Implement masking and overriding of generators

Sometimes it is necessary to stop a generator from running. Either
because of a bug, or for testing, or some other reason. The only way
to do that would be to rename or chmod the generator binary, which is
inconvenient and does not survive upgrades. Allow masking and
overriding generators similarly to units and other configuration
files.

For the systemd instance, masking would be more common, rather than
overriding generators. For the user instances, it may also be useful
for users to have generators in $XDG_CONFIG_HOME to augment or
override system-wide generators.

Directories are searched according to the usual scheme (/usr/lib,
/usr/local/lib, /run, /etc), and files with the same name in higher
priority directories override files with the same name in lower
priority directories. Empty files and links to /dev/null mask a given
name.

https://bugs.freedesktop.org/show_bug.cgi?id=87230

1 files changed, 79 insertions, 49 deletions

diff --git a/src/shared/path-lookup.c b/src/shared/path-lookup.c
index 051f1a4835..1e5bb858e8 100644
--- a/src/shared/path-lookup.c
+++ b/src/shared/path-lookup.c
@@ -78,6 +78,33 @@ int user_runtime_dir(char **runtime_dir) {
         return 0;
 }
 
+static int user_data_home_dir(char **dir, const char *suffix) {
+        const char *e;
+        char *res;
+
+        /* We don't treat /etc/xdg/systemd here as the spec
+         * suggests because we assume that that is a link to
+         * /etc/systemd/ anyway. */
+
+        e = getenv("XDG_DATA_HOME");
+        if (e)
+                res = strappend(e, suffix);
+        else {
+                const char *home;
+
+                home = getenv("HOME");
+                if (home)
+                        res = strjoin(home, "/.local/share", suffix, NULL);
+                else
+                        return 0;
+        }
+        if (!res)
+                return -ENOMEM;
+
+        *dir = res;
+        return 0;
+}
+
 static char** user_dirs(
                 const char *generator,
                 const char *generator_early,
@@ -100,10 +127,12 @@ static char** user_dirs(
                 NULL
         };
 
-        const char *home, *e;
+        const char *e;
         _cleanup_free_ char *config_home = NULL, *runtime_dir = NULL, *data_home = NULL;
         _cleanup_strv_free_ char **config_dirs = NULL, **data_dirs = NULL;
-        char **r = NULL;
+        _cleanup_free_ char **res = NULL;
+        char **tmp;
+        int r;
 
         /* Implement the mechanisms defined in
          *
@@ -115,33 +144,21 @@ static char** user_dirs(
          */
 
         if (user_config_home(&config_home) < 0)
-                goto fail;
+                return NULL;
 
         if (user_runtime_dir(&runtime_dir) < 0)
-                goto fail;
-
-        home = getenv("HOME");
+                return NULL;
 
         e = getenv("XDG_CONFIG_DIRS");
         if (e) {
                 config_dirs = strv_split(e, ":");
                 if (!config_dirs)
-                        goto fail;
+                        return NULL;
         }
 
-        /* We don't treat /etc/xdg/systemd here as the spec
-         * suggests because we assume that that is a link to
-         * /etc/systemd/ anyway. */
-
-        e = getenv("XDG_DATA_HOME");
-        if (e) {
-                if (asprintf(&data_home, "%s/systemd/user", e) < 0)
-                        goto fail;
-
-        } else if (home) {
-                if (asprintf(&data_home, "%s/.local/share/systemd/user", home) < 0)
-                        goto fail;
-        }
+        r = user_data_home_dir(&data_home, "/systemd/user");
+        if (r < 0)
+                return NULL;
 
         e = getenv("XDG_DATA_DIRS");
         if (e)
@@ -151,58 +168,71 @@ static char** user_dirs(
                                      "/usr/share",
                                      NULL);
         if (!data_dirs)
-                goto fail;
+                return NULL;
 
         /* Now merge everything we found. */
         if (generator_early)
-                if (strv_extend(&r, generator_early) < 0)
-                        goto fail;
+                if (strv_extend(&res, generator_early) < 0)
+                        return NULL;
 
         if (config_home)
-                if (strv_extend(&r, config_home) < 0)
-                        goto fail;
+                if (strv_extend(&res, config_home) < 0)
+                        return NULL;
 
         if (!strv_isempty(config_dirs))
-                if (strv_extend_strv_concat(&r, config_dirs, "/systemd/user") < 0)
-                        goto fail;
+                if (strv_extend_strv_concat(&res, config_dirs, "/systemd/user") < 0)
+                        return NULL;
 
-        if (strv_extend_strv(&r, (char**) config_unit_paths) < 0)
-                goto fail;
+        if (strv_extend_strv(&res, (char**) config_unit_paths) < 0)
+                return NULL;
 
         if (runtime_dir)
-                if (strv_extend(&r, runtime_dir) < 0)
-                        goto fail;
+                if (strv_extend(&res, runtime_dir) < 0)
+                        return NULL;
 
-        if (strv_extend(&r, runtime_unit_path) < 0)
-                goto fail;
+        if (strv_extend(&res, runtime_unit_path) < 0)
+                return NULL;
 
         if (generator)
-                if (strv_extend(&r, generator) < 0)
-                        goto fail;
+                if (strv_extend(&res, generator) < 0)
+                        return NULL;
 
         if (data_home)
-                if (strv_extend(&r, data_home) < 0)
-                        goto fail;
+                if (strv_extend(&res, data_home) < 0)
+                        return NULL;
 
         if (!strv_isempty(data_dirs))
-                if (strv_extend_strv_concat(&r, data_dirs, "/systemd/user") < 0)
-                        goto fail;
+                if (strv_extend_strv_concat(&res, data_dirs, "/systemd/user") < 0)
+                        return NULL;
 
-        if (strv_extend_strv(&r, (char**) data_unit_paths) < 0)
-                goto fail;
+        if (strv_extend_strv(&res, (char**) data_unit_paths) < 0)
+                return NULL;
 
         if (generator_late)
-                if (strv_extend(&r, generator_late) < 0)
-                        goto fail;
+                if (strv_extend(&res, generator_late) < 0)
+                        return NULL;
 
-        if (!path_strv_make_absolute_cwd(r))
-                goto fail;
+        if (!path_strv_make_absolute_cwd(res))
+                return NULL;
 
-        return r;
+        tmp = res;
+        res = NULL;
+        return tmp;
+}
 
-fail:
-        strv_free(r);
-        return NULL;
+char **generator_paths(SystemdRunningAs running_as) {
+        if (running_as == SYSTEMD_USER)
+                return strv_new("/etc/systemd/user-generators",
+                                "/run/systemd/user-generators",
+                                "/usr/local/lib/systemd/user-generators",
+                                USER_GENERATOR_PATH,
+                                NULL);
+        else
+                return strv_new("/etc/systemd/system-generators",
+                                "/run/systemd/system-generators",
+                                "/usr/local/lib/systemd/system-generators",
+                                SYSTEM_GENERATOR_PATH,
+                                NULL);
 }
 
 int lookup_paths_init(