Merge bitcoin/bitcoin#30156: fuzz: More accurate coverage reports

949abeb [fuzz] Avoid collecting initialization coverage (dergoegge)

Pull request description:

  Our coverage reports include coverage of initialization code, which can be misleading when trying to evaluate the coverage a fuzz harness achieves through fuzzing alone.

  This PR proposes to make fuzz coverage reports more accurate by resetting coverage counters after initialization code has been run. This makes it easier to evaluate which code was actually reached through fuzzing (e.g. to spot fuzz blockers).

ACKs for top commit:
  maflcko:
    utACK 949abeb
  brunoerg:
    nice, utACK 949abeb

Tree-SHA512: c8579bda4f3d71d199b9331fbe6316fce375a906743d0bc216bb94958dc03fdc9a951ea50cfeb487494a75668ae3c16471a82f7e5fdd912d781dc29d063e2c5b

Author: fanquake

src/test/fuzz/fuzz.cpp

@@ -79,6 +79,26 @@ void FuzzFrameworkRegisterTarget(std::string_view name, TypeTestOneInput target,
 static std::string_view g_fuzz_target;
 static const TypeTestOneInput* g_test_one_input{nullptr};
 
+
+#if defined(__clang__) && defined(__linux__)
+extern "C" void __llvm_profile_reset_counters(void) __attribute__((weak));
+extern "C" void __gcov_reset(void) __attribute__((weak));
+
+void ResetCoverageCounters()
+{
+    if (__llvm_profile_reset_counters) {
+        __llvm_profile_reset_counters();
+    }
+
+    if (__gcov_reset) {
+        __gcov_reset();
+    }
+}
+#else
+void ResetCoverageCounters() {}
+#endif
+
+
 void initialize()
 {
     // Terminate immediately if a fuzzing harness ever tries to create a TCP socket.
@@ -129,6 +149,8 @@ void initialize()
     Assert(!g_test_one_input);
     g_test_one_input = &it->second.test_one_input;
     it->second.opts.init();
+
+    ResetCoverageCounters();
 }
 
 #if defined(PROVIDE_FUZZ_MAIN_FUNCTION)