Shake cmd does not deal with async exceptions

[facundominguez]

The following program terminates when the action for a produces a failure, without waiting for action b to finish. Shouldn't shake wait until b completes?

$ cat test.hs
import Control.Concurrent
import Development.Shake
import System.Environment

main = withArgs ["-j"] $ shakeArgs shakeOptions $ do
    want ["a", "b"]
    "a" %> \_out -> do
      liftIO $ threadDelay (100 * 1000)
      fail "ugh"
    "b" %> \_out ->
      cmd Shell "sleep 3 && echo hello"

$ ghc-8.0.2 --make -threaded test.hs
[1 of 1] Compiling Main             ( test.hs, test.o )
Linking test ...
$ ./test
# sleep (for b)
Error when running Shake build system:
* a
ugh

$ hello

[ndmitchell]

When one action fails, Shake attempts to kill all the other actions that are underway, to return an error message to the user as fast as possible. From your trace, that seems to be what is happening - can you confirm?

[facundominguez]

That's not what's happening. Shake seems to return immediately, and the action for b completes afterwards. Note that the hello message is printed after the new bash prompt is printed.

If the action for b were killed, the hello message would never print.

[ndmitchell]

Actually, it's caused by Shake not dealing with async exceptions properly in cmd - Shake does clean up (using async exceptions) and cmd incorrectly ignores them. I've found a nice test case and added it to the test suite, now to fix it's behaviour...

[ndmitchell]

Looking at the code, there's way too much try_, ignore, forkIO etc in the code. I don't know which one is causing the problem, but I'm tempted to remove most/all of them, and then see if the bug goes away - and in the process will probably also squash a lot of other bugs.

[ndmitchell]

I've fixed a bunch of stuff in Shake, but come down to underlying bugs in Process now: haskell/process#107