Some execution functions lookup files incorrectly

[Blaisorblade]

To reproduce:

1. Add to the PATH a non-executable git file
2. Watch stack try and fail to execute that file, if needed when fetching.

$ > ~/.local/bin/git
$ ls -l ~/.local/bin/git
-rw-r--r--+ 1 pgiarrusso staff 0 Aug 14 11:22 /Users/pgiarrusso/.local/bin/git
$ stack build --dry-run
/Users/pgiarrusso/.local/bin/git: streamingProcess: runInteractiveProcess: exec: permission denied (Permission denied)
$ stack upgrade
Fetching package index .../Users/pgiarrusso/.local/bin/git: createProcess: runInteractiveProcess: exec: permission denied (Permission denied)

For the failure to occur on build, the package needs git dependencies, for instance:

resolver: lts-6.6

packages:
- '.'
- location:
    git: https://github.com/agda/agda
    commit: e3472d7c1441c0d6db66be9e094e9dbcb7c33735

I noticed this by chance while debugging #2241. I can't remove Git from OS X 10.11, but I tried shadowing it with a failing one and made a mistake.

The bug is thankfully not in the process package. Nor is stack itself doing the lookup—the bug is somewhere between System.Process.Read and its supporting libraries. BTW, System.Process.Read appears confusing to me.

$ stack runghc ./runProcess.hs
git version 2.9.0
First git done
git version 2.9.0

runProcess.hs:

import System.IO
import System.Process

args = ["--version"]
main = do
  (_, _, _, ph) <- createProcess (proc "git" args)
  ec <- waitForProcess ph
  hPutStrLn stderr "First git done"
  ph <- runProcess "git" args Nothing Nothing Nothing Nothing Nothing
  ec <- waitForProcess ph
  return ()

[Blaisorblade]

BTW, all that was on OS X, I haven't rechecked on Linux yet.

[Blaisorblade]

Ah, I had missed stack's findExecutable, the bug must be there!