fix frame count reporting

Summary:
We're hiding internal frames during dump, but forget to adjust
the reported count, so the output looks clowny.

Reviewed By: luciang

Differential Revision: D32340094

fbshipit-source-id: 86a02c54178a03a83fa64c29faf6d82abfa45bf8

folly/experimental/exception_tracer/ExceptionTracer.cpp

@@ -66,17 +66,20 @@ void printExceptionInfo(
   } else {
     out << "(unknown type)";
   }
-  out << " (" << info.frames.size()
-      << (info.frames.size() == 1 ? " frame" : " frames") << ")\n";
-  try {
-    size_t frameCount = info.frames.size();
-
-    // Skip our own internal frames
   static constexpr size_t kInternalFramesNumber = 3;
-    if (frameCount > kInternalFramesNumber) {
+
+  // Skip our own internal frames.
+  size_t frameCount = info.frames.size();
+  if (frameCount <= kInternalFramesNumber) {
+    out << "\n";
+    return;
+  }
   auto addresses = info.frames.data() + kInternalFramesNumber;
   frameCount -= kInternalFramesNumber;
 
+  out << " (" << frameCount << (frameCount == 1 ? " frame" : " frames")
+      << ")\n";
+  try {
     std::vector<SymbolizedFrame> frames;
     frames.resize(frameCount);
 
@@ -88,7 +91,6 @@ void printExceptionInfo(
 
     OStreamSymbolizePrinter osp(out, options);
     osp.println(frames.data(), frameCount);
-    }
   } catch (const std::exception& e) {
     out << "\n !! caught " << folly::exceptionStr(e) << "\n";
   } catch (...) {

folly/experimental/exception_tracer/test/SmartExceptionTracerTest.cpp

@@ -64,8 +64,7 @@ TEST(SmartExceptionTracer, EmptyExceptionWrapper) {
   std::ostringstream ss;
   ss << info;
   ASSERT_TRUE(
-      ss.str().find("Exception type: (unknown type) (0 frames)") !=
-      std::string::npos);
+      ss.str().find("Exception type: (unknown type)") != std::string::npos);
 }
 
 TEST(SmartExceptionTracer, InvalidException) {
@@ -77,8 +76,7 @@ TEST(SmartExceptionTracer, InvalidException) {
     std::ostringstream ss;
     ss << info;
     ASSERT_TRUE(
-        ss.str().find("Exception type: (unknown type) (0 frames)") !=
-        std::string::npos);
+        ss.str().find("Exception type: (unknown type)") != std::string::npos);
   }
 }
 
@@ -89,8 +87,7 @@ TEST(SmartExceptionTracer, UnthrownException) {
   std::ostringstream ss;
   ss << info;
   ASSERT_TRUE(
-      ss.str().find("Exception type: std::runtime_error (0 frames)") !=
-      std::string::npos);
+      ss.str().find("Exception type: std::runtime_error") != std::string::npos);
 }
 
 namespace {