Improve string(RANDOM) default seed

The naive time(0) seed is unique only within one second.  Instead try to
read a real source of entropy and otherwise fall back to a combination
of the process id and high-resolution time.

Source/cmStringCommand.cxx

@@ -770,7 +770,7 @@ bool cmStringCommand
 
   static bool seeded = false;
   bool force_seed = false;
-  int seed = (int) time(NULL);
+  int seed = 0;
   int length = 5;
   const char cmStringCommandDefaultAlphabet[] = "qwertyuiopasdfghjklzxcvbnm"
     "QWERTYUIOPASDFGHJKLZXCVBNM"
@@ -825,7 +825,7 @@ bool cmStringCommand
   if (!seeded || force_seed)
     {
     seeded = true;
-    srand(seed);
+    srand(force_seed? seed : cmSystemTools::RandomSeed());
     }
 
   const char* alphaPtr = alphabet.c_str();

Source/cmSystemTools.cxx

@@ -31,7 +31,9 @@
 
 #if defined(_WIN32)
 # include <windows.h>
+# include <wincrypt.h>
 #else
+# include <sys/time.h>
 # include <sys/types.h>
 # include <unistd.h>
 # include <utime.h>
@@ -2232,6 +2234,68 @@ bool cmSystemTools::FileTimeSet(const char* fname, cmSystemToolsFileTime* t)
   return true;
 }
 
+//----------------------------------------------------------------------------
+#ifdef _WIN32
+static int WinCryptRandom(void* data, size_t size)
+{
+  int result = 0;
+  HCRYPTPROV hProvider = 0;
+  if(CryptAcquireContextW(&hProvider, 0, 0, PROV_RSA_FULL,
+                          CRYPT_VERIFYCONTEXT | CRYPT_SILENT))
+    {
+    result = CryptGenRandom(hProvider, (DWORD)size, (BYTE*)data)? 1:0;
+    CryptReleaseContext(hProvider, 0);
+    }
+  return result;
+}
+#endif
+
+//----------------------------------------------------------------------------
+unsigned int cmSystemTools::RandomSeed()
+{
+#if defined(_WIN32) && !defined(__CYGWIN__)
+  unsigned int seed = 0;
+
+  // Try using a real random source.
+  if(WinCryptRandom(&seed, sizeof(seed)))
+    {
+    return seed;
+    }
+
+  // Fall back to the time and pid.
+  FILETIME ft;
+  GetSystemTimeAsFileTime(&ft);
+  unsigned int t1 = static_cast<unsigned int>(ft.dwHighDateTime);
+  unsigned int t2 = static_cast<unsigned int>(ft.dwLowDateTime);
+  unsigned int pid = static_cast<unsigned int>(GetCurrentProcessId());
+  return t1 ^ t2 ^ pid;
+#else
+  union
+  {
+    unsigned int integer;
+    char bytes[sizeof(unsigned int)];
+  } seed;
+
+  // Try using a real random source.
+  std::ifstream fin("/dev/urandom");
+  if(fin && fin.read(seed.bytes, sizeof(seed)) &&
+     fin.gcount() == sizeof(seed))
+    {
+    return seed.integer;
+    }
+
+  // Fall back to the time and pid.
+  struct timeval t;
+  gettimeofday(&t, 0);
+  unsigned int pid = static_cast<unsigned int>(getpid());
+  unsigned int tv_sec = t.tv_sec;
+  unsigned int tv_usec = t.tv_usec;
+  // Since tv_usec never fills more than 11 bits we shift it to fill
+  // in the slow-changing high-order bits of tv_sec.
+  return tv_sec ^ (tv_usec << 21) ^ pid;
+#endif
+}
+
 //----------------------------------------------------------------------------
 static std::string cmSystemToolsExecutableDirectory;
 void cmSystemTools::FindExecutableDirectory(const char* argv0)

Source/cmSystemTools.h

@@ -402,6 +402,9 @@ public:
   static bool FileTimeGet(const char* fname, cmSystemToolsFileTime* t);
   static bool FileTimeSet(const char* fname, cmSystemToolsFileTime* t);
 
+  /** Random seed generation.  */
+  static unsigned int RandomSeed();
+
   /** Find the directory containing the running executable.  Save it
    in a global location to be queried by GetExecutableDirectory
    later.  */