WASI: implement in terms of getentropy

This isn't really WASI-specific; it should work for any platform that has
`getentropy`.

System/Entropy.hs

@@ -27,6 +27,8 @@ module System.Entropy
 import System.EntropyGhcjs
 #elif defined(isWindows)
 import System.EntropyWindows
+#elif wasi_HOST_OS
+import System.EntropyWasi
 #else
 import System.EntropyNix
 #endif

System/EntropyWasi.hs

@@ -0,0 +1,39 @@
+{-# LANGUAGE CApiFFI #-}
+
+module System.EntropyWasi where
+
+import Control.Monad (when)
+import Data.ByteString (ByteString)
+import Data.ByteString.Internal as B
+import Data.Word (Word8)
+import Foreign.C.Types
+import Foreign.Ptr
+
+data CryptHandle = BogusCryptHandle
+
+openHandle :: IO CryptHandle
+openHandle = pure BogusCryptHandle
+
+hGetEntropy :: CryptHandle -> Int -> IO ByteString
+hGetEntropy BogusCryptHandle = \n -> B.create n $ go (fromIntegral n)
+  where
+    go :: CSize -> Ptr Word8 -> IO ()
+    go n ptr
+      | n <= 256 = getentropy' ptr n
+      | otherwise = do
+          getentropy' ptr 256
+          go (n - 256) (ptr `plusPtr` 256)
+
+    getentropy' ptr n = do
+      res <- getentropy ptr n
+      when (res /= 0) $
+        fail "getentropy failed"
+
+foreign import capi safe "unistd.h getentropy"
+  getentropy :: Ptr Word8 -> CSize -> IO CInt
+
+closeHandle :: CryptHandle -> IO ()
+closeHandle BogusCryptHandle = pure ()
+
+hardwareRandom :: Int -> IO (Maybe ByteString)
+hardwareRandom _ = pure Nothing

entropy.cabal

@@ -1,4 +1,4 @@
-cabal-version:  >=1.10
+cabal-version:  3.8
 name:           entropy
 version:        0.4.1.10
 x-revision:     2
@@ -7,7 +7,7 @@ description:    A mostly platform independent method to obtain cryptographically
                 Users looking for cryptographically strong (number-theoretically
                 sound) PRNGs should see the 'DRBG' package too.
 synopsis:       A platform independent entropy source
-license:        BSD3
+license:        BSD-3-Clause
 license-file:   LICENSE
 copyright:      Thomas DuBuisson <[email protected]>
 author:         Thomas DuBuisson <[email protected]>
@@ -61,6 +61,8 @@ library
   else {
     if os(windows)
       other-modules: System.EntropyWindows
+    elif os(wasi)
+      other-modules: System.EntropyWasi
     else {
       other-modules: System.EntropyNix
     }
@@ -90,6 +92,7 @@ library
       cpp-options: -DisWindows
       cc-options:  -DisWindows
       extra-libraries: advapi32
+    elif os(wasi)
     else
       Build-Depends: unix
       c-sources: cbits/getrandom.c cbits/random_initialized.c