Fix memory leak from #12664

This fixes the leak with `setProgArgv`. The problem was
that `setProgArgv` would not free the objects pointed
to by `prog_argc`, `prog_argv` when the globals were
changed resulting in a leak.

The only strictly necessary change is in `rts/RtsFlags.c`, but
the code in `System.Environment` was a bit confusing and not
exception safe, so I refactored it.

Test Plan: ./validate

Reviewers: simonmar, ezyang, austin, hvr, bgamari, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2576

GHC Trac Issues: #12664

1 files changed, 18 insertions, 0 deletions

diff --git a/libraries/base/GHC/Foreign.hs b/libraries/base/GHC/Foreign.hs
index e8553d8061..7d2f915920 100644
--- a/libraries/base/GHC/Foreign.hs
+++ b/libraries/base/GHC/Foreign.hs
@@ -32,6 +32,7 @@ module GHC.Foreign (
     --
     withCString,
     withCStringLen,
+    withCStringsLen,
 
     charIsRepresentable,
   ) where
@@ -134,6 +135,23 @@ withCString enc s act = withEncodedCString enc True s $ \(cp, _sz) -> act cp
 withCStringLen         :: TextEncoding -> String -> (CStringLen -> IO a) -> IO a
 withCStringLen enc = withEncodedCString enc False
 
+-- | Marshal a list of Haskell strings into an array of NUL terminated C strings
+-- using temporary storage.
+--
+-- * the Haskell strings may /not/ contain any NUL characters
+--
+-- * the memory is freed when the subcomputation terminates (either
+--   normally or via an exception), so the pointer to the temporary
+--   storage must /not/ be used after this.
+--
+withCStringsLen :: TextEncoding
+                -> [String]
+                -> (Int -> Ptr CString -> IO a)
+                -> IO a
+withCStringsLen enc strs f = go [] strs
+  where
+  go cs (s:ss) = withCString enc s $ \c -> go (c:cs) ss
+  go cs [] = withArrayLen (reverse cs) f
 
 -- | Determines whether a character can be accurately encoded in a 'CString'.
 --