core: add new logic for services to store file descriptors in PID 1

With this change it is possible to send file descriptors to PID 1, via
sd_pid_notify_with_fds() which PID 1 will store individually for each
service, and pass via the usual fd passing logic on next invocation.
This is useful for enable daemon reload schemes where daemons serialize
their state to /run, push their fds into PID 1 and terminate, restoring
their state on next start from the data in /run and passed in from PID
1.

The fds are kept by PID 1 as long as no POLLHUP or POLLERR is seen on
them, and the service they belong to are either not dead or failed, or
have a job queued.

1 files changed, 148 insertions, 18 deletions

diff --git a/src/core/service.c b/src/core/service.c
index bfbe959edb..78232ee71c 100644
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -242,6 +242,42 @@ static void service_reset_watchdog(Service *s) {
         service_start_watchdog(s);
 }
 
+static void service_fd_store_unlink(ServiceFDStore *fs) {
+
+        if (!fs)
+                return;
+
+        if (fs->service) {
+                assert(fs->service->n_fd_store > 0);
+                LIST_REMOVE(fd_store, fs->service->fd_store, fs);
+                fs->service->n_fd_store--;
+        }
+
+        if (fs->event_source) {
+                sd_event_source_set_enabled(fs->event_source, SD_EVENT_OFF);
+                sd_event_source_unref(fs->event_source);
+        }
+
+        safe_close(fs->fd);
+        free(fs);
+}
+
+static void service_release_resources(Unit *u) {
+        Service *s = SERVICE(u);
+
+        assert(s);
+
+        if (!s->fd_store)
+                return;
+
+        log_debug("Releasing all resources for %s", u->id);
+
+        while (s->fd_store)
+                service_fd_store_unlink(s->fd_store);
+
+        assert(s->n_fd_store == 0);
+}
+
 static void service_done(Unit *u) {
         Service *s = SERVICE(u);
 
@@ -286,6 +322,8 @@ static void service_done(Unit *u) {
         service_stop_watchdog(s);
 
         s->timer_event_source = sd_event_source_unref(s->timer_event_source);
+
+        service_release_resources(u);
 }
 
 static int service_arm_timer(Service *s, usec_t usec) {
@@ -549,6 +587,14 @@ static void service_dump(Unit *u, FILE *f, const char *prefix) {
         if (s->status_text)
                 fprintf(f, "%sStatus Text: %s\n",
                         prefix, s->status_text);
+
+        if (s->n_fd_store_max > 0) {
+                fprintf(f,
+                        "%sFile Descriptor Store Max: %u\n"
+                        "%sFile Descriptor Store Current: %u\n",
+                        prefix, s->n_fd_store_max,
+                        prefix, s->n_fd_store);
+        }
 }
 
 static int service_load_pid_file(Service *s, bool may_warn) {
@@ -806,10 +852,10 @@ static int service_coldplug(Unit *u) {
 }
 
 static int service_collect_fds(Service *s, int **fds, unsigned *n_fds) {
+        _cleanup_free_ int *rfds = NULL;
+        unsigned rn_fds = 0;
         Iterator i;
         int r;
-        int *rfds = NULL;
-        unsigned rn_fds = 0;
         Unit *u;
 
         assert(s);
@@ -831,10 +877,12 @@ static int service_collect_fds(Service *s, int **fds, unsigned *n_fds) {
 
                 r = socket_collect_fds(sock, &cfds, &cn_fds);
                 if (r < 0)
-                        goto fail;
+                        return r;
 
-                if (!cfds)
+                if (cn_fds <= 0) {
+                        free(cfds);
                         continue;
+                }
 
                 if (!rfds) {
                         rfds = cfds;
@@ -842,32 +890,39 @@ static int service_collect_fds(Service *s, int **fds, unsigned *n_fds) {
                 } else {
                         int *t;
 
-                        t = new(int, rn_fds+cn_fds);
+                        t = realloc(rfds, (rn_fds + cn_fds) * sizeof(int));
                         if (!t) {
                                 free(cfds);
-                                r = -ENOMEM;
-                                goto fail;
+                                return -ENOMEM;
                         }
 
-                        memcpy(t, rfds, rn_fds * sizeof(int));
-                        memcpy(t+rn_fds, cfds, cn_fds * sizeof(int));
-                        free(rfds);
+                        memcpy(t + rn_fds, cfds, cn_fds * sizeof(int));
+                        rfds = t;
+                        rn_fds += cn_fds;
+
                         free(cfds);
 
