Add getPid

This allows fetching a process' ID without using `unsafeProcessHandle`.

src/System/Process/Typed.hs

@@ -109,6 +109,9 @@ module System.Process.Typed
     , checkExitCode
     , checkExitCodeSTM
 
+      -- ** Process ID
+    , getPid
+
       -- ** Process streams
     , getStdin
     , getStdout
@@ -138,6 +141,7 @@ import Control.Concurrent (threadDelay)
 import Control.Concurrent.Async (asyncWithUnmask, cancel, waitCatch)
 import Control.Concurrent.STM (newEmptyTMVarIO, atomically, putTMVar, TMVar, readTMVar, tryReadTMVar, STM, tryPutTMVar, throwSTM, catchSTM)
 import System.Exit (ExitCode (ExitSuccess, ExitFailure))
+import System.Posix.Types (ProcessID)
 import System.Process.Typed.Internal
 import qualified Data.ByteString.Lazy as L
 import GHC.RTS.Flags (getConcFlags, ctxtSwitchTime)
@@ -641,6 +645,17 @@ checkExitCodeSTM p = do
             , eceStderr = L.empty
             }
 
+-- | Returns the PID (process ID) of a subprocess.
+--
+-- 'Nothing' is returned if the underlying 'P.ProcessHandle' was already closed.
+-- Otherwise a PID is returned that remains valid as long as the handle is
+-- open. The operating system may reuse the PID as soon as the last handle to
+-- the process is closed.
+--
+-- @since 0.2.12.0
+getPid :: Process stdin stdout stderr -> IO (Maybe ProcessID)
+getPid = P.getPid . pHandle
+
 -- | Internal
 clearStreams :: ProcessConfig stdin stdout stderr -> ProcessConfig () () ()
 clearStreams pc = pc