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

3 files changed, 33 insertions, 21 deletions

diff --git a/src/core/manager.c b/src/core/manager.c
index 4918091126..e53f22215d 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -93,6 +93,8 @@ static int manager_dispatch_time_change_fd(sd_event_source *source, int fd, uint
 static int manager_dispatch_idle_pipe_fd(sd_event_source *source, int fd, uint32_t revents, void *userdata);
 static int manager_dispatch_jobs_in_progress(sd_event_source *source, usec_t usec, void *userdata);
 static int manager_dispatch_run_queue(sd_event_source *source, void *userdata);
+static int manager_run_generators(Manager *m);
+static void manager_undo_generators(Manager *m);
 
 static int manager_watch_jobs_in_progress(Manager *m) {
         usec_t next;
@@ -1086,8 +1088,10 @@ int manager_startup(Manager *m, FILE *serialization, FDSet *fds) {
         assert(m);
 
         dual_timestamp_get(&m->generators_start_timestamp);
-        manager_run_generators(m);
+        r = manager_run_generators(m);
         dual_timestamp_get(&m->generators_finish_timestamp);
+        if (r < 0)
+                return r;
 
         r = lookup_paths_init(
                         &m->lookup_paths, m->running_as, true,
@@ -2517,7 +2521,9 @@ int manager_reload(Manager *m) {
         lookup_paths_free(&m->lookup_paths);
 
         /* Find new unit paths */
-        manager_run_generators(m);
+        q = manager_run_generators(m);
+        if (q < 0 && r >= 0)
+                r = q;
 
         q = lookup_paths_init(
                         &m->lookup_paths, m->running_as, true,
@@ -2525,19 +2531,19 @@ int manager_reload(Manager *m) {
                         m->generator_unit_path,
                         m->generator_unit_path_early,
                         m->generator_unit_path_late);
-        if (q < 0)
+        if (q < 0 && r >= 0)
                 r = q;
 
         manager_build_unit_path_cache(m);
 
         /* First, enumerate what we can from all config files */
         q = manager_enumerate(m);
-        if (q < 0)
+        if (q < 0 && r >= 0)
                 r = q;
 
         /* Second, deserialize our stored data */
         q = manager_deserialize(m, f, fds);
-        if (q < 0)
+        if (q < 0 && r >= 0)
                 r = q;
 
         fclose(f);
@@ -2545,12 +2551,12 @@ int manager_reload(Manager *m) {
 
         /* Re-register notify_fd as event source */
         q = manager_setup_notify(m);
-        if (q < 0)
+        if (q < 0 && r >= 0)
                 r = q;
 
         /* Third, fire things up! */
         q = manager_coldplug(m);
-        if (q < 0)
+        if (q < 0 && r >= 0)
                 r = q;
 
         assert(m->n_reloading > 0);
@@ -2775,27 +2781,33 @@ static void trim_generator_dir(Manager *m, char **generator) {
         return;
 }
 
-void manager_run_generators(Manager *m) {
-        const char *generator_path;
+static int manager_run_generators(Manager *m) {
+        _cleanup_free_ char **paths = NULL;
         const char *argv[5];
+        char **path;
         int r;
 
         assert(m);
 
         if (m->test_run)
-                return;
+                return 0;
 
-        generator_path = m->running_as == SYSTEMD_SYSTEM ? SYSTEM_GENERATOR_PATH : USER_GENERATOR_PATH;
+        paths = generator_paths(m->running_as);
+        if (!paths)
+                return log_oom();
 
         /* Optimize by skipping the whole process by not creating output directories
          * if no generators are found. */
-        if (access(generator_path, F_OK) != 0) {
+        STRV_FOREACH(path, paths) {
+                r = access(*path, F_OK);
+                if (r == 0)
+                        goto found;
                 if (errno != ENOENT)
-                        log_error_errno(errno, "Failed to open generator directory %s: %m",
-                                        generator_path);
-                return;
+                        log_warning_errno(errno, "Failed to open generator directory %s: %m", *path);
         }
+        return 0;
 
+ found:
         r = create_generator_dir(m, &m->generator_unit_path, "generator");
         if (r < 0)
                 goto finish;
@@ -2815,12 +2827,13 @@ void manager_run_generators(Manager *m) {
         argv[4] = NULL;
 
         RUN_WITH_UMASK(0022)
-                execute_directory(generator_path, DEFAULT_TIMEOUT_USEC, (char**) argv);
+                execute_directories((const char* const*) paths, DEFAULT_TIMEOUT_USEC, (char**) argv);
 
 finish:
         trim_generator_dir(m, &m->generator_unit_path);
         trim_generator_dir(m, &m->generator_unit_path_early);
         trim_generator_dir(m, &m->generator_unit_path_late);
+        return r;
 }
 
 static void remove_generator_dir(Manager *m, char **generator) {
@@ -2837,7 +2850,7 @@ static void remove_generator_dir(Manager *m, char **generator) {
         *generator = NULL;
 }
 
-void manager_undo_generators(Manager *m) {
+static void manager_undo_generators(Manager *m) {
         assert(m);
 
         remove_generator_dir(m, &m->generator_unit_path);
diff --git a/src/core/manager.h b/src/core/manager.h
index ab75f902e5..19fb0a901d 100644
--- a/src/core/manager.h
+++ b/src/core/manager.h
@@ -349,9 +349,6 @@ bool manager_unit_inactive_or_pending(Manager *m, const char *name);
 
 void manager_check_finished(Manager *m);
 
-void manager_run_generators(Manager *m);
-void manager_undo_generators(Manager *m);
-
 void manager_recheck_journal(Manager *m);
 
 void manager_set_show_status(Manager *m, ShowStatus mode);
diff --git a/src/core/shutdown.c b/src/core/shutdown.c
index 4cbdf12587..71f001ac13 100644
--- a/src/core/shutdown.c
+++ b/src/core/shutdown.c
@@ -49,6 +49,7 @@
 #include "cgroup-util.h"
 #include "def.h"
 #include "switch-root.h"
+#include "strv.h"
 
 #define FINALIZE_ATTEMPTS 50
 
@@ -159,6 +160,7 @@ int main(int argc, char *argv[]) {
         char *arguments[3];
         unsigned retries;
         int cmd, r;
+        static const char* const dirs[] = {SYSTEM_SHUTDOWN_PATH, NULL};
 
         log_parse_environment();
         r = parse_argv(argc, argv);
@@ -308,7 +310,7 @@ int main(int argc, char *argv[]) {
         arguments[0] = NULL;
         arguments[1] = arg_verb;
         arguments[2] = NULL;
-        execute_directory(SYSTEM_SHUTDOWN_PATH, DEFAULT_TIMEOUT_USEC, arguments);
+        execute_directories(dirs, DEFAULT_TIMEOUT_USEC, arguments);
 
         if (!in_container && !in_initrd() &&
             access("/run/initramfs/shutdown", X_OK) == 0) {