core: fix priority ordering in notify-handling

Currently, we dispatch NOTIFY messages in a tight loop. Regardless how
much data is incoming, we always dispatch everything that is queued.
This, however, completely breaks priority event-handling of sd-event.
When dispatching one NOTIFY event, another completely different event
might fire, or might be queued by the NOTIFY handling. However, this
event will not get dispatched until all other further NOTIFY messages are
handled. Those might even arrive _after_ the other event fired, and as
such completely break priority ordering of sd-event (which several code
paths rely on).

Break this by never dispatching multiple messages. Just return after each
message that was read and let sd-event handle everything else.

(The patch looks scarier that it is. It basically just drops the for(;;)
 loop and re-indents the loop-content.)

1 files changed, 77 insertions, 79 deletions

diff --git a/src/core/manager.c b/src/core/manager.c
index 04111091e7..d161e6c57b 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -1509,9 +1509,33 @@ static void manager_invoke_notify_message(Manager *m, Unit *u, pid_t pid, char *
 }
 
 static int manager_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t revents, void *userdata) {
+        _cleanup_fdset_free_ FDSet *fds = NULL;
         Manager *m = userdata;
+
+        char buf[NOTIFY_BUFFER_MAX+1];
+        struct iovec iovec = {
+                .iov_base = buf,
+                .iov_len = sizeof(buf)-1,
+        };
+        union {
+                struct cmsghdr cmsghdr;
+                uint8_t buf[CMSG_SPACE(sizeof(struct ucred)) +
+                            CMSG_SPACE(sizeof(int) * NOTIFY_FD_MAX)];
+        } control = {};
+        struct msghdr msghdr = {
+                .msg_iov = &iovec,
+                .msg_iovlen = 1,
+                .msg_control = &control,
+                .msg_controllen = sizeof(control),
+        };
+
+        struct cmsghdr *cmsg;
+        struct ucred *ucred = NULL;
+        bool found = false;
+        Unit *u1, *u2, *u3;
+        int r, *fd_array = NULL;
+        unsigned n_fds = 0;
         ssize_t n;
-        int r;
 
         assert(m);
         assert(m->notify_fd == fd);
@@ -1521,101 +1545,75 @@ static int manager_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t
                 return 0;
         }
 
-        for (;;) {
-                _cleanup_fdset_free_ FDSet *fds = NULL;
-                char buf[NOTIFY_BUFFER_MAX+1];
-                struct iovec iovec = {
-                        .iov_base = buf,
-                        .iov_len = sizeof(buf)-1,
-                };
-                union {
-                        struct cmsghdr cmsghdr;
-                        uint8_t buf[CMSG_SPACE(sizeof(struct ucred)) +
-                                    CMSG_SPACE(sizeof(int) * NOTIFY_FD_MAX)];
-                } control = {};
-                struct msghdr msghdr = {
-                        .msg_iov = &iovec,
-                        .msg_iovlen = 1,
-                        .msg_control = &control,
-                        .msg_controllen = sizeof(control),
-                };
-                struct cmsghdr *cmsg;
-                struct ucred *ucred = NULL;
-                bool found = false;
-                Unit *u1, *u2, *u3;
-                int *fd_array = NULL;
-                unsigned n_fds = 0;
-
-                n = recvmsg(m->notify_fd, &msghdr, MSG_DONTWAIT|MSG_CMSG_CLOEXEC);
-                if (n < 0) {
-                        if (errno == EAGAIN || errno == EINTR)
-                                break;
+        n = recvmsg(m->notify_fd, &msghdr, MSG_DONTWAIT|MSG_CMSG_CLOEXEC);
+        if (n < 0) {
+                if (errno == EAGAIN || errno == EINTR)
+                        return 0;
 
-                        return -errno;
-                }
+                return -errno;
+        }
 
-                CMSG_FOREACH(cmsg, &msghdr) {
-                        if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS) {
+        CMSG_FOREACH(cmsg, &msghdr) {
+                if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS) {
 
-                                fd_array = (int*) CMSG_DATA(cmsg);
-                                n_fds = (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int);
+                        fd_array = (int*) CMSG_DATA(cmsg);
+                        n_fds = (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int);
 
