core: rework how we flush incoming traffic when a socket unit goes down

Previously, we'd simply close and reopen the socket file descriptors. This is
problematic however, as we won't transition through the SOCKET_CHOWN state
then, and thus the file ownership won't be correct for the sockets.

Rework the flushing logic, and actually read any queued data from the sockets
for flushing, and accept any queued messages and disconnect them.

src/basic/io-util.c

@@ -33,6 +33,11 @@ int flush_fd(int fd) {
                 .events = POLLIN,
         };
 
+        /* Read from the specified file descriptor, until POLLIN is not set anymore, throwing away everything
+         * read. Note that some file descriptors (notable IP sockets) will trigger POLLIN even when no data can be read
+         * (due to IP packet checksum mismatches), hence this function is only safe to be non-blocking if the fd used
+         * was set to non-blocking too. */
+
         for (;;) {
                 char buf[LINE_MAX];
                 ssize_t l;

src/basic/socket-util.c

@@ -23,6 +23,7 @@
 #include <net/if.h>
 #include <netdb.h>
 #include <netinet/ip.h>
+#include <poll.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -970,3 +971,42 @@ fallback:
 
         return (ssize_t) k;
 }
+
+int flush_accept(int fd) {
+
+        struct pollfd pollfd = {
+                .fd = fd,
+                .events = POLLIN,
+        };
+        int r;
+
+
+        /* Similar to flush_fd() but flushes all incoming connection by accepting them and immediately closing them. */
+
+        for (;;) {
+                int cfd;
+
+                r = poll(&pollfd, 1, 0);
+                if (r < 0) {
+                        if (errno == EINTR)
+                                continue;
+
+                        return -errno;
+
+                } else if (r == 0)
+                        return 0;
+
+                cfd = accept4(fd, NULL, NULL, SOCK_NONBLOCK|SOCK_CLOEXEC);
+                if (cfd < 0) {
+                        if (errno == EINTR)
+                                continue;
+
+                        if (errno == EAGAIN)
+                                return 0;
+
+                        return -errno;
+                }
+
+                close(cfd);
+        }
+}

src/basic/socket-util.h

@@ -135,5 +135,7 @@ int receive_one_fd(int transport_fd, int flags);
 
 ssize_t next_datagram_size_fd(int fd);
 
+int flush_accept(int fd);
+
 #define CMSG_FOREACH(cmsg, mh)                                          \
         for ((cmsg) = CMSG_FIRSTHDR(mh); (cmsg); (cmsg) = CMSG_NXTHDR((mh), (cmsg)))