win32: allow putenv() to set empty values

WIN32 _putenv() can't set an environment variable with an empty
value because 'NAME=' is treated as a request to delete the variable.

Allow mingw_putenv() to set an empty value by first setting a fake
value and then truncating it.

This problem has always been present in mingw_putenv() but it became
particularly pressing when ash started treating all applets as NOEXEC.
The environment of NOEXEC applets is created by clearing the current
environment and using putenv() to set a new one from the shell
variables.  Empty variable were not being set.

See GitHub issue #197.

1 files changed, 18 insertions, 4 deletions

diff --git a/win32/env.c b/win32/env.c
index a7ab7851a..4d4e9c8fd 100644
--- a/win32/env.c
+++ b/win32/env.c
@@ -78,16 +78,30 @@ int clearenv(void)
 
 int mingw_putenv(const char *env)
 {
-        char *s;
+        char *s, **envp;
+        int ret = 0;
 
         if ( (s=strchr(env, '=')) == NULL ) {
                 return unsetenv(env);
         }
 
-        if ( s[1] != '\0' ) {
+        if (s[1] != '\0') {
+                /* setting non-empty value is fine */
                 return _putenv(env);
         }
+        else {
+                /* set empty value by setting a non-empty one then truncating */
+                char *envstr = xasprintf("%s0", env);
+                ret = _putenv(envstr);
+
+                for (envp = environ; *envp; ++envp) {
+                        if (strcmp(*envp, envstr) == 0) {
+                                (*envp)[s - env + 1] = '\0';
+                                break;
+                        }
+                }
+                free(envstr);
+        }
 
-        /* can't set empty value */
-        return 0;
+        return ret;
 }