Add FFI generator for Windows (#18)

spec/rng_spec.lua

@@ -64,6 +64,25 @@ describe("rng's", function()
 
 
 
+  describe("win_ffi()", function()
+
+    it("fails on Posix", function()
+      local old_package_config = package.config
+      finally(function()
+        package.config = old_package_config -- luacheck: ignore
+      end)
+      -- make it think this is Windows
+      package.config = "/" .. package.config:sub(2,-1) -- luacheck: ignore
+
+      local rng, err = require("uuid").rng.win_ffi()
+      assert.is.falsy(rng)
+      assert.is.equal("win-ffi is only available on Windows", err)
+    end)
+
+  end)
+
+
+
   describe("urandom()", function()
 
     it("generates 1 byte", function()

src/uuid/rng/init.lua

@@ -80,6 +80,60 @@ end
 
 
 
+do
+  local win_ffi_rng
+
+  ----------------------------------------------------------------------------
+  -- Returns an rng that implements Windows' RtlGenRandom function which is
+  -- a good source of randomness on all modern versions of Windows.
+  -- Requires LuaJIT or [`cffi-lua`](https://github.com/q66/cffi-lua).
+  -- @treturn function A function that returns `n` random bytes, signature: `byte_string, err = func(n)`
+  -- @usage
+  -- local uuid = require "uuid"
+  -- uuid.set_rng(uuid.rng.win_ffi())
+  function rng.win_ffi()
+    if package.config:sub(1, 1) ~= "\\" then
+      return nil, "win-ffi is only available on Windows"
+    end
+
+    -- return cahced rng function if we already created it
+    if win_ffi_rng then
+      return win_ffi_rng
+    end
+
+    local ok, ffi = pcall(require, "ffi")
+    if not ok then
+        ok, ffi = pcall(require, "cffi")
+    end
+    if not ok then
+      return nil, "ffi not available"
+    end
+
+    local ffi_defs = [[
+      unsigned char SystemFunction036(void *RandomBuffer, unsigned long RandomBufferLength);
+    ]]
+    ffi.cdef(ffi_defs)
+
+    local ok, advapi32 = pcall(ffi.load, "advapi32.dll")
+    if not ok then
+      return nil, "failed to load advapi32.dll"
+    end
+
+    win_ffi_rng = function(n)
+      local buffer = ffi.new("unsigned char[?]", n)
+      local ok = advapi32.SystemFunction036(buffer, n)
+      if ok == 0 then
+        return nil, "call to RtlGenRandom failed"
+      end
+      return ffi.string(buffer, n)
+    end
+
+    return win_ffi_rng
+  end
+end
+
+
+
 do
   local rnd = function(n)
     local file, err = io.open("/dev/urandom", "rb")
@@ -186,6 +240,7 @@ if lua_version >= 5.4 then
 end
 
 
+
 do
   -- create a table, on modern CPUs the ASLR will make this unique.
   -- we keep the table around, to prevent it being GC'ed and the address being reused
@@ -230,6 +285,13 @@ do
       end
     end
 
+    do -- try RtlGenRandom() via FFI on windows
+      local wf = rng.win_ffi()
+      if wf then
+        return wf(40)  -- this is crypto level, so good enough
+      end
+    end
+
     -- fallback to sha1 of a combination of values
     local seed = {
       unique_table_id,
@@ -298,4 +360,5 @@ do
 end
 
 
+
 return rng