Demonstrate one aspect of issue haskell#187

Due to the nature of directory-based locking, if the process exits
before exception handlers are run, the directory is never removed and
the system remains in a "locked" state indefinitely. This can happen due
to SIGKILL or power failure, as well as a blocked thread when main
exits. This repro follows that last approach, forking a thread and then
letting main exit while that locked thread delays.

On my machine, I get the following output for running issue-187.sh:

$ ./issue-187.sh
+ rm -rf tmp
+ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.2.2
+ cabal --version
cabal-install version 2.0.0.1
compiled using version 2.0.1.1 of the Cabal library
+ cabal sandbox init
Writing a default package environment file to
/Users/michael/Documents/hackage-security/cabal.sandbox.config
Using an existing sandbox located at
/Users/michael/Documents/hackage-security/.cabal-sandbox
+ cabal install ./hackage-security
Resolving dependencies...
In order, the following will be installed:
hackage-security-0.5.2.2 (reinstall)
Warning: Note that reinstalls are always dangerous. Continuing anyway...
Notice: installing into a sandbox located at
/Users/michael/Documents/hackage-security/.cabal-sandbox
Configuring hackage-security-0.5.2.2...
Building hackage-security-0.5.2.2...
Installed hackage-security-0.5.2.2
+ cabal exec -- ghc -hide-all-packages -package hackage-security
-package base -package directory -Wall -Werror issue-187.hs
[1 of 1] Compiling Main             ( issue-187.hs, issue-187.o )
Linking issue-187 ...
+ ./issue-187
+ ./issue-187
issue-187:
/Users/michael/Documents/hackage-security/tmp/hackage-security-lock:
createDirectory: already exists (File exists)

issue-187.hs

@@ -0,0 +1,28 @@
+import           Control.Concurrent                       (MVar, forkIO,
+                                                           newEmptyMVar,
+                                                           putMVar, takeMVar,
+                                                           threadDelay)
+import           Control.Exception                        (finally)
+import qualified Hackage.Security.Client                  as HS
+import qualified Hackage.Security.Client.Repository.Cache as HS
+import qualified Hackage.Security.Util.Path               as HS
+import           System.Directory                         (canonicalizePath, createDirectoryIfMissing)
+
+update :: MVar () -> FilePath -> IO ()
+update baton path = HS.lockCache cache (do
+  putMVar baton ()
+  threadDelay 5000000
+  putStrLn "Leaving update") `finally` putMVar baton ()
+  where
+    cache = HS.Cache
+      { HS.cacheRoot = HS.fromAbsoluteFilePath path
+      , HS.cacheLayout = HS.cabalCacheLayout
+      }
+
+main :: IO ()
+main = do
+  createDirectoryIfMissing True "tmp"
+  path <- canonicalizePath "tmp"
+  baton <- newEmptyMVar
+  _ <- forkIO $ update baton path
+  takeMVar baton

issue-187.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+set -eux
+
+rm -rf tmp
+
+ghc --version
+cabal --version
+
+cabal sandbox init || true
+cabal install ./hackage-security
+
+cabal exec -- ghc \
+  -hide-all-packages \
+  -package hackage-security \
+  -package base \
+  -package directory \
+  -Wall -Werror \
+  issue-187.hs
+
+./issue-187
+./issue-187