-                        } else if (cmsg->cmsg_level == SOL_SOCKET &&
-                                   cmsg->cmsg_type == SCM_CREDENTIALS &&
-                                   cmsg->cmsg_len == CMSG_LEN(sizeof(struct ucred))) {
+                } else if (cmsg->cmsg_level == SOL_SOCKET &&
+                           cmsg->cmsg_type == SCM_CREDENTIALS &&
+                           cmsg->cmsg_len == CMSG_LEN(sizeof(struct ucred))) {
 
-                                ucred = (struct ucred*) CMSG_DATA(cmsg);
-                        }
+                        ucred = (struct ucred*) CMSG_DATA(cmsg);
                 }
+        }
 
-                if (n_fds > 0) {
-                        assert(fd_array);
+        if (n_fds > 0) {
+                assert(fd_array);
 
-                        r = fdset_new_array(&fds, fd_array, n_fds);
-                        if (r < 0) {
-                                close_many(fd_array, n_fds);
-                                return log_oom();
-                        }
+                r = fdset_new_array(&fds, fd_array, n_fds);
+                if (r < 0) {
+                        close_many(fd_array, n_fds);
+                        return log_oom();
                 }
+        }
 
-                if (!ucred || ucred->pid <= 0) {
-                        log_warning("Received notify message without valid credentials. Ignoring.");
-                        continue;
-                }
+        if (!ucred || ucred->pid <= 0) {
+                log_warning("Received notify message without valid credentials. Ignoring.");
+                return 0;
+        }
 
-                if ((size_t) n >= sizeof(buf)) {
-                        log_warning("Received notify message exceeded maximum size. Ignoring.");
-                        continue;
-                }
+        if ((size_t) n >= sizeof(buf)) {
+                log_warning("Received notify message exceeded maximum size. Ignoring.");
+                return 0;
+        }
 
-                buf[n] = 0;
+        buf[n] = 0;
 
-                /* Notify every unit that might be interested, but try
-                 * to avoid notifying the same one multiple times. */
-                u1 = manager_get_unit_by_pid_cgroup(m, ucred->pid);
-                if (u1) {
-                        manager_invoke_notify_message(m, u1, ucred->pid, buf, n, fds);
-                        found = true;
-                }
+        /* Notify every unit that might be interested, but try
+         * to avoid notifying the same one multiple times. */
+        u1 = manager_get_unit_by_pid_cgroup(m, ucred->pid);
+        if (u1) {
+                manager_invoke_notify_message(m, u1, ucred->pid, buf, n, fds);
+                found = true;
+        }
 
-                u2 = hashmap_get(m->watch_pids1, PID_TO_PTR(ucred->pid));
-                if (u2 && u2 != u1) {
-                        manager_invoke_notify_message(m, u2, ucred->pid, buf, n, fds);
-                        found = true;
-                }
+        u2 = hashmap_get(m->watch_pids1, PID_TO_PTR(ucred->pid));
+        if (u2 && u2 != u1) {
+                manager_invoke_notify_message(m, u2, ucred->pid, buf, n, fds);
+                found = true;
+        }
 
-                u3 = hashmap_get(m->watch_pids2, PID_TO_PTR(ucred->pid));
-                if (u3 && u3 != u2 && u3 != u1) {
-                        manager_invoke_notify_message(m, u3, ucred->pid, buf, n, fds);
-                        found = true;
-                }
+        u3 = hashmap_get(m->watch_pids2, PID_TO_PTR(ucred->pid));
+        if (u3 && u3 != u2 && u3 != u1) {
+                manager_invoke_notify_message(m, u3, ucred->pid, buf, n, fds);
+                found = true;
+        }
 
-                if (!found)
-                        log_warning("Cannot find unit for notify message of PID "PID_FMT".", ucred->pid);
+        if (!found)
+                log_warning("Cannot find unit for notify message of PID "PID_FMT".", ucred->pid);
 
-                if (fdset_size(fds) > 0)
-                        log_warning("Got auxiliary fds with notification message, closing all.");
-        }
+        if (fdset_size(fds) > 0)
+                log_warning("Got auxiliary fds with notification message, closing all.");
 
         return 0;
 }