More recompilations than expected with variable rules

[takano-akio]

I expect the following program to print "updated" just once, but with shake HEAD (but not with shake-0.15.11) it prints "updated" twice. This problem looks similar to #523 and may be related, but it is about more recompilations than expected, not less.

import Development.Shake

main :: IO ()
main = do
  writeFile "0.in" "foo"
  writeFile "1.in" "foo"
  run $ rules "0.out" -- prints "updated"
  writeFile "0.in" "bar"
  run $ rules "1.out" -- prints "updated" again
  where
    run r = do
      putStrLn "Running shake"
      shake shakeOptions r

rules :: FilePath -> Rules ()
rules wanted = do
  want ["top"]

  "0.out" %> \f -> do
    alwaysRerun
    writeFile' f =<< readFile' "0.in"
    liftIO $ putStrLn "updated"
  "1.out" %> \f -> writeFile' f =<< readFile' "1.in"

  phony "top" $ do
    -- alwaysRerun -- uncommenting this line fixes the issue
    need [wanted]

One suspicious point in this program is the rule for "top", whose content changes depending on the value of the argument wanted. I had thought this would be ok because it is a phony target, and thus merely defines an abbreviation. But perhaps I'm not supposed to do this?

[ndmitchell]

Thanks! I've pushed a potential fix to HEAD. I'll describe it in detail later (just rushing out), but you might want to retry the original problem you were having. Should fix both tickets.

[pacak]

Nope, extra recompilations I showed logs for in #519 are still there with this fix applied.

[ndmitchell]

So the issue that I fixed is that as part of #427 I moved checking whether the output was dirty from before rebuilding the dependencies to after. For normal file rules the two effects are roughly the same. For phony rules, the output is always dirty, so it has changed the order to something counter intuitive. I've fixed that by inserting a fake alwaysRerun at the beginning of every phony, restoring the old behaviour. The fact #519 doesn't get fixed is a separate issue then.