Reload .cabal files when they are modified

[fendor]

Partially addresses #4236

It doesn't work with sessionLoading multipleComponents because hie-bios doesn't give us the correct cradle dependencies.

A workaround is to declare the cradle dependencies in the hie.yaml. We need to fix this in hie-bios proper, but it is no longer an HLS bug.

[soulomoon]

I don't understand how ignoring version in the VFS help us here🤔

[fendor]

Because cabal only works with the physical file system. If the VFS changes, then cabal simply can't perform a reload, as cabal repl will still observe the same .cabal file. That's why we only want to invalidate the shake session, if the .cabal file on disk has been modified.

Further, let's have a look at getModificationTimeImpl:

getModificationTimeImpl
  :: Bool
  -> NormalizedFilePath
  -> Action (Maybe BS.ByteString, ([FileDiagnostic], Maybe FileVersion))
getModificationTimeImpl missingFileDiags file = do
    let file' = fromNormalizedFilePath file
    let wrap time = (Just $ LBS.toStrict $ B.encode $ toRational time, ([], Just $ ModificationTime time))
    mbVf <- getVirtualFile file
    case mbVf of
        Just (virtualFileVersion -> ver) -> do
            alwaysRerun
            pure (Just $ LBS.toStrict $ B.encode ver, ([], Just $ VFSVersion ver))
        Nothing -> do
            isWF <- use_ AddWatchedFile file
            if isWF
                then -- the file is watched so we can rely on FileWatched notifications,
                        -- but also need a dependency on IsFileOfInterest to reinstall
                        -- alwaysRerun when the file becomes VFS
                    void (use_ IsFileOfInterest file)
                else if isInterface file
                    then -- interface files are tracked specially using the closed world assumption
                        pure ()
                    else -- in all other cases we will need to freshly check the file system
                        alwaysRerun
                        
           ...

First, we don't care about the isInterface file check. Then, if the file isn't open in the VFS, we will never rerun this rule, unless the IsFileOfInterest rule is marked dirty, which it never will for .cabal files.

In my understanding, isWF will be True for .cabal files.

[soulomoon]

Thanks for the explanation, much clear to me now.

[soulomoon]

Just some more interesting thoughts, do you think we can make GetModificationTime rule use GetPhysicalModificationTime rule in the end to remove some duplicated file time query?

[fendor]

I am not sure, maybe. The whole file watcher architecture requires a thorough overhaul, it feels very brittle.