fuzz: Abort when calling system time without setting mock time

src/test/fuzz/util/check_globals.cpp

@@ -5,6 +5,7 @@
 #include <test/fuzz/util/check_globals.h>
 
 #include <test/util/random.h>
+#include <util/time.h>
 
 #include <iostream>
 #include <memory>
@@ -16,6 +17,8 @@ struct CheckGlobalsImpl {
     {
         g_used_g_prng = false;
         g_seeded_g_prng_zero = false;
+        g_used_system_time = false;
+        SetMockTime(0s);
     }
     ~CheckGlobalsImpl()
     {
@@ -34,6 +37,19 @@ struct CheckGlobalsImpl {
                       << std::endl;
             std::abort(); // Abort, because AFL may try to recover from a std::exit
         }
+
+        if (g_used_system_time) {
+            std::cerr << "\n\n"
+                         "The current fuzz target accessed system time.\n\n"
+
+                         "This is acceptable, but requires the fuzz target to call \n"
+                         "SetMockTime() at the beginning of processing the fuzz input.\n\n"
+
+                         "Without setting mock time, time-dependent behavior can lead \n"
+                         "to non-reproducible bugs or inefficient fuzzing.\n\n"
+                      << std::endl;
+            std::abort();
+        }
     }
 };
 

src/test/fuzz/util/check_globals.h

@@ -5,10 +5,13 @@
 #ifndef BITCOIN_TEST_FUZZ_UTIL_CHECK_GLOBALS_H
 #define BITCOIN_TEST_FUZZ_UTIL_CHECK_GLOBALS_H
 
+#include <atomic>
 #include <memory>
 #include <optional>
 #include <string>
 
+extern std::atomic<bool> g_used_system_time;
+
 struct CheckGlobalsImpl;
 struct CheckGlobals {
     CheckGlobals();

src/util/time.cpp

@@ -20,10 +20,14 @@
 void UninterruptibleSleep(const std::chrono::microseconds& n) { std::this_thread::sleep_for(n); }
 
 static std::atomic<std::chrono::seconds> g_mock_time{}; //!< For testing
+std::atomic<bool> g_used_system_time{false};
 
 NodeClock::time_point NodeClock::now() noexcept
 {
     const auto mocktime{g_mock_time.load(std::memory_order_relaxed)};
+    if (!mocktime.count()) {
+        g_used_system_time = true;
+    }
     const auto ret{
         mocktime.count() ?
             mocktime :