pid1: drop unit caches only based on mtime

v2:
- do not watch mtime of transient and generated dirs

  We'd reload the map after every transient unit we created, which we don't
  need to do, since we create those units ourselves and know their fragment
  path.

src/analyze/analyze.c

@@ -1568,7 +1568,7 @@ static int do_unit_files(int argc, char *argv[], void *userdata) {
         if (r < 0)
                 return log_error_errno(r, "lookup_paths_init() failed: %m");
 
-        r = unit_file_build_name_map(&lp, &unit_ids, &unit_names, NULL);
+        r = unit_file_build_name_map(&lp, NULL, &unit_ids, &unit_names, NULL);
         if (r < 0)
                 return log_error_errno(r, "unit_file_build_name_map() failed: %m");
 

src/core/load-fragment.c

@@ -4611,6 +4611,15 @@ int unit_load_fragment(Unit *u) {
                 return 0;
         }
 
+        /* Possibly rebuild the fragment map to catch new units */
+        r = unit_file_build_name_map(&u->manager->lookup_paths,
+                                     &u->manager->unit_cache_mtime,
+                                     &u->manager->unit_id_map,
+                                     &u->manager->unit_name_map,
+                                     &u->manager->unit_path_cache);
+        if (r < 0)
+                log_error_errno(r, "Failed to rebuild name map: %m");
+
         r = unit_file_find_fragment(u->manager->unit_id_map,
                                     u->manager->unit_name_map,
                                     u->id,

src/core/manager.c

@@ -678,6 +678,7 @@ static void manager_free_unit_name_maps(Manager *m) {
         m->unit_id_map = hashmap_free(m->unit_id_map);
         m->unit_name_map = hashmap_free(m->unit_name_map);
         m->unit_path_cache = set_free_free(m->unit_path_cache);
+        m->unit_cache_mtime =  0;
 }
 
 static int manager_setup_run_queue(Manager *m) {
@@ -1626,11 +1627,6 @@ int manager_startup(Manager *m, FILE *serialization, FDSet *fds) {
         if (r < 0)
                 log_warning_errno(r, "Failed to reduce unit file paths, ignoring: %m");
 
-        manager_free_unit_name_maps(m);
-        r = unit_file_build_name_map(&m->lookup_paths, &m->unit_id_map, &m->unit_name_map, &m->unit_path_cache);
-        if (r < 0)
-                return log_error_errno(r, "Failed to build name map: %m");
-
         {
                 /* This block is (optionally) done with the reloading counter bumped */
                 _cleanup_(manager_reloading_stopp) Manager *reloading = NULL;
@@ -2847,10 +2843,6 @@ int manager_loop(Manager *m) {
         assert(m);
         assert(m->objective == MANAGER_OK); /* Ensure manager_startup() has been called */
 
-        /* Release the path and unit name caches */
-        manager_free_unit_name_maps(m);
-        // FIXME: once this happens, we cannot load any more units
-
         manager_check_finished(m);
 
         /* There might still be some zombies hanging around from before we were exec()'ed. Let's reap them. */
@@ -3526,10 +3518,8 @@ int manager_reload(Manager *m) {
         if (r < 0)
                 log_warning_errno(r, "Failed to reduce unit file paths, ignoring: %m");
 
+        /* We flushed out generated files, for which we don't watch mtime, so we should flush the old map. */
         manager_free_unit_name_maps(m);
-        r = unit_file_build_name_map(&m->lookup_paths, &m->unit_id_map, &m->unit_name_map, &m->unit_path_cache);
-        if (r < 0)
-                log_warning_errno(r, "Failed to build name map: %m");
 
         /* First, enumerate what we can from kernel and suchlike */
         manager_enumerate_perpetual(m);

src/core/manager.h

@@ -224,6 +224,7 @@ struct Manager {
         Hashmap *unit_id_map;
         Hashmap *unit_name_map;
         Set *unit_path_cache;
+        usec_t unit_cache_mtime;
 
         char **transient_environment;  /* The environment, as determined from config files, kernel cmdline and environment generators */
         char **client_environment;     /* Environment variables created by clients through the bus API */

