Fix input-output-hk#42: use patches for revisions

app/Foliage/CmdBuild.hs

@@ -6,7 +6,7 @@ module Foliage.CmdBuild (cmdBuild) where
 import Codec.Archive.Tar qualified as Tar
 import Codec.Archive.Tar.Entry qualified as Tar
 import Codec.Compression.GZip qualified as GZip
-import Control.Monad (unless, void, when)
+import Control.Monad (foldM, unless, void, when)
 import Data.Aeson qualified as Aeson
 import Data.ByteString qualified as BS
 import Data.ByteString.Lazy qualified as BL
@@ -113,9 +113,22 @@ buildAction
             cf <- prepareIndexPkgCabal pkgId cabalFileTimestamp originalCabalFilePath
 
             -- all revised cabal files, with their timestamp
-            revcf <- for cabalFileRevisions $ uncurry (prepareIndexPkgCabal pkgId)
-
-            return $ cf : revcf
+            revcf <-
+              foldM
+                ( \(lastRevisionPath, cabalFiles) (timestamp, revisionPath) -> do
+                    if takeExtension originalCabalFilePath `elem` [".diff", ".patch"]
+                      then do
+                        patchedRevisionPath <- applyPatch lastRevisionPath revisionPath
+                        preparedCabal <- prepareIndexPkgCabal pkgId timestamp patchedRevisionPath
+                        pure (patchedRevisionPath, cabalFiles ++ [preparedCabal])
+                      else do
+                        preparedCabal <- prepareIndexPkgCabal pkgId timestamp revisionPath
+                        pure (revisionPath, cabalFiles ++ [preparedCabal])
+                )
+                (originalCabalFilePath, [])
+                (sortOn fst cabalFileRevisions)
+
+            return $ cf : snd revcf
         )
         packageVersions
 
@@ -312,3 +325,9 @@ mkTarEntry contents indexFile timestamp =
 anchorPath :: Path Absolute -> (RepoLayout -> RepoPath) -> FilePath
 anchorPath outputDirRoot p =
   toFilePath $ anchorRepoPathLocally outputDirRoot $ p hackageRepoLayout
+
+applyPatch :: FilePath -> FilePath -> Action FilePath
+applyPatch originalFilePath patchFilePath = do
+  withTempFile $ \outputFilePath -> do
+    cmd_ ["patch", "-i", patchFilePath, "-o", outputFilePath, originalFilePath]
+    return outputFilePath