Use shorter hash for script-builds directories

Using a Base64 hash and truncating it to 26 characters, saves 38 chars,
which helps avoid long paths issues on Windows, while still providing
130 bits of hash in order to avoid collisions.

Bug haskell#8841

cabal-install/cabal-install.cabal

@@ -205,6 +205,7 @@ library
         async      >= 2.0      && < 2.3,
         array      >= 0.4      && < 0.6,
         base16-bytestring >= 0.1.1 && < 1.1.0.0,
+        base64-bytestring >= 1.0 && < 1.3,
         binary     >= 0.7.3    && < 0.9,
         bytestring >= 0.10.6.0 && < 0.12,
         containers >= 0.5.6.2  && < 0.7,

cabal-install/src/Distribution/Client/HashValue.hs

@@ -6,6 +6,7 @@ module Distribution.Client.HashValue (
     hashValue,
     truncateHash,
     showHashValue,
+    showHashValueBase64,
     readFileHashValue,
     hashFromTUF,
     ) where
@@ -17,6 +18,7 @@ import qualified Hackage.Security.Client as Sec
 
 import qualified Crypto.Hash.SHA256         as SHA256
 import qualified Data.ByteString.Base16     as Base16
+import qualified Data.ByteString.Base64     as Base64
 import qualified Data.ByteString.Char8      as BS
 import qualified Data.ByteString.Lazy.Char8 as LBS
 
@@ -55,6 +57,9 @@ hashValue = HashValue . SHA256.hashlazy
 showHashValue :: HashValue -> String
 showHashValue (HashValue digest) = BS.unpack (Base16.encode digest)
 
+showHashValueBase64 :: HashValue -> String
+showHashValueBase64 (HashValue digest) = BS.unpack (Base64.encode digest)
+
 -- | Hash the content of a file. Uses SHA256.
 --
 readFileHashValue :: FilePath -> IO HashValue

cabal-install/src/Distribution/Client/ScriptUtils.hs

@@ -26,7 +26,7 @@ import Distribution.Client.Config
 import Distribution.Client.DistDirLayout
     ( DistDirLayout(..) )
 import Distribution.Client.HashValue
-    ( hashValue, showHashValue )
+    ( hashValue, showHashValueBase64 )
 import Distribution.Client.HttpUtils
          ( HttpTransport, configureTransport )
 import Distribution.Client.NixStyleOptions
@@ -125,7 +125,12 @@ import qualified Text.Parsec as P
 -- Two hashes will be the same as long as the absolute paths
 -- are the same.
 getScriptHash :: FilePath -> IO String
-getScriptHash script = showHashValue . hashValue . fromString <$> canonicalizePath script
+getScriptHash script
+  -- Base64 is shorter than Base16, which helps avoid long path issues on windows
+  -- but it can contain /'s which aren't valid in file paths so replace them with
+  -- %'s. 26 chars / 130 bits is enough to practically avoid collisions.
+  = map (\c -> if c == '/' then '%' else c) . take 26
+  . showHashValueBase64 . hashValue . fromString <$> canonicalizePath script
 
 -- | Get the directory for caching a script build.
 --

cabal-testsuite/cabal-testsuite.cabal

@@ -60,7 +60,7 @@ library
     , aeson                 ^>= 1.4.2.0 || ^>=1.5.0.0 || ^>= 2.0.0.0 || ^>= 2.1.0.0
     , async                 ^>= 2.2.1
     , attoparsec            ^>= 0.13.2.2 || ^>=0.14.1
-    , base16-bytestring     ^>= 0.1.1.6 || ^>= 1.0.0.0
+    , base64-bytestring     ^>= 1.0.0.0 || ^>= 1.1.0.0 || ^>= 1.2.0.0
     , bytestring            ^>= 0.10.0.2 || ^>= 0.11.0.0
     , containers            ^>= 0.5.0.0 || ^>= 0.6.0.1
     , cryptohash-sha256     ^>= 0.11.101.0

cabal-testsuite/src/Test/Cabal/Prelude.hs

@@ -52,7 +52,7 @@ import Control.Monad (unless, when, void, forM_, liftM2, liftM4)
 import Control.Monad.Trans.Reader (withReaderT, runReaderT)
 import Control.Monad.IO.Class (MonadIO (..))
 import qualified Crypto.Hash.SHA256 as SHA256
-import qualified Data.ByteString.Base16 as Base16
+import qualified Data.ByteString.Base64 as Base64
 import qualified Data.ByteString.Char8 as C
 import Data.List (isInfixOf, stripPrefix, isPrefixOf, intercalate)
 import Data.List.NonEmpty (NonEmpty (..))
@@ -837,7 +837,8 @@ getScriptCacheDirectory :: FilePath -> TestM FilePath
 getScriptCacheDirectory script = do
     cabalDir <- testCabalDir `fmap` getTestEnv
     hashinput <- liftIO $ canonicalizePath script
-    let hash = C.unpack . Base16.encode . SHA256.hash . C.pack $ hashinput
+    let hash = map (\c -> if c == '/' then '%' else c) . take 26
+             . C.unpack . Base64.encode . SHA256.hash . C.pack $ hashinput
     return $ cabalDir </> "script-builds" </> hash
 
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