTQ5NjgxMDc4NzQAAAACAgIC/WuIwIckAgICAgEA
  AAAAAAUAAAAAAv7///MAeAL0AAAAAAAAaW52Ow==' | base64 --decode > /tmp/fuzz.input

  FUZZ=process_messages ./bld-cmake/bin/fuzz /tmp/fuzz.input --printtoconsole=1 | grep -A99 'POTENTIAL DEADLOCK DETECTED'
  ```

  ```
  [test] [sync.cpp:108] [potential_deadlock_detected] [error] POTENTIAL DEADLOCK DETECTED
  [test] [sync.cpp:109] [potential_deadlock_detected] [error] Previous lock order was:
  [test] [sync.cpp:118] [potential_deadlock_detected] [error]  'NetEventsInterface::g_msgproc_mutex' in test/fuzz/process_messages.cpp:89 (in thread 'test')
  [test] [sync.cpp:118] [potential_deadlock_detected] [error]  'm_chainstate_mutex' in validation.cpp:3351 (in thread 'test')
  [test] [sync.cpp:118] [potential_deadlock_detected] [error]  'cs_main' in validation.cpp:3373 (in thread 'test')
  [test] [sync.cpp:118] [potential_deadlock_detected] [error]  (2) 'MempoolMutex()' in validation.cpp:3376 (in thread 'test')
  [test] [sync.cpp:118] [potential_deadlock_detected] [error]  (1) 'm_tx_download_mutex' in net_processing.cpp:2216 (in thread 'test')
  [test] [sync.cpp:122] [potential_deadlock_detected] [error] Current lock order is:
  [test] [sync.cpp:133] [potential_deadlock_detected] [error]  'NetEventsInterface::g_msgproc_mutex' in test/fuzz/process_messages.cpp:89 (in thread 'test')
  [test] [sync.cpp:133] [potential_deadlock_detected] [error]  'cs_main' in net_processing.cpp:4725 (in thread 'test')
  [test] [sync.cpp:133] [potential_deadlock_detected] [error]  (1) 'm_tx_download_mutex' in net_processing.cpp:4725 (in thread 'test')
  [test] [sync.cpp:133] [potential_deadlock_detected] [error]  (2) 'cs' in txmempool.h:521 (in thread 'test')
  ```

  Fix this by using the `ImmediateBackgroundTaskRunner` from `src/test/fuzz/cmpctblock.cpp`.

ACKs for top commit:
  Crypt-iQ:
    ACK fae6665f01
  sedited:
    ACK fae6665f01
  marcofleon:
    tACK fae6665f01
  frankomosh:
    Tested ACK fae6665f01

Tree-SHA512: 9cee43aa72495abfd69211004b27ee6857d3a1a6bbab9fdc5a8b5349159a54e270236f198a516a7d9087917af8c95ee626e8307155ded8d08314a6b606dd0c33

src/test/fuzz/cmpctblock.cpp

@@ -34,7 +34,6 @@
 #include <txmempool.h>
 #include <uint256.h>
 #include <util/check.h>
-#include <util/task_runner.h>
 #include <util/time.h>
 #include <validation.h>
 #include <validationinterface.h>
@@ -48,7 +47,6 @@
 #include <memory>
 #include <optional>
 #include <string>
-#include <thread>
 #include <utility>
 #include <vector>
 
@@ -104,15 +102,6 @@ public:
 };
 
 
-//! Used to run tasks in a std::thread to avoid DEBUG_LOCKORDER false positives.
-class ImmediateBackgroundTaskRunner : public util::TaskRunnerInterface
-{
-public:
-    void insert(std::function<void()> func) override { std::thread(std::move(func)).join(); }
-    void flush() override {}
-    size_t size() override { return 0; }
-};
-
 } // namespace
 
 extern void MakeRandDeterministicDANGEROUS(const uint256& seed) noexcept;

src/test/fuzz/process_messages.cpp

@@ -50,6 +50,8 @@ void initialize_process_messages()
             {}),
     };
     g_setup = testing_setup.get();
+    // Replace validation_signals before creating chainman and mempool so they use it.
+    g_setup->m_node.validation_signals = std::make_unique<ValidationSignals>(std::make_unique<ImmediateBackgroundTaskRunner>());
     ResetChainmanAndMempool(*g_setup, init_clock);
 }
 

src/test/util/validation.h

@@ -7,8 +7,12 @@
 
 #include <consensus/amount.h>
 #include <primitives/transaction.h>
+#include <util/task_runner.h>
 #include <validation.h>
 
+#include <cstddef>
+#include <functional>
+#include <thread>
 #include <utility>
 #include <vector>
 
@@ -19,6 +23,15 @@ class CValidationInterface;
 class FakeNodeClock;
 struct TestingSetup;
 
+/// Runs callbacks synchronously and deterministically, while avoiding DEBUG_LOCKORDER false positives.
+class ImmediateBackgroundTaskRunner : public util::TaskRunnerInterface
+{
+public:
+    void insert(std::function<void()> func) override { std::thread(std::move(func)).join(); }
+    void flush() override {}
+    size_t size() override { return 0; }
+};
+
 struct TestBlockManager : public node::BlockManager {
     /** Test-only method to clear internal state for fuzzing */
     void CleanupForFuzzing();