Use lock-free lazy initialization for static histogram references

net/base/mime_sniffer.cc

Index: net/base/mime_sniffer.cc
===================================================================
--- net/base/mime_sniffer.cc	(revision 80220)
+++ net/base/mime_sniffer.cc	(working copy)
@@ -209,9 +209,9 @@
   MAGIC_HTML_TAG("p")  // Mozilla
 };
 
-static scoped_refptr<base::Histogram> UMASnifferHistogramGet(const char* name,
-                                                             int array_size) {
-  scoped_refptr<base::Histogram> counter =
+static base::Histogram* UMASnifferHistogramGet(const char* name,
+                                               int array_size) {
+  base::Histogram* counter =
       base::LinearHistogram::FactoryGet(name, 1, array_size - 1, array_size,
       base::Histogram::kUmaTargetedHistogramFlag);
   return counter;
@@ -308,13 +308,14 @@
     if (!IsAsciiWhitespace(*pos))
       break;
   }
-  scoped_refptr<base::Histogram> counter =
-      UMASnifferHistogramGet("mime_sniffer.kSniffableTags2",
-                             arraysize(kSniffableTags));
+  static base::Histogram* counter(NULL);
+  if (!counter)
+    counter = UMASnifferHistogramGet("mime_sniffer.kSniffableTags2",
+                                     arraysize(kSniffableTags));
   // |pos| now points to first non-whitespace character (or at end).
   return CheckForMagicNumbers(pos, end - pos,
                               kSniffableTags, arraysize(kSniffableTags),
-                              counter.get(), result);
+                              counter, result);
 }
 
 // Returns true and sets result if the content matches any of kMagicNumbers.
@@ -326,12 +327,13 @@
   *have_enough_content &= TruncateSize(kBytesRequiredForMagic, &size);
 
   // Check our big table of Magic Numbers
-  scoped_refptr<base::Histogram> counter =
-      UMASnifferHistogramGet("mime_sniffer.kMagicNumbers2",
-                             arraysize(kMagicNumbers));
+  static base::Histogram* counter(NULL);
+  if (!counter)
+    counter = UMASnifferHistogramGet("mime_sniffer.kMagicNumbers2",
+                                     arraysize(kMagicNumbers));
   return CheckForMagicNumbers(content, size,
                               kMagicNumbers, arraysize(kMagicNumbers),
-                              counter.get(), result);
+                              counter, result);
 }
 
 // Byte order marks
@@ -367,9 +369,10 @@
   // We want to skip XML processing instructions (of the form "<?xml ...")
   // and stop at the first "plain" tag, then make a decision on the mime-type
   // based on the name (or possibly attributes) of that tag.
-  scoped_refptr<base::Histogram> counter =
-      UMASnifferHistogramGet("mime_sniffer.kMagicXML2",
-                             arraysize(kMagicXML));
+  static base::Histogram* counter(NULL);
+  if (!counter)
+    counter = UMASnifferHistogramGet("mime_sniffer.kMagicXML2",
+                                     arraysize(kMagicXML));
   const int kMaxTagIterations = 5;
   for (int i = 0; i < kMaxTagIterations && pos < end; ++i) {
     pos = reinterpret_cast<const char*>(memchr(pos, '<', end - pos));
@@ -389,7 +392,7 @@
 
     if (CheckForMagicNumbers(pos, end - pos,
                              kMagicXML, arraysize(kMagicXML),
-                             counter.get(), result))
+                             counter, result))
       return true;
 
     // TODO(evanm): handle RSS 1.0, which is an RDF format and more difficult
@@ -451,13 +454,14 @@
   const bool is_truncated = TruncateSize(kMaxBytesToSniff, &size);
 
   // First, we look for a BOM.
-  scoped_refptr<base::Histogram> counter =
-      UMASnifferHistogramGet("mime_sniffer.kByteOrderMark2",
-                             arraysize(kByteOrderMark));
+  static base::Histogram* counter(NULL);
+  if (!counter)
+    counter = UMASnifferHistogramGet("mime_sniffer.kByteOrderMark2",
+                                     arraysize(kByteOrderMark));
   std::string unused;
   if (CheckForMagicNumbers(content, size,
                            kByteOrderMark, arraysize(kByteOrderMark),
-                           counter.get(), &unused)) {
+                           counter, &unused)) {
     // If there is BOM, we think the buffer is not binary.
     result->assign("text/plain");
     return false;
@@ -493,9 +497,10 @@
     // Firefox rejects a mime type if it is exactly */*
     "*/*",
   };
-  scoped_refptr<base::Histogram> counter =
-      UMASnifferHistogramGet("mime_sniffer.kUnknownMimeTypes2",
-                             arraysize(kUnknownMimeTypes) + 1);
+  static base::Histogram* counter(NULL);
+  if (!counter)
+    counter = UMASnifferHistogramGet("mime_sniffer.kUnknownMimeTypes2",
+                                     arraysize(kUnknownMimeTypes) + 1);
   for (size_t i = 0; i < arraysize(kUnknownMimeTypes); ++i) {
     if (mime_type == kUnknownMimeTypes[i]) {
       counter->Add(i);
@@ -519,8 +524,9 @@
                      const std::string& type_hint,
                      bool* have_enough_content,
                      std::string* result) {
-  scoped_refptr<base::Histogram> counter =
-      UMASnifferHistogramGet("mime_sniffer.kSniffCRX", 3);
+  static base::Histogram* counter(NULL);
+  if (!counter)
+    counter = UMASnifferHistogramGet("mime_sniffer.kSniffCRX", 3);
 
   // Technically, the crx magic number is just Cr24, but the bytes after that
   // are a version number which changes infrequently. Including it in the
@@ -557,8 +563,10 @@
 }
 
 bool ShouldSniffMimeType(const GURL& url, const std::string& mime_type) {
-  scoped_refptr<base::Histogram> should_sniff_counter =
-      UMASnifferHistogramGet("mime_sniffer.ShouldSniffMimeType2", 3);
+  static base::Histogram* should_sniff_counter(NULL);
+  if (!should_sniff_counter)
+    should_sniff_counter =
+        UMASnifferHistogramGet("mime_sniffer.ShouldSniffMimeType2", 3);
   // We are willing to sniff the mime type for HTTP, HTTPS, and FTP
   bool sniffable_scheme = url.is_empty() ||
                           url.SchemeIs("http") ||
@@ -582,9 +590,10 @@
     "text/xml",
     "application/xml",
   };
-  scoped_refptr<base::Histogram> counter =
-      UMASnifferHistogramGet("mime_sniffer.kSniffableTypes2",
-                             arraysize(kSniffableTypes) + 1);
+  static base::Histogram* counter(NULL);
+  if (!counter)
+    counter = UMASnifferHistogramGet("mime_sniffer.kSniffableTypes2",
+                                     arraysize(kSniffableTypes) + 1);
   for (size_t i = 0; i < arraysize(kSniffableTypes); ++i) {
     if (mime_type == kSniffableTypes[i]) {
       counter->Add(i);