win32: better fix for empty environment variables

It appears the CRT and OS each have a copy of the environment.
mingw_putenv() fools the CRT into accepting an empty environment
variable by calling _putenv("V=0") then truncating the new value
by hand.  But _putenv() also updates the OS environment with the
fake 'V=0' value.  Commit 5b48ca53b (win32: pass NULL to spawnve,
not environ) resulted in this fake value being used and hence empty
variables getting the value '0'.

- Add a call to SetEnvironmentVariable() in mingw_putenv() to update
  the OS environment.

- Restore the use of NULL environment pointers in mingw_spawnvp().

- Add a test.

(GitHub issue #250)

1 files changed, 4 insertions, 0 deletions

diff --git a/win32/env.c b/win32/env.c
index 4d4e9c8fd..8e54c4c5e 100644
--- a/win32/env.c
+++ b/win32/env.c
@@ -100,6 +100,10 @@ int mingw_putenv(const char *env)
                                 break;
                         }
                 }
+
+                /* tell the OS environment about the change */
+                envstr[s - env] = '\0';
+                SetEnvironmentVariable(envstr, "");
                 free(envstr);
         }