slave: unsetenv() env vars that have been set to the empty string

That is, users can now *clear* environment variables by doing:

  [environment]
  VAR=””

Note that the quotes are required.

Closes #1225

CHANGELOG.md

@@ -51,8 +51,11 @@
 * `name` capability to `XTGETTCAP`.
 * String values in `foot.ini` may now be quoted. This can be used to
   set a value to the empty string, for example.
+* Environment variables can now be **unset**, by setting
+  `[environment].<variable>=""` (quotes are required) ([#1225][1225])
 
 [1136]: https://codeberg.org/dnkl/foot/issues/1136
+[1225]: https://codeberg.org/dnkl/foot/issues/1225
 
 
 ### Changed

config.c

@@ -2295,21 +2295,22 @@ parse_section_environment(struct context *ctx)
 {
     struct config *conf = ctx->conf;
     const char *key = ctx->key;
-    const char *value = ctx->value;
 
+    /* Check for pre-existing env variable */
     tll_foreach(conf->env_vars, it) {
-        if (strcmp(it->item.name, key) == 0) {
-            free(it->item.value);
-            it->item.value = xstrdup(value);
-            return true;
-        }
+        if (strcmp(it->item.name, key) == 0)
+            return value_to_str(ctx, &it->item.value);
     }
 
-    struct env_var var = {
-        .name = xstrdup(key),
-        .value = xstrdup(value),
-    };
-    tll_push_back(conf->env_vars, var);
+    /*
+     * No pre-existing variable - allocate a new one
+     */
+
+    char *value = NULL;
+    if (!value_to_str(ctx, &value))
+        return false;
+
+    tll_push_back(conf->env_vars, ((struct env_var){xstrdup(key), value}));
     return true;
 }
 

slave.c

@@ -359,8 +359,15 @@ slave_spawn(int ptmx, int argc, const char *cwd, char *const *argv,
 #endif
 
         if (extra_env_vars != NULL) {
-            tll_foreach(*extra_env_vars, it)
-                setenv(it->item.name, it->item.value, 1);
+            tll_foreach(*extra_env_vars, it) {
+                const char *name = it->item.name;
+                const char *value = it->item.value;
+
+                if (strlen(value) == 0)
+                    unsetenv(name);
+                else
+                    setenv(name, value, 1);
+            }
         }
 
         char **_shell_argv = NULL;