resolved: instead of closing DNS UDP transaction fds right-away, add them to a socket "graveyard"

The "socket graveyard" shall contain sockets we have sent a question out
of, but not received a reply. If we'd close thus sockets immediately
when we are not interested anymore, we'd trigger ICMP port unreachable
messages once we after all *do* get a reply. Let's avoid that, by
leaving the fds open for a bit longer, until a timeout is reached or a
reply datagram received.

Fixes: #17421

1 files changed, 133 insertions, 0 deletions

diff --git a/src/resolve/resolved-socket-graveyard.c b/src/resolve/resolved-socket-graveyard.c
new file mode 100644
index 0000000000..067cb666d4
--- /dev/null
+++ b/src/resolve/resolved-socket-graveyard.c
@@ -0,0 +1,133 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include "resolved-socket-graveyard.h"
+
+#define SOCKET_GRAVEYARD_USEC (5 * USEC_PER_SEC)
+#define SOCKET_GRAVEYARD_MAX 100
+
+/* This implements a socket "graveyard" for UDP sockets. If a socket fd is added to the graveyard it is kept
+ * open for a couple of more seconds, expecting one reply. Once the reply is received the fd is closed
+ * immediately, or if none is received it is closed after the timeout. Why all this? So that if we contact a
+ * DNS server, and it doesn't reply instantly, and we lose interest in the response and thus close the fd, we
+ * don't end up sending back an ICMP error once the server responds but we aren't listening anymore. (See
+ * https://github.com/systemd/systemd/issues/17421 for further information.)
+ *
+ * Note that we don't allocate any timer event source to clear up the graveyard once the socket's timeout is
+ * reached. Instead we operate lazily: we close old entries when adding a new fd to the graveyard, or
+ * whenever any code runs manager_socket_graveyard_process() — which the DNS transaction code does right
+ * before allocating a new UDP socket. */
+
+static SocketGraveyard* socket_graveyard_free(SocketGraveyard *g) {
+        if (!g)
+                return NULL;
+
+        if (g->manager) {
+                assert(g->manager->n_socket_graveyard > 0);
+                g->manager->n_socket_graveyard--;
+
+                if (g->manager->socket_graveyard_oldest == g)
+                        g->manager->socket_graveyard_oldest = g->graveyard_prev;
+
+                LIST_REMOVE(graveyard, g->manager->socket_graveyard, g);
+
+                assert((g->manager->n_socket_graveyard > 0) == !!g->manager->socket_graveyard);
+                assert((g->manager->n_socket_graveyard > 0) == !!g->manager->socket_graveyard_oldest);
+        }
+
+        if (g->io_event_source) {
+                log_debug("Closing graveyard socket fd %i", sd_event_source_get_io_fd(g->io_event_source));
+                sd_event_source_unref(g->io_event_source);
+        }
+
+        return mfree(g);
+}
+
+DEFINE_TRIVIAL_CLEANUP_FUNC(SocketGraveyard*, socket_graveyard_free);
+
+void manager_socket_graveyard_process(Manager *m) {
+        usec_t n = USEC_INFINITY;
+
+        assert(m);
+
+        while (m->socket_graveyard_oldest) {
+                SocketGraveyard *g = m->socket_graveyard_oldest;
+
+                if (n == USEC_INFINITY)
+                        assert_se(sd_event_now(m->event, clock_boottime_or_monotonic(), &n) >= 0);
+
+                if (g->deadline > n)
+                        break;
+
+                socket_graveyard_free(g);
+        }
+}
+
+void manager_socket_graveyard_clear(Manager *m) {
+        assert(m);
+
+        while (m->socket_graveyard)
+                socket_graveyard_free(m->socket_graveyard);
+}
+
+static int on_io_event(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
+        SocketGraveyard *g = userdata;
+
+        assert(g);
+
+        /* An IO event happened on the graveyard fd. We don't actually care which event that is, and we don't
+         * read any incoming packet off the socket. We just close the fd, that's enough to not trigger the
+         * ICMP unreachable port event */
+
+        socket_graveyard_free(g);
+        return 0;
+}
+
+static void manager_socket_graveyard_make_room(Manager *m) {
+        assert(m);
+
+        while (m->n_socket_graveyard >= SOCKET_GRAVEYARD_MAX)
+                socket_graveyard_free(m->socket_graveyard_oldest);
+}
+
+int manager_add_socket_to_graveyard(Manager *m, int fd) {
+        _cleanup_(socket_graveyard_freep) SocketGraveyard *g = NULL;
+        int r;
+
+        assert(m);
+        assert(fd >= 0);
+
+        manager_socket_graveyard_process(m);
+        manager_socket_graveyard_make_room(m);
+
+        g = new(SocketGraveyard, 1);
+        if (!g)
+                return log_oom();
+
+        *g = (SocketGraveyard) {
+                .manager = m,
+        };
+
+        LIST_PREPEND(graveyard, m->socket_graveyard, g);
+        if (!m->socket_graveyard_oldest)
+                m->socket_graveyard_oldest = g;
+
+        m->n_socket_graveyard++;
+
+        assert_se(sd_event_now(m->event, clock_boottime_or_monotonic(), &g->deadline) >= 0);
+        g->deadline += SOCKET_GRAVEYARD_USEC;
+
+        r = sd_event_add_io(m->event, &g->io_event_source, fd, EPOLLIN, on_io_event, g);
+        if (r < 0)
+                return log_error_errno(r, "Failed to create graveyard IO source: %m");
+
+        r = sd_event_source_set_io_fd_own(g->io_event_source, true);
+        if (r < 0)
+                return log_error_errno(r, "Failed to enable graveyard IO source fd ownership: %m");
+
+        (void) sd_event_source_set_description(g->io_event_source, "graveyard");
+
+        log_debug("Added socket %i to graveyard", fd);
+
+        TAKE_PTR(g);
+        return 0;
+}