Use Base64 hash for script-builds directories

This saves 21 chars in the build path, which should help avoid long path
issues on Windows.

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 <$> canonicalizePath script
+getScriptHash script
+  -- Base64 is shorter than Base16, which helps avoid long path issues on windows
+  -- but it can contain /'s which aren't valid in file paths so replace them with
+  -- %'s. Also drop unnecessary padding.
+  = map (\c -> if c == '/' then '%' else c) . dropWhileEnd (== '=')
+  . showHashValueBase64 . hashValue . fromString <$> canonicalizePath script
 
 -- | Get the directory for caching a script build.
 --