core: Make cog_modules_add_directory() honor COG_MODULEDIR

Move handling of the COG_MODULEDIR environment variable into the
cog_modules_add_directory() function, fix the bug that skipped using
the environment variable when cog_init() is not used, and improve
robustness:

- The path passed is checked to be a valid directory location.
- The environment variable is checked only once for the whole
  execution of the process, the first time the function is called
  with a NULL argument.
- The path in the environment variable is checked for validity as
  well. A warning is produced for invalid locations.
- If either the environment variable or the compiled-in default
  path are used, it is considered that the "default" path was set,
  and will not be set again to avoid re-scanning directories
  unnecessarily.

While at it, sprinkle comments on the code to explain the logic.

core/cog-modules.c

@@ -183,27 +183,75 @@ cog_modules_foreach(GIOExtensionPoint *extension_point, void (*callback)(GIOExte
 void
 cog_modules_add_directory(const char *directory_path)
 {
-    if (!directory_path)
-        directory_path = COG_MODULEDIR;
+    g_autoptr(GFile) path_file = NULL;
+
+    /*
+     * 1. If a path was assed, validate that it is a directory,
+     *    and produce and API usage warning otherwise.
+     */
+    if (directory_path) {
+        path_file = g_file_new_for_path(directory_path);
+        GFileType path_type = g_file_query_file_type(path_file, G_FILE_QUERY_INFO_NONE, NULL);
+        g_return_if_fail(path_type == G_FILE_TYPE_DIRECTORY);
+        directory_path = g_file_peek_path(path_file);
+    }
 
+    // The ensure_extension_points() function has its own GOnce lock,
+    // it does *NOT* need to run guarded by the "module_scan" lock.
     ensure_extension_points();
 
     G_LOCK_DEFINE_STATIC(module_scan);
     G_LOCK(module_scan);
 
-    static GIOModuleScope *scope = NULL;
-    if (!scope)
-        scope = g_io_module_scope_new(G_IO_MODULE_SCOPE_BLOCK_DUPLICATES);
-
+    /*
+     * 2. The "default" path is either the value of the COG_MODULEDIR
+     *    environment variable, or the compiled-in default path. If any
+     *    of them has already been scanned, bail out early.
+     */
     static gboolean default_path_added = FALSE;
-    if (!strcmp(COG_MODULEDIR, directory_path)) {
-        if (default_path_added) {
-            g_debug("%s: Default path already added, skipping.", G_STRFUNC);
-            goto out; // Ensure that the the lock is released.
-        }
+    if (default_path_added) {
+        g_debug("%s: Default path already added, skipping.", G_STRFUNC);
+        goto out; // Ensure that the the lock is released.
+    }
+
+    /*
+     * 3. For a NULL path, try using the COG_MODULEDIR environment variable,
+     *    or fall back to the compiled-in module path if undefined/invalid.
+     */
+    if (directory_path) {
+        // If the passed path matches the built-in default, mark it as added.
+        default_path_added = !strcmp(COG_MODULEDIR, directory_path);
+    } else {
+        g_assert(!path_file);
+
+        directory_path = COG_MODULEDIR;
         default_path_added = TRUE;
+
+        // Replace the path with the one from the environment variable,
+        // if it actually points to a valid directory.
+        static gboolean env_path_checked = FALSE;
+        if (!env_path_checked) {
+            env_path_checked = TRUE;
+            const char *env_value = g_getenv("COG_MODULEDIR");
+            if (env_value) {
+                path_file = g_file_new_for_path(env_value);
+                if (g_file_query_file_type(path_file, G_FILE_QUERY_INFO_NONE, NULL) == G_FILE_TYPE_DIRECTORY) {
+                    directory_path = g_file_peek_path(path_file);
+                } else {
+                    g_warning("Path '%s' is not a directory.", env_value);
+                }
+            }
+        }
     }
 
+    /*
+     * 4. At this point, "directory_path" contains the path to scan.
+     *    Create a scope if needed to avoid loading duplicate plug-ins.
+     */
+    static GIOModuleScope *scope = NULL;
+    if (!scope)
+        scope = g_io_module_scope_new(G_IO_MODULE_SCOPE_BLOCK_DUPLICATES);
+
     g_debug("%s: Scanning '%s'", G_STRFUNC, directory_path);
     g_io_modules_scan_all_in_directory_with_scope(directory_path, scope);
 

core/cog-platform.c

@@ -90,7 +90,7 @@ cog_platform_ensure_singleton(const char *name)
 void
 cog_init(const char *platform_name, const char *module_path)
 {
-    cog_modules_add_directory(module_path ?: g_getenv("COG_MODULEDIR"));
+    cog_modules_add_directory(module_path);
     gboolean already_initialized = cog_platform_ensure_singleton(platform_name ?: g_getenv("COG_PLATFORM_NAME"));
     g_return_if_fail(!already_initialized);
 }