Nh2 fix streaming process on nonthreaded

Data/Streaming/Process.hs

@@ -32,7 +32,7 @@ module Data.Streaming.Process
     ) where
 
 import           Control.Applicative             as A ((<$>), (<*>))
-import           Control.Concurrent              (forkIOWithUnmask)
+import           Control.Concurrent              (forkIOWithUnmask, threadWaitRead)
 import           Control.Concurrent.STM          (STM, TMVar, atomically,
                                                   newEmptyTMVar, putTMVar,
                                                   readTMVar)
@@ -44,6 +44,7 @@ import           Data.Streaming.Process.Internal
 import           Data.Typeable                   (Typeable)
 import           System.Exit                     (ExitCode (ExitSuccess))
 import           System.IO                       (hClose)
+import           System.Posix.IO (handleToFd)
 import           System.Process
 
 #if MIN_VERSION_process(1,2,0)
@@ -148,6 +149,19 @@ streamingProcess cp = liftIO $ do
         (getStdout, stdoutStream) = osStdStream
         (getStderr, stderrStream) = osStdStream
 
+    -- We use a pipe to the child process to determine when it's dead.
+    -- In Unix, when there is a Unix pipe between two processes, then
+    --   "When the child process terminates, its end of the pipe will be closed"
+    -- (see https://stackoverflow.com/questions/8976004/using-waitpid-or-sigaction/8976461#8976461)
+    -- See also http://tldp.org/LDP/lpg/node11.html about Unix pipes.
+    -- Making this decision based on a pipe FD is better than `waitpid()` because
+    -- we can use GHC IO manager's `threadWaitRead` function to wait in a
+    -- non-blocking, non-polling way.
+    -- TODO: Use `createPipeFd` instead of `createPipe` once this package
+    --       requries process >= 1.4.2.0; then we don't have to use
+    --       `handleToFd` below.
+    (readHandle, writeHandle) <- createPipe
+
 #if MIN_VERSION_process(1,2,0)
     (stdinH, stdoutH, stderrH, ph) <- PI.createProcess_ "streamingProcess" cp
 #else
@@ -158,12 +172,19 @@ streamingProcess cp = liftIO $ do
         , std_err = fromMaybe (std_err cp) stderrStream
         }
 
+    -- Close pipe write end from parent process (we don't need it).
+    hClose writeHandle
+    -- When the child process closes its write end (e.g. by terminating),
+    -- we'll read EOF on our read end, and we wait for that to happen with
+    -- the `threadWaitRead readFd` below.
+    readFd <- handleToFd readHandle
+
     ec <- atomically newEmptyTMVar
     -- Apparently waitForProcess can throw an exception itself when
     -- delegate_ctlc is True, so to avoid this TMVar from being left empty, we
     -- capture any exceptions and store them as an impure exception in the
     -- TMVar
-    _ <- forkIOWithUnmask $ \_unmask -> try (waitForProcess ph)
+    _ <- forkIOWithUnmask $ \_unmask -> try (threadWaitRead readFd >> waitForProcess ph)
         >>= atomically
           . putTMVar ec
           . either