Relax CHECK condition in benchmark_runner.cc (#938)

* Add State::error_occurred()

* Relax CHECK condition in benchmark_runner.cc

If the benchmark state contains an error, do not expect any iterations has been run.
This allows using SkipWithError() and return early from the benchmark function.

* README.md: document new possible usage of SkipWithError()

README.md

@@ -1184,7 +1184,9 @@ Users must explicitly exit the loop, otherwise all iterations will be performed.
 Users may explicitly return to exit the benchmark immediately.
 
 The `SkipWithError(...)` function may be used at any point within the benchmark,
-including before and after the benchmark loop.
+including before and after the benchmark loop. Moreover, if `SkipWithError(...)`
+has been used, it is not required to reach the benchmark loop and one may return
+from the benchmark function early.
 
 For example:
 
@@ -1192,24 +1194,32 @@ For example:
 static void BM_test(benchmark::State& state) {
   auto resource = GetResource();
   if (!resource.good()) {
-      state.SkipWithError("Resource is not good!");
-      // KeepRunning() loop will not be entered.
+    state.SkipWithError("Resource is not good!");
+    // KeepRunning() loop will not be entered.
   }
   while (state.KeepRunning()) {
-      auto data = resource.read_data();
-      if (!resource.good()) {
-        state.SkipWithError("Failed to read data!");
-        break; // Needed to skip the rest of the iteration.
-     }
-     do_stuff(data);
+    auto data = resource.read_data();
+    if (!resource.good()) {
+      state.SkipWithError("Failed to read data!");
+      break; // Needed to skip the rest of the iteration.
+    }
+    do_stuff(data);
   }
 }
 
 static void BM_test_ranged_fo(benchmark::State & state) {
-  state.SkipWithError("test will not be entered");
+  auto resource = GetResource();
+  if (!resource.good()) {
+    state.SkipWithError("Resource is not good!");
+    return; // Early return is allowed when SkipWithError() has been used.
+  }
   for (auto _ : state) {
-    state.SkipWithError("Failed!");
-    break; // REQUIRED to prevent all further iterations.
+    auto data = resource.read_data();
+    if (!resource.good()) {
+      state.SkipWithError("Failed to read data!");
+      break; // REQUIRED to prevent all further iterations.
+    }
+    do_stuff(data);
   }
 }
 ```

include/benchmark/benchmark.h

@@ -541,6 +541,9 @@ class State {
   // responsibility to exit the scope as needed.
   void SkipWithError(const char* msg);
 
+  // Returns true if an error has been reported with 'SkipWithError(...)'.
+  bool error_occurred() const { return error_occurred_; }
+
   // REQUIRES: called exactly once per iteration of the benchmarking loop.
   // Set the manually measured time for this benchmark iteration, which
   // is used instead of automatically measured time if UseManualTime() was

src/benchmark_runner.cc

@@ -117,7 +117,7 @@ void RunInThread(const BenchmarkInstance* b, IterationCount iters,
           ? internal::ThreadTimer::CreateProcessCpuTime()
           : internal::ThreadTimer::Create());
   State st = b->Run(iters, thread_id, &timer, manager);
-  CHECK(st.iterations() >= st.max_iterations)
+  CHECK(st.error_occurred() || st.iterations() >= st.max_iterations)
       << "Benchmark returned before State::KeepRunning() returned false!";
   {
     MutexLock l(manager->GetBenchmarkMutex());

test/skip_with_error_test.cc

@@ -61,6 +61,12 @@ int AddCases(const char* base_name, std::initializer_list<TestCase> const& v) {
 
 }  // end namespace
 
+void BM_error_no_running(benchmark::State& state) {
+  state.SkipWithError("error message");
+}
+BENCHMARK(BM_error_no_running);
+ADD_CASES("BM_error_no_running", {{"", true, "error message"}});
+
 void BM_error_before_running(benchmark::State& state) {
   state.SkipWithError("error message");
   while (state.KeepRunning()) {