Make GUID generation on POSIX compliant with GUID v4 format.

base/guid_posix.cc

Index: base/guid_posix.cc
diff --git a/base/guid_posix.cc b/base/guid_posix.cc
index 1b6d56fdea2d2ce6702826487c3a4595623eef83..9681b084fa300b204f9c80afd01b1c6d3a14f63c 100644
--- a/base/guid_posix.cc
+++ b/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);
 }