Fix interface hashes including time stamp of dependent files.

Fixes #8144.

Before, the modification time of e.g. #included files (and everything
that ends up as a UsageFile, e.g. via addDependentFile) was taken as
input for the interface hash of a module.

This lead to different hashes for identical inputs on every compilation.

We now use file content hashes instead.

This changes the interface file format.
You will get "Binary.get(Usage): 50" when you try to do an incremental
using .hi files that were created with a GHC 7.7 (only) older than this commit.

To calculate the md5 hash (`Fingerprint`) of a file in constant space,
there now is GHC.Fingerprint.getFileHash, and a fallback version
for older GHCs that needs to load the file into memory completely
(only used when compiling stage1 with an older GHC).

Signed-off-by: Austin Seipp <aseipp@pobox.com>

1 files changed, 29 insertions, 2 deletions

diff --git a/compiler/utils/Fingerprint.hsc b/compiler/utils/Fingerprint.hsc
index 95f31c08bb..895906a3c6 100644
--- a/compiler/utils/Fingerprint.hsc
+++ b/compiler/utils/Fingerprint.hsc
@@ -8,18 +8,22 @@
 --
 -- ----------------------------------------------------------------------------
 
-{-# OPTIONS_GHC -fno-warn-orphans #-}
 module Fingerprint (
         Fingerprint(..), fingerprint0,
         readHexFingerprint,
         fingerprintData,
-        fingerprintString
+        fingerprintString,
+        -- Re-exported rom GHC.Fingerprint for GHC >= 7.7, local otherwise
+        getFileHash
    ) where
 
 #include "md5.h"
 ##include "HsVersions.h"
 
 import Numeric          ( readHex )
+import Foreign
+import Panic
+import System.IO
 
 import GHC.Fingerprint
 
@@ -30,3 +34,26 @@ readHexFingerprint s = Fingerprint w1 w2
        [(w1,"")] = readHex s1
        [(w2,"")] = readHex (take 16 s2)
 
+
+#if __GLASGOW_HASKELL__ < 707
+-- Only use this if we're smaller than GHC 7.7, otherwise
+-- GHC.Fingerprint exports a better version of this function.
+
+-- | Computes the hash of a given file.
+-- It loads the full file into memory an does not work with files bigger than
+-- MAXINT.
+getFileHash :: FilePath -> IO Fingerprint
+getFileHash path = withBinaryFile path ReadMode $ \h -> do
+
+  fileSize <- toIntFileSize `fmap` hFileSize h
+
+  allocaBytes fileSize (\bufPtr -> fingerprintData bufPtr fileSize)
+
+  where
+    toIntFileSize :: Integer -> Int
+    toIntFileSize size
+      | size > fromIntegral (maxBound :: Int) = throwGhcException $
+          Sorry $ "Fingerprint.getFileHash: Tried to calculate hash of file "
+                  ++ path ++ " with size > maxBound :: Int. This is not supported."
+      | otherwise = fromIntegral size
+#endif