Handle backlog of UNIX socket too

The cheaper subsystem specifies that "backlog is only available on Linux
and only on TCP sockets (not UNIX domain sockets)."
This commit specifically implement this: UNIX domain socket support for
backlog on Linux, using Netlink to call the kernel and get the queue status.

core/master.c

@@ -259,6 +259,13 @@ static void get_tcp_info(struct uwsgi_socket *uwsgi_sock) {
 
 #ifdef UNBIT
 #define SIOBKLGQ 0x8908
+#else
+
+#include <linux/netlink.h>
+#include <linux/rtnetlink.h>
+#include <linux/unix_diag.h>
+#include <linux/sock_diag.h>
+
 #endif
 
 #ifdef SIOBKLGQ
@@ -272,7 +279,141 @@ static void get_linux_unbit_SIOBKLGQ(struct uwsgi_socket *uwsgi_sock) {
 		uwsgi_sock->max_queue = (uint64_t) uwsgi.listen_queue;
 	}
 }
+#else
+
+static int get_socket_inode_from_fd(int fd) {
+	int inode = -1;
+	char source_link_path[32];
+	char link_target[32];
+	sprintf(source_link_path, "/proc/self/fd/%i", fd);
+	if (readlink(source_link_path, link_target, 32) > 0)
+		sscanf(link_target, "socket:[%i]", &inode);
+	return inode;
+}
+
+static void send_netlink_query_for_inode(int fd, int target_ino) {
+	struct sockaddr_nl nladdr = {.nl_family = AF_NETLINK};
+	struct {
+		struct nlmsghdr nlh;
+		struct unix_diag_req udr;
+	} req = {
+		.nlh = {
+			.nlmsg_len = sizeof(req),
+			.nlmsg_type = SOCK_DIAG_BY_FAMILY,
+			.nlmsg_flags = NLM_F_REQUEST
+		},
+		.udr = {
+			.sdiag_family = AF_UNIX,
+			.udiag_show = UDIAG_SHOW_RQLEN,
+			.udiag_ino = target_ino,
+			.udiag_cookie = {-1, -1}
+		}
+	};
+	struct iovec iov = {.iov_base = &req, .iov_len = sizeof(req)};
+	struct msghdr msg = {
+		.msg_name = &nladdr,
+		.msg_namelen = sizeof(nladdr),
+		.msg_iov = &iov,
+		.msg_iovlen = 1
+	};
+
+	for (;;) {
+		if (sendmsg(fd, &msg, 0) < 0) {
+			if (errno == EINTR)
+				continue;
+
+			perror("sendmsg");
+			return;
+		}
+
+		return;
+	}
+}
+
+static int receive_netlink_answers(int fd, uint64_t *current_queue, uint64_t *max_queue) {
+	long buf[8192 / sizeof(long)];
+	struct sockaddr_nl nladdr;
+	struct iovec iov = {.iov_base = buf, .iov_len = sizeof(buf)};
+	int flags = 0;
+
+	for (;;) {
+		struct msghdr msg = {
+			.msg_name = &nladdr,
+			.msg_namelen = sizeof(nladdr),
+			.msg_iov = &iov,
+			.msg_iovlen = 1
+		};
+
+		ssize_t ret = recvmsg(fd, &msg, flags);
+
+		if (ret < 0) {
+			if (errno == EINTR)
+				continue;
+
+			perror("recvmsg");
+			return -1;
+		}
+		if (ret == 0)
+			return 0;
+
+		if (nladdr.nl_family != AF_NETLINK) {
+			fputs("!AF_NETLINK\n", stderr);
+			return -1;
+		}
+
+		const struct nlmsghdr *h = (struct nlmsghdr *)buf;
+
+		if (!NLMSG_OK(h, ret)) {
+			fputs("!NLMSG_OK\n", stderr);
+			return -1;
+		}
+
+		for (; NLMSG_OK(h, ret); h = NLMSG_NEXT(h, ret)) {
+			if (h->nlmsg_type == NLMSG_DONE)
+				return 0;
+			if (h->nlmsg_type == NLMSG_ERROR) {
+				const struct nlmsgerr *err = NLMSG_DATA(h);
+				if (h->nlmsg_len < NLMSG_LENGTH(sizeof(*err))) {
+					fputs("NLMSG_ERROR\n", stderr);
+				} else {
+					errno = -err->error;
+					perror("NLMSG_ERROR");
+				}
+				return -1;
+			}
+			if (h->nlmsg_type != SOCK_DIAG_BY_FAMILY) {
+				fprintf(stderr, "unexpected nlmsg_type %u\n", (unsigned)h->nlmsg_type);
+		  		return -1;
+			}
+
+			// Now extract queue len from results
+			const struct unix_diag_msg *diag = NLMSG_DATA(h);
+			unsigned int rta_len = h->nlmsg_len - NLMSG_LENGTH(sizeof(*diag));
+
+			for (struct rtattr *attr = (struct rtattr *)(diag + 1); RTA_OK(attr, rta_len); attr = RTA_NEXT(attr, rta_len)) {
+				switch (attr->rta_type) {
+				case UNIX_DIAG_RQLEN:
+					*current_queue = ((struct unix_diag_rqlen *) RTA_DATA(attr))->udiag_rqueue;
+					*max_queue = ((struct unix_diag_rqlen *) RTA_DATA(attr))->udiag_wqueue;
+					break;
+				}
+			}
+		}
+		return 0;
+	}
+}
+
+static void get_linux_unix_socket_queue(struct uwsgi_socket *uwsgi_sock) {
+	int fd = uwsgi_sock->fd;
+	int inode = get_socket_inode_from_fd(fd);
+	int diag_socket = socket(AF_NETLINK, SOCK_RAW | SOCK_CLOEXEC, NETLINK_SOCK_DIAG);
+	send_netlink_query_for_inode(diag_socket, inode);
+	receive_netlink_answers(diag_socket, &uwsgi_sock->queue, &uwsgi_sock->max_queue);
+	close(diag_socket);
+}
+
 #endif
+
 #endif
 
 static void master_check_listen_queue() {
@@ -282,15 +423,16 @@ static void master_check_listen_queue() {
 	while(uwsgi_sock) {
 		if (uwsgi_sock->family == AF_INET) {
 			get_tcp_info(uwsgi_sock);
-                }
+		}
+		else if (uwsgi_sock->family == AF_UNIX) {
 #ifdef __linux__
 #ifdef SIOBKLGQ
-                else if (uwsgi_sock->family == AF_UNIX) {
-                	get_linux_unbit_SIOBKLGQ(uwsgi_sock);
-                }
+			get_linux_unbit_SIOBKLGQ(uwsgi_sock);
+#else
+			get_linux_unix_socket_queue(uwsgi_sock);
 #endif
 #endif
-
+		}
 		if (uwsgi_sock->queue > backlog) {
 			backlog = uwsgi_sock->queue;
 		}