socket-proxyd: rework to support multiple sockets and splice()-based zero-copy network IO

This also drops --ignore-env, which can't really work anymore if we
allow multiple fds. Also adds support for pretty printing of peer
identities for debug purposes, and abstract namespace UNIX sockets. Also
ensures that we never take more connections than a certain limit.

src/shared/socket-util.c

@@ -568,6 +568,89 @@ bool socket_address_matches_fd(const SocketAddress *a, int fd) {
         return false;
 }
 
+int getpeername_pretty(int fd, char **ret) {
+
+        union {
+                struct sockaddr sa;
+                struct sockaddr_un un;
+                struct sockaddr_in in;
+                struct sockaddr_in6 in6;
+                struct sockaddr_storage storage;
+        } sa;
+
+        socklen_t salen;
+        char *p;
+
+        assert(fd >= 0);
+        assert(ret);
+
+        salen = sizeof(sa);
+        if (getpeername(fd, &sa.sa, &salen) < 0)
+                return -errno;
+
+        switch (sa.sa.sa_family) {
+
+        case AF_INET: {
+                uint32_t a;
+
+                a = ntohl(sa.in.sin_addr.s_addr);
+
+                if (asprintf(&p,
+                             "%u.%u.%u.%u:%u",
+                             a >> 24, (a >> 16) & 0xFF, (a >> 8) & 0xFF, a & 0xFF,
+                             ntohs(sa.in.sin_port)) < 0)
+                        return -ENOMEM;
+
+                break;
+        }
+
+        case AF_INET6: {
+                static const unsigned char ipv4_prefix[] = {
+                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF
+                };
+
+                if (memcmp(&sa.in6.sin6_addr, ipv4_prefix, sizeof(ipv4_prefix)) == 0) {
+                        const uint8_t *a = sa.in6.sin6_addr.s6_addr+12;
+
+                        if (asprintf(&p,
+                                     "%u.%u.%u.%u:%u",
+                                     a[0], a[1], a[2], a[3],
+                                     ntohs(sa.in6.sin6_port)) < 0)
+                                return -ENOMEM;
+                } else {
+                        char a[INET6_ADDRSTRLEN];
+
+                        if (asprintf(&p,
+                                     "%s:%u",
+                                     inet_ntop(AF_INET6, &sa.in6.sin6_addr, a, sizeof(a)),
+                                     ntohs(sa.in6.sin6_port)) < 0)
+                                return -ENOMEM;
+                }
+
+                break;
+        }
+
+        case AF_UNIX: {
+                struct ucred ucred;
+
+                salen = sizeof(ucred);
+                if (getsockopt(fd, SOL_SOCKET, SO_PEERCRED, &ucred, &salen) < 0)
+                        return -errno;
+
+                if (asprintf(&p, "PID %lu/UID %lu", (unsigned long) ucred.pid, (unsigned long) ucred.pid) < 0)
+                        return -ENOMEM;
+
+                break;
+        }
+
+        default:
+                return -ENOTSUP;
+        }
+
+        *ret = p;
+        return 0;
+}
+
 static const char* const netlink_family_table[] = {
         [NETLINK_ROUTE] = "route",
         [NETLINK_FIREWALL] = "firewall",

src/shared/socket-util.h

@@ -99,3 +99,5 @@ int netlink_family_to_string_alloc(int b, char **s);
 int netlink_family_from_string(const char *s);
 
 bool socket_ipv6_is_supported(void);
+
+int getpeername_pretty(int fd, char **ret);