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)

3 files changed, 75 insertions, 9 deletions

diff --git a/testsuite/env.tests b/testsuite/env.tests
new file mode 100755
index 000000000..456808728
--- /dev/null
+++ b/testsuite/env.tests
@@ -0,0 +1,67 @@
+#!/bin/sh
+# Copyright 2022 by Ron Yorston
+# Licensed under GPLv2, see file LICENSE in this source tree.
+
+. ./testing.sh
+
+# testing "test name" "commands" "expected result" "file input" "stdin"
+
+# Not so much a test of 'env' as of whether environment variables
+# (or the lack thereof) are correctly passed to child processes.
+testing "environment variables 1a" \
+        "V=set env sh -c 'env | grep ^V='" \
+        "V=set
+" "" ""
+
+testing "environment variables 1b" \
+        "V= env sh -c 'env | grep ^V='" \
+        "V=
+" "" ""
+
+testing "environment variables 1c" \
+        "env sh -c 'env | grep ^V='" \
+        "" "" ""
+
+testing "environment variables 2a" \
+        "V=set sh -c 'env | grep ^V='" \
+        "V=set
+" "" ""
+
+testing "environment variables 2b" \
+        "V= sh -c 'env | grep ^V='" \
+        "V=
+" "" ""
+
+testing "environment variables 2c" \
+        "sh -c 'env | grep ^V='" \
+        "" "" ""
+
+testing "environment variables 3a" \
+        "V=set env sh -c 'echo \${V-unset}'" \
+        "set
+" "" ""
+
+testing "environment variables 3b" \
+        "V= env sh -c 'echo \${V-unset}'" \
+        "
+" "" ""
+
+testing "environment variables 3c" \
+        "env sh -c 'echo \${V-unset}'" \
+        "unset
+" "" ""
+
+testing "environment variables 4a" \
+        "V=set sh -c 'echo \${V-unset}'" \
+        "set
+" "" ""
+
+testing "environment variables 4b" \
+        "V= sh -c 'echo \${V-unset}'" \
+        "
+" "" ""
+
+testing "environment variables 4c" \
+        "sh -c 'echo \${V-unset}'" \
+        "unset
+" "" ""
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);
         }
 
diff --git a/win32/process.c b/win32/process.c
index 5978226f0..d4ab07ad8 100644
--- a/win32/process.c
+++ b/win32/process.c
@@ -345,32 +345,27 @@ mingw_spawnvp(int mode, const char *cmd, char *const *argv)
 {
         char *prog;
         intptr_t ret;
-#if !defined(_UCRT)
-        char *const *envp = environ;
-#else
-        char *const *envp = NULL;
-#endif
 
 #if ENABLE_FEATURE_PREFER_APPLETS && NUM_APPLETS > 1
         if (find_applet_by_name(cmd) >= 0)
-                return mingw_spawn_applet(mode, argv, envp);
+                return mingw_spawn_applet(mode, argv, NULL);
         else
 #endif
         if (has_path(cmd)) {
                 char *path = alloc_system_drive(cmd);
                 add_win32_extension(path);
-                ret = mingw_spawn_interpreter(mode, path, argv, envp, 0);
+                ret = mingw_spawn_interpreter(mode, path, argv, NULL, 0);
                 free(path);
 #if ENABLE_FEATURE_PREFER_APPLETS && NUM_APPLETS > 1
                 if (ret == -1 && unix_path(cmd) &&
                                 find_applet_by_name(bb_basename(cmd)) >= 0) {
-                        return mingw_spawn_applet(mode, argv, envp);
+                        return mingw_spawn_applet(mode, argv, NULL);
                 }
 #endif
                 return ret;
         }
         else if ((prog=find_first_executable(cmd)) != NULL) {
-                ret = mingw_spawn_interpreter(mode, prog, argv, envp, 0);
+                ret = mingw_spawn_interpreter(mode, prog, argv, NULL, 0);
                 free(prog);
                 return ret;
         }