hadrian: Handle broken symlinks properly when creating source dist di…

…rectories

If we have a broken symlink in the repository, don't try to `need` the symlink
or the target of the symlink. Attempting to do so has `shake` attempt to read the
target to compute its hash, which fails because the target doesn't exist.

hadrian/src/Rules/SourceDist.hs

@@ -74,7 +74,9 @@ copyFileSourceDist source target = do
  error ("source-dist: tried to create non-relative symlink in source dist: " ++ show link_target)
  putProgressInfo =<< renderAction ("Create symlink (" ++ link_target ++ ")") source target
  isDirectory <- liftIO $ IO.doesDirectoryExist source
- when (not isDirectory) $
+ -- We don't want to call `need` on broken symlinks
+ linkTargetExists <- liftIO $ IO.doesPathExist link_target
+ when (not isDirectory && linkTargetExists) $
  need [source]
  let createLink src tgt
  | isDirectory = liftIO $ IO.createDirectoryLink src tgt