Fix WinEnv::NowMicrosec

* std::chrono does not provide enough granularity for microsecs and periodically emits
    duplicates
  * the bug is manifested in log rotation logic where we get duplicate
   log file names and loose previous log content
  * msvc does not imlement COW on std::strings adjusted the test to use
    refs in the loops as auto does not retain ref info
  * adjust auto_log rotation test with Windows specific command to remove
    a folder. The test previously worked because we have unix utils installed
    in house but this may not be the case for everyone.

port/win/env_win.cc

@@ -1976,9 +1976,16 @@ class WinEnv : public Env {
   }
 
   virtual uint64_t NowMicros() override {
-    using namespace std::chrono;
+    // all std::chrono clocks on windows have the same resolution that is only
-    return duration_cast<microseconds>(system_clock::now().time_since_epoch())
+    // On Windows 8 and Windows 2012 Server
-        .count();
+    // GetSystemTimePreciseAsFileTime(&current_time) can be used
+    LARGE_INTEGER li;
+    QueryPerformanceCounter(&li);
+    // Convert to nanoseconds first to avoid loss of precision
+    // and divide by frequency
+    li.QuadPart *= std::micro::den;
+    li.QuadPart /= perf_counter_frequency_;
+    return li.QuadPart;
   }
 
   virtual uint64_t NowNanos() override {

util/auto_roll_logger.cc

@@ -91,7 +91,7 @@ void AutoRollLogger::Logv(const char* format, va_list ap) {
 
 void AutoRollLogger::WriteHeaderInfo() {
   mutex_.AssertHeld();
-  for (auto header : headers_) {
+  for (auto& header : headers_) {
     LogInternal("%s", header.c_str());
   }
 }

util/auto_roll_logger_test.cc

@@ -23,7 +23,17 @@ namespace rocksdb {
 class AutoRollLoggerTest : public testing::Test {
  public:
   static void InitTestDb() {
-    string deleteCmd = "rm -rf " + kTestDir;
+#ifdef OS_WIN
+    // Replace all slashes in the path so windows CompSpec does not
+    // become confused
+    std::string testDir(kTestDir);
+    std::replace_if(testDir.begin(), testDir.end(),
+      [](char ch) { return ch == '/'; },
+      '\\');
+    std::string deleteCmd = "if exist " + testDir + " rd /s /q " + testDir;
+#else
+    std::string deleteCmd = "rm -rf " + kTestDir;
+#endif
     ASSERT_TRUE(system(deleteCmd.c_str()) == 0);
     Env::Default()->CreateDir(kTestDir);
   }
@@ -296,17 +306,18 @@ TEST_F(AutoRollLoggerTest, InfoLogLevel) {
 
 // Test the logger Header function for roll over logs
 // We expect the new logs creates as roll over to carry the headers specified
-static list<string> GetOldFileNames(const string& path) {
+static std::vector<string> GetOldFileNames(const string& path) {
+  std::vector<string> ret;
+
   const string dirname = path.substr(/*start=*/ 0, path.find_last_of("/"));
   const string fname = path.substr(path.find_last_of("/") + 1);
 
-  vector<string> children;
+  std::vector<string> children;
   Env::Default()->GetChildren(dirname, &children);
 
   // We know that the old log files are named [path]<something>
   // Return all entities that match the pattern
-  list<string> ret;
+  for (auto& child : children) {
-  for (auto child : children) {
     if (fname != child && child.find(fname) == 0) {
       ret.push_back(dirname + "/" + child);
     }
@@ -360,7 +371,7 @@ TEST_F(AutoRollLoggerTest, LogHeaderTest) {
       }
     }
 
-    const string& newfname = logger.TEST_log_fname().c_str();
+    const string newfname = logger.TEST_log_fname();
 
     // Log enough data to cause a roll over
     int i = 0;
@@ -376,11 +387,11 @@ TEST_F(AutoRollLoggerTest, LogHeaderTest) {
     // Flush the log for the latest file
     LogFlush(&logger);
 
-    const list<string> oldfiles = GetOldFileNames(newfname);
+    const auto oldfiles = GetOldFileNames(newfname);
 
     ASSERT_EQ(oldfiles.size(), (size_t) 2);
 
-    for (auto oldfname : oldfiles) {
+    for (auto& oldfname : oldfiles) {
       // verify that the files rolled over
       ASSERT_NE(oldfname, newfname);
       // verify that the old log contains all the header logs