Fix os.getenv of lua on Windows

Change to use os_getenv instead of getenv because environment variable
set by uv_os_setenv can not be get with getenv.

src/nvim/lua/executor.c

@@ -24,6 +24,10 @@
 #include "nvim/undo.h"
 #include "nvim/ascii.h"
 
+#ifdef WIN32
+#include "nvim/os/env.h"
+#endif
+
 #include "nvim/lua/executor.h"
 #include "nvim/lua/converter.h"
 
@@ -118,6 +122,14 @@ static int nlua_state_init(lua_State *const lstate) FUNC_ATTR_NONNULL_ALL
   lua_setfield(lstate, -2, "debug");
   lua_pop(lstate, 1);
 
+#ifdef WIN32
+  // os.getenv
+  lua_getglobal(lstate, "os");
+  lua_pushcfunction(lstate, &nlua_getenv);
+  lua_setfield(lstate, -2, "getenv");
+  lua_pop(lstate, 1);
+#endif
+
   // vim
   if (luaL_dostring(lstate, (char *)&vim_module[0])) {
     nlua_error(lstate, _("E5106: Error while creating vim module: %.*s"));
@@ -337,6 +349,19 @@ int nlua_debug(lua_State *lstate)
   return 0;
 }
 
+#ifdef WIN32
+/// os.getenv implementation: On Windows, uv_os_setenv does not update _environ,
+/// so we need to use os_getenv instead of getenv. Therefore we will apply a
+/// monkey patch here.
+///
+/// @param  lstate  Lua interpreter state.
+static int nlua_getenv(lua_State *lstate)
+{
+  lua_pushstring(lstate, os_getenv(luaL_checkstring(lstate, 1)));
+  return 1;
+}
+#endif
+
 /// Evaluate lua string
 ///
 /// Used for luaeval().

src/nvim/os/env.h

@@ -0,0 +1,7 @@
+#ifndef NVIM_OS_ENV_H
+#define NVIM_OS_ENV_H
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/env.h.generated.h"
+#endif
+#endif  // NVIM_OS_ENV_H

test/functional/lua/overrides_spec.lua

@@ -300,3 +300,19 @@ describe('package.path/package.cpath', function()
     eq(new_paths_str, eval_lua('package.path'):sub(1, #new_paths_str))
   end)
 end)
+
+describe('os.getenv', function()
+  it('returns nothing for not set env var', function()
+    eq(NIL, funcs.luaeval('os.getenv("XTEST_1")'))
+  end)
+  it('returns env var set by the parent process', function()
+    local  value = 'foo'
+    clear({env = {['XTEST_1']=value}})
+    eq(value, funcs.luaeval('os.getenv("XTEST_1")'))
+  end)
+  it('returns env var set by let', function()
+    local value = 'foo'
+    meths.command('let $XTEST_1 = "'..value..'"')
+    eq(value, funcs.luaeval('os.getenv("XTEST_1")'))
+  end)
+end)