Make GUID generation on POSIX compliant with GUID v4 format.

Per RFC 4122 section 4.4, the format of GUID version 4 must be
xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx, where y is one of [8, 9, A, B].
This CL updates base::GenerateGUID() on POSIX to be compliant with the
GUID v4 format.

This fix is proposed by Alex Vakulenko <avakulenko@chromium.org>

BUG=None
TEST=base_unittests

Review URL: https://codereview.chromium.org/541633005

Cr-Commit-Position: refs/heads/master@{#293518}

base/guid_posix.cc

@@ -11,6 +11,20 @@ namespace base {
 
 std::string GenerateGUID() {
   uint64 sixteen_bytes[2] = { base::RandUint64(), base::RandUint64() };
+
+  // Set the GUID to version 4 as described in RFC 4122, section 4.4.
+  // The format of GUID version 4 must be xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx,
+  // where y is one of [8, 9, A, B].
+
+  // Clear the version bits and set the version to 4:
+  sixteen_bytes[0] &= 0xffffffffffff0fffULL;
+  sixteen_bytes[0] |= 0x0000000000004000ULL;
+
+  // Set the two most significant bits (bits 6 and 7) of the
+  // clock_seq_hi_and_reserved to zero and one, respectively:
+  sixteen_bytes[1] &= 0x3fffffffffffffffULL;
+  sixteen_bytes[1] |= 0x8000000000000000ULL;
+
   return RandomDataToGUIDString(sixteen_bytes);
 }
 

base/guid_unittest.cc

@@ -10,6 +10,20 @@
 #include "testing/gtest/include/gtest/gtest.h"
 
 #if defined(OS_POSIX)
+
+namespace {
+
+bool IsGUIDv4(const std::string& guid) {
+  // The format of GUID version 4 must be xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx,
+  // where y is one of [8, 9, A, B].
+  return base::IsValidGUID(guid) && guid[14] == '4' &&
+         (guid[19] == '8' || guid[19] == '9' || guid[19] == 'A' ||
+          guid[19] == 'a' || guid[19] == 'B' || guid[19] == 'b');
+}
+
+}  // namespace
+
+
 TEST(GUIDTest, GUIDGeneratesAllZeroes) {
   uint64 bytes[] = { 0, 0 };
   std::string clientid = base::RandomDataToGUIDString(bytes);
@@ -41,5 +55,9 @@ TEST(GUIDTest, GUIDBasicUniqueness) {
     EXPECT_EQ(36U, guid1.length());
     EXPECT_EQ(36U, guid2.length());
     EXPECT_NE(guid1, guid2);
+#if defined(OS_POSIX)
+    EXPECT_TRUE(IsGUIDv4(guid1));
+    EXPECT_TRUE(IsGUIDv4(guid2));
+#endif
   }
 }