src/shared/unit-file.c

@@ -152,8 +152,47 @@ static int unit_ids_map_get(
         return -ELOOP;
 }
 
+static bool lookup_paths_mtime_exclude(const LookupPaths *lp, const char *path) {
+        /* Paths that are under our exclusive control. Users shall not alter those directly. */
+
+        return streq_ptr(path, lp->generator) ||
+               streq_ptr(path, lp->generator_early) ||
+               streq_ptr(path, lp->generator_late) ||
+               streq_ptr(path, lp->transient) ||
+               streq_ptr(path, lp->persistent_control) ||
+               streq_ptr(path, lp->runtime_control);
+}
+
+static bool lookup_paths_mtime_good(const LookupPaths *lp, usec_t mtime) {
+        char **dir;
+
+        STRV_FOREACH(dir, (char**) lp->search_path) {
+                struct stat st;
+
+                if (lookup_paths_mtime_exclude(lp, *dir))
+                        continue;
+
+                /* Determine the latest lookup path modification time */
+                if (stat(*dir, &st) < 0) {
+                        if (errno == ENOENT)
+                                continue;
+
+                        log_debug_errno(errno, "Failed to stat %s, ignoring: %m", *dir);
+                        continue;
+                }
+
+                if (timespec_load(&st.st_mtim) > mtime) {
+                        log_debug_errno(errno, "Unit dir %s has changed, need to update cache.", *dir);
+                        return false;
+                }
+        }
+
+        return true;
+}
+
 int unit_file_build_name_map(
                 const LookupPaths *lp,
+                usec_t *cache_mtime,
                 Hashmap **ret_unit_ids_map,
                 Hashmap **ret_unit_names_map,
                 Set **ret_path_cache) {
@@ -171,6 +210,12 @@ int unit_file_build_name_map(
         _cleanup_set_free_free_ Set *paths = NULL;
         char **dir;
         int r;
+        usec_t mtime = 0;
+
+        /* Before doing anything, check if the mtime that was passed is still valid. If
+         * yes, do nothing. If *cache_time == 0, always build the cache. */
+        if (cache_mtime && *cache_mtime > 0 && lookup_paths_mtime_good(lp, *cache_mtime))
+                return 0;
 
         if (ret_path_cache) {
                 paths = set_new(&path_hash_ops);
@@ -181,6 +226,7 @@ int unit_file_build_name_map(
         STRV_FOREACH(dir, (char**) lp->search_path) {
                 struct dirent *de;
                 _cleanup_closedir_ DIR *d = NULL;
+                struct stat st;
 
                 d = opendir(*dir);
                 if (!d) {
@@ -189,6 +235,13 @@ int unit_file_build_name_map(
                         continue;
                 }
 
+                /* Determine the latest lookup path modification time */
+                if (fstat(dirfd(d), &st) < 0)
+                        return log_error_errno(errno, "Failed to fstat %s: %m", *dir);
+
+                if (!lookup_paths_mtime_exclude(lp, *dir))
+                        mtime = MAX(mtime, timespec_load(&st.st_mtim));
+
                 FOREACH_DIRENT(de, d, log_warning_errno(errno, "Failed to read \"%s\", ignoring: %m", *dir)) {
                         char *filename;
                         _cleanup_free_ char *_filename_free = NULL, *simplified = NULL;
@@ -316,12 +369,14 @@ int unit_file_build_name_map(
                                                  basename(dst), src);
         }
 
+        if (cache_mtime)
+                *cache_mtime = mtime;
         *ret_unit_ids_map = TAKE_PTR(ids);
         *ret_unit_names_map = TAKE_PTR(names);
         if (ret_path_cache)
                 *ret_path_cache = TAKE_PTR(paths);
 
-        return 0;
+        return 1;
 }
 
 int unit_file_find_fragment(

src/shared/unit-file.h

@@ -4,6 +4,7 @@
 #include <stdbool.h>
 
 #include "hashmap.h"
+#include "time-util.h"
 #include "unit-name.h"
 
 typedef enum UnitFileState UnitFileState;
@@ -42,6 +43,7 @@ int unit_validate_alias_symlink_and_warn(const char *filename, const char *targe
 
 int unit_file_build_name_map(
                 const LookupPaths *lp,
+                usec_t *ret_time,
                 Hashmap **ret_unit_ids_map,
                 Hashmap **ret_unit_names_map,
                 Set **ret_path_cache);

src/systemctl/systemctl.c

@@ -2593,7 +2593,7 @@ static int unit_find_paths(
                 _cleanup_set_free_free_ Set *names = NULL;
 
                 if (!cached_name_map) {
-                        r = unit_file_build_name_map(lp, &cached_id_map, &cached_name_map, NULL);
+                        r = unit_file_build_name_map(lp, NULL, &cached_id_map, &cached_name_map, NULL);
                         if (r < 0)
                                 return r;
                 }

src/test/test-unit-file.c

@@ -31,10 +31,12 @@ static void test_unit_file_build_name_map(void) {
         Iterator i;
         const char *k, *dst;
         char **v;
+        usec_t mtime = 0;
+        int r;
 
         assert_se(lookup_paths_init(&lp, UNIT_FILE_SYSTEM, 0, NULL) >= 0);
 
-        assert_se(unit_file_build_name_map(&lp, &unit_ids, &unit_names, NULL) == 0);
+        assert_se(unit_file_build_name_map(&lp, &mtime, &unit_ids, &unit_names, NULL) == 1);
 
         HASHMAP_FOREACH_KEY(dst, k, unit_ids, i)
                 log_info("ids: %s → %s", k, dst);
@@ -43,6 +45,14 @@ static void test_unit_file_build_name_map(void) {
                 _cleanup_free_ char *j = strv_join(v, ", ");
                 log_info("aliases: %s ← %s", k, j);
         }
+
+        char buf[FORMAT_TIMESTAMP_MAX];
+        log_debug("Last modification time: %s", format_timestamp(buf, sizeof buf, mtime));
+
+        r = unit_file_build_name_map(&lp, &mtime, &unit_ids, &unit_names, NULL);
+        assert_se(IN_SET(r, 0, 1));
+        if (r == 0)
+                log_debug("Cache rebuild skipped based on mtime.");
 }
 
 int main(int argc, char **argv) {