Can do either, or both... up to you guys - let me know.

I hate to pollute the API, but lets do both.

Alright. Since they only differ by one line, I refactored.

found_pos tracks the position as it goes through the loop, so is incremented every pass.  found_paths tracks how many have been found, and is only incremented when a path is found.  found_pos is used to check that the 'pre' paths are actually first and the 'post' paths are actaully last.

Ah, the names are a bit too similar then. Maybe just call this one index? And then you should increment at the end of the loop for consistency with how indexing works in C. I'd also flip the order of the comparison in the asserts as well.

renamed it to paths_pos

which build directory?  I'm using 'convey' to run tests and such. convey clang-format seems to run, but writes no changes and doesn't seem to product any actionable output (eg. a list of fixes it wants). should it?

Using convey is good too, but it only prints out the problems:
diff -r 464e53233e3b gplugin/gplugin-manager.c
--- a/gplugin/gplugin-manager.c Fri Oct 22 22:53:27 2021 -0400
+++ b/gplugin/gplugin-manager.c Sat Oct 23 02:56:16 2021 +0000
@@ -133,15 +133,15 @@
    gchar **paths;
    gint i;
    const gchar *from_env;
-   void (*changer)(GPluginManager*, const gchar*);
+   void (*changer)(GPluginManager *, const gchar *);

    from_env = g_getenv(name);
-   if (from_env == NULL) {
+   if(from_env == NULL) {
        return;
    }

    paths = g_strsplit(from_env, G_SEARCHPATH_SEPARATOR_S, 0);
-   if (prepend) {
+   if(prepend) {
        changer = &gplugin_manager_prepend_path;
    } else {
        changer = &gplugin_manager_append_path;
@@ -727,10 +727,7 @@
    GPluginManager *manager,
    const gchar *name)
 {
-   gplugin_manager_change_paths_from_environment(
-           manager,
-           name,
-           FALSE);
+   gplugin_manager_change_paths_from_environment(manager, name, FALSE);
 }

 /**
@@ -746,10 +743,7 @@
    GPluginManager *manager,
    const gchar *name)
 {
-   gplugin_manager_change_paths_from_environment(
-           manager,
-           name,
-           TRUE);
+   gplugin_manager_change_paths_from_environment(manager, name, TRUE);
 }

 /**
diff -r 464e53233e3b gplugin/tests/test-plugin-manager-paths.c
--- a/gplugin/tests/test-plugin-manager-paths.c Fri Oct 22 22:53:27 2021 -0400
+++ b/gplugin/tests/test-plugin-manager-paths.c Sat Oct 23 02:56:16 2021 +0000
@@ -221,7 +221,6 @@
    g_assert_cmpuint(0, ==, size);
 }

-
 static void
 test_gplugin_manager_add_app_paths_via_environ(void)
 {
@@ -240,26 +239,24 @@
    path_count = g_list_length(paths);

    /* build contents of environment vars */
-   pre_paths = g_strdup_printf("%s%s%s",
-           "/prefoo",
-           G_SEARCHPATH_SEPARATOR_S,
-           "/prebar/"
-           );
-   post_paths = g_strdup_printf("%s%s%s",
-           "/postfoo",
-           G_SEARCHPATH_SEPARATOR_S,
-           "/postbar/"
-           );
+   pre_paths = g_strdup_printf(
+       "%s%s%s",
+       "/prefoo",
+       G_SEARCHPATH_SEPARATOR_S,
+       "/prebar/");
+   post_paths = g_strdup_printf(
+       "%s%s%s",
+       "/postfoo",
+       G_SEARCHPATH_SEPARATOR_S,
+       "/postbar/");

    /* put /envfoo and /envbar as paths in an environment variable */
    g_setenv(pre_env_var_name, pre_paths, TRUE);
    g_setenv(post_env_var_name, post_paths, TRUE);

    /* have the manager add them */
-   gplugin_manager_prepend_paths_from_environment(manager,
-           pre_env_var_name);
-   gplugin_manager_append_paths_from_environment(manager,
-           post_env_var_name);
+   gplugin_manager_prepend_paths_from_environment(manager, pre_env_var_name);
+   gplugin_manager_append_paths_from_environment(manager, post_env_var_name);

    /* now get the manager's current paths */
    paths = gplugin_manager_get_paths(manager);
@@ -271,16 +268,16 @@
    path_pos = 0;
    for(l = paths; l != NULL; l = l->next) {
        path_pos += 1;
-       if (g_strcmp0(l->data, "/prefoo/") == 0) {
+       if(g_strcmp0(l->data, "/prefoo/") == 0) {
            g_assert_cmpint(2, >=, path_pos);
            found_paths += 1;
-       } else if (g_strcmp0(l->data, "/prebar/") == 0) {
+       } else if(g_strcmp0(l->data, "/prebar/") == 0) {
            g_assert_cmpint(2, >=, path_pos);
            found_paths += 1;
-       } else if (g_strcmp0(l->data, "/postfoo/") == 0) {
+       } else if(g_strcmp0(l->data, "/postfoo/") == 0) {
            g_assert_cmpint(2, <, path_pos);
            found_paths += 1;
-       } else if (g_strcmp0(l->data, "/postbar/") == 0) {
+       } else if(g_strcmp0(l->data, "/postbar/") == 0) {
            g_assert_cmpint(2, <, path_pos);
            found_paths += 1;
        }