-                        rfds = t;
-                        rn_fds = rn_fds+cn_fds;
                 }
         }
 
+        if (s->n_fd_store > 0) {
+                ServiceFDStore *fs;
+                int *t;
+
+                t = realloc(rfds, (rn_fds + s->n_fd_store) * sizeof(int));
+                if (!t)
+                        return -ENOMEM;
+
+                rfds = t;
+                LIST_FOREACH(fd_store, fs, s->fd_store)
+                        rfds[rn_fds++] = fs->fd;
+        }
+
         *fds = rfds;
         *n_fds = rn_fds;
 
+        rfds = NULL;
         return 0;
-
-fail:
-        free(rfds);
-
-        return r;
 }
 
 static int service_spawn(
@@ -2543,7 +2598,75 @@ static int service_dispatch_watchdog(sd_event_source *source, usec_t usec, void
         return 0;
 }
 
-static void service_notify_message(Unit *u, pid_t pid, char **tags) {
+static int on_fd_store_io(sd_event_source *e, int fd, uint32_t revents, void *userdata) {
+        ServiceFDStore *fs = userdata;
+
+        assert(e);
+        assert(fs);
+
+        /* If we get either EPOLLHUP or EPOLLERR, it's time to remove this entry from the fd store */
+        service_fd_store_unlink(fs);
+        return 0;
+}
+
+static int service_add_fd_set(Service *s, FDSet *fds) {
+        int r;
+
+        assert(s);
+
+        if (fdset_size(fds) <= 0)
+                return 0;
+
+        while (s->n_fd_store < s->n_fd_store_max) {
+                _cleanup_close_ int fd = -1;
+                ServiceFDStore *fs;
+                bool same = false;
+
+                fd = fdset_steal_first(fds);
+                if (fd < 0)
+                        break;
+
+                LIST_FOREACH(fd_store, fs, s->fd_store) {
+                        r = same_fd(fs->fd, fd);
+                        if (r < 0)
+                                return log_unit_error_errno(UNIT(s)->id, r, "%s: Couldn't check if same fd: %m", UNIT(s)->id);
+                        if (r > 0) {
+                                same = true;
+                                break;
+                        }
+                }
+
+                if (same)
+                        continue;
+
+                fs = new0(ServiceFDStore, 1);
+                if (!fs)
+                        return log_oom();
+
+                fs->fd = fd;
+                fs->service = s;
+
+                r = sd_event_add_io(UNIT(s)->manager->event, &fs->event_source, fd, 0, on_fd_store_io, fs);
+                if (r < 0) {
+                        free(fs);
+                        return log_unit_error_errno(UNIT(s)->id, r, "%s: Failed to add even source: %m", UNIT(s)->id);
+                }
+
+                LIST_PREPEND(fd_store, s->fd_store, fs);
+                s->n_fd_store++;
+
+                fd = -1;
+
+                log_unit_debug(UNIT(s)->id, "%s: added fd to fd store.", UNIT(s)->id);
+        }
+
+        if (fdset_size(fds) > 0)
+                log_unit_warning(UNIT(s)->id, "%s: tried to store more fds than FDStoreMax=%u allows, closing remaining.", UNIT(s)->id, s->n_fd_store_max);
+
+        return 0;
+}
+
+static void service_notify_message(Unit *u, pid_t pid, char **tags, FDSet *fds) {
         Service *s = SERVICE(u);
         _cleanup_free_ char *cc = NULL;
         bool notify_dbus = false;
@@ -2675,6 +2798,12 @@ static void service_notify_message(Unit *u, pid_t pid, char **tags) {
                 service_reset_watchdog(s);
         }
 
+        /* Add the passed fds to the fd store */
+        if (strv_find(tags, "FDSTORE=1")) {
+                log_unit_debug(u->id, "%s: got FDSTORE=1", u->id);
+                service_add_fd_set(s, fds);
+        }
+
         /* Notify clients about changed status or main pid */
         if (notify_dbus)
                 unit_add_to_dbus_queue(u);
@@ -2917,6 +3046,7 @@ const UnitVTable service_vtable = {
         .init = service_init,
         .done = service_done,
         .load = service_load,
+        .release_resources = service_release_resources,
 
         .coldplug = service_coldplug,