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

4 files changed, 158 insertions, 96 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(
diff --git a/src/shared/path-lookup.h b/src/shared/path-lookup.h
index 655e4549ca..2ec888da81 100644
--- a/src/shared/path-lookup.h
+++ b/src/shared/path-lookup.h
@@ -42,6 +42,8 @@ typedef enum SystemdRunningAs {
 int user_config_home(char **config_home);
 int user_runtime_dir(char **runtime_dir);
 
+char **generator_paths(SystemdRunningAs running_as);
+
 int lookup_paths_init(LookupPaths *p,
                       SystemdRunningAs running_as,
                       bool personal,
diff --git a/src/shared/util.c b/src/shared/util.c
index 06bd1b9f04..6520e511f0 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -4037,10 +4037,10 @@ bool dirent_is_file_with_suffix(const struct dirent *de, const char *suffix) {
         return endswith(de->d_name, suffix);
 }
 
-static int do_execute(const char *directory, usec_t timeout, char *argv[]) {
+static int do_execute(char **directories, usec_t timeout, char *argv[]) {
         _cleanup_hashmap_free_free_ Hashmap *pids = NULL;
-        _cleanup_closedir_ DIR *d;
-        struct dirent *de;
+        _cleanup_set_free_free_ Set *seen = NULL;
+        char **directory;
 
         /* We fork this all off from a child process so that we can
          * somewhat cleanly make use of SIGALRM to set a time limit */
@@ -4050,57 +4050,80 @@ static int do_execute(const char *directory, usec_t timeout, char *argv[]) {
 
         assert_se(prctl(PR_SET_PDEATHSIG, SIGTERM) == 0);
 
-        d = opendir(directory);
-        if (!d) {
-                if (errno == ENOENT)
-                        return 0;
-
-                return log_error_errno(errno, "Failed to open directory %s: %m", directory);
-        }
-
         pids = hashmap_new(NULL);
         if (!pids)
                 return log_oom();
 
-        FOREACH_DIRENT(de, d, break) {
-                _cleanup_free_ char *path = NULL;
-                pid_t pid;
-                int r;
+        seen = set_new(&string_hash_ops);
+        if (!seen)
+                return log_oom();
 
-                if (!dirent_is_file(de))
-                        continue;
+        STRV_FOREACH(directory, directories) {
+                _cleanup_closedir_ DIR *d;
+                struct dirent *de;
 
-                path = strjoin(directory, "/", de->d_name, NULL);
-                if (!path)
-                        return log_oom();
+                d = opendir(*directory);
+                if (!d) {
+                        if (errno == ENOENT)
+                                continue;
 
-                pid = fork();
-                if (pid < 0) {
-                        log_error_errno(errno, "Failed to fork: %m");
-                        continue;
-                } else if (pid == 0) {
-                        char *_argv[2];
+                        return log_error_errno(errno, "Failed to open directory %s: %m", *directory);
+                }
 
-                        assert_se(prctl(PR_SET_PDEATHSIG, SIGTERM) == 0);
+                FOREACH_DIRENT(de, d, break) {
+                        _cleanup_free_ char *path = NULL;
+                        pid_t pid;
+                        int r;
 
-                        if (!argv) {
-                                _argv[0] = path;
-                                _argv[1] = NULL;
-                                argv = _argv;
+                        if (!dirent_is_file(de))
+                                continue;
+
+                        if (set_contains(seen, de->d_name)) {
+                                log_debug("%1$s/%2$s skipped (%2$s was already seen).", *directory, de->d_name);
+                                continue;
+                        }
+
+                        r = set_put_strdup(seen, de->d_name);
+                        if (r < 0)
+                                return log_oom();
+
+                        path = strjoin(*directory, "/", de->d_name, NULL);
+                        if (!path)
+                                return log_oom();
+
+                        if (null_or_empty_path(path)) {
+                                log_debug("%s is empty (a mask).", path);
+                                continue;
                         } else
-                                argv[0] = path;
+                                log_debug("%s will be executed.", path);
 
-                        execv(path, argv);
-                        return log_error_errno(errno, "Failed to execute %s: %m", path);
-                }
+                        pid = fork();
+                        if (pid < 0) {
+                                log_error_errno(errno, "Failed to fork: %m");
+                                continue;
+                        } else if (pid == 0) {
+                                char *_argv[2];
 
-                log_debug("Spawned %s as " PID_FMT ".", path, pid);
+                                assert_se(prctl(PR_SET_PDEATHSIG, SIGTERM) == 0);
 
-                r = hashmap_put(pids, UINT_TO_PTR(pid), path);
-                if (r < 0)
-                        return log_oom();
+                                if (!argv) {
+                                        _argv[0] = path;
+                                        _argv[1] = NULL;
+                                        argv = _argv;
+                                } else
+                                        argv[0] = path;
+
+                                execv(path, argv);
+                                return log_error_errno(errno, "Failed to execute %s: %m", path);
+                        }
+
+                        log_debug("Spawned %s as " PID_FMT ".", path, pid);
 
-                path = NULL;
+                        r = hashmap_put(pids, UINT_TO_PTR(pid), path);
+                        if (r < 0)
+                                return log_oom();
+                        path = NULL;
+                }
         }
 
         /* Abort execution of this process after the timout. We simply
@@ -4126,14 +4149,21 @@ static int do_execute(const char *directory, usec_t timeout, char *argv[]) {
         return 0;
 }
 
-void execute_directory(const char *directory, usec_t timeout, char *argv[]) {
+void execute_directories(const char* const* directories, usec_t timeout, char *argv[]) {
         pid_t executor_pid;
         int r;
+        char *name;
+        char **dirs = (char**) directories;
+
+        assert(!strv_isempty(dirs));
 
-        assert(directory);
+        name = basename(dirs[0]);
+        assert(!isempty(name));
 
-        /* Executes all binaries in the directory in parallel and waits
-         * for them to finish. Optionally a timeout is applied. */
+        /* Executes all binaries in the directories in parallel and waits
+         * for them to finish. Optionally a timeout is applied. If a file
+         * with the same name exists in more than one directory, the
+         * earliest one wins. */
 
         executor_pid = fork();
         if (executor_pid < 0) {
@@ -4141,11 +4171,11 @@ void execute_directory(const char *directory, usec_t timeout, char *argv[]) {
                 return;
 
         } else if (executor_pid == 0) {
-                r = do_execute(directory, timeout, argv);
+                r = do_execute(dirs, timeout, argv);
                 _exit(r < 0 ? EXIT_FAILURE : EXIT_SUCCESS);
         }
 
-        wait_for_terminate_and_warn(directory, executor_pid, true);
+        wait_for_terminate_and_warn(name, executor_pid, true);
 }
 
 int kill_and_sigcont(pid_t pid, int sig) {
diff --git a/src/shared/util.h b/src/shared/util.h
index 3d31cb3f04..141a3feab6 100644
--- a/src/shared/util.h
+++ b/src/shared/util.h
@@ -534,7 +534,7 @@ bool tty_is_console(const char *tty) _pure_;
 int vtnr_from_tty(const char *tty);
 const char *default_term_for_tty(const char *tty);
 
-void execute_directory(const char *directory, usec_t timeout, char *argv[]);
+void execute_directories(const char* const* directories, usec_t timeout, char *argv[]);
 
 int kill_and_sigcont(pid_t pid, int sig);