Add hook for creating ghci external interpreter

Summary:
The external interpreter is launched by calling
'System.Process.createProcess' with a 'CreateProcess' parameter.

The current value for this has the 'std_in', 'std_out' and 'std_err'
fields use the default of 'Inherit', meaning that the remote interpreter
shares the stdio with the original ghc/ghci process.

This patch introduces a new hook to the DynFlags, which has an
opportunity to override the 'CreateProcess' fields, launch the process,
and retrieve the stdio handles actually used.

So if a ghci external interpreter session is launched from the GHC API
the stdio can be redirected if required, which is useful for tooling/IDE
integration.

Test Plan: ./validate

Reviewers: austin, hvr, simonmar, bgamari

Reviewed By: simonmar, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2518

1 files changed, 4 insertions, 0 deletions

diff --git a/compiler/main/Hooks.hs b/compiler/main/Hooks.hs
index 237101bce0..8d706d8fa5 100644
--- a/compiler/main/Hooks.hs
+++ b/compiler/main/Hooks.hs
@@ -25,6 +25,7 @@ module Hooks ( Hooks
              , runRnSpliceHook
 #ifdef GHCI
              , getValueSafelyHook
+             , createIservProcessHook
 #endif
              ) where
 
@@ -45,6 +46,7 @@ import CoreSyn
 import GHCi.RemoteTypes
 import SrcLoc
 import Type
+import System.Process
 #endif
 import BasicTypes
 
@@ -78,6 +80,7 @@ emptyHooks = Hooks
   , runRnSpliceHook        = Nothing
 #ifdef GHCI
   , getValueSafelyHook     = Nothing
+  , createIservProcessHook = Nothing
 #endif
   }
 
@@ -96,6 +99,7 @@ data Hooks = Hooks
   , runRnSpliceHook        :: Maybe (HsSplice Name -> RnM (HsSplice Name))
 #ifdef GHCI
   , getValueSafelyHook     :: Maybe (HscEnv -> Name -> Type -> IO (Maybe HValue))
+  , createIservProcessHook :: Maybe (CreateProcess -> IO ProcessHandle)
 #endif
   }