Make foot able to receive a socket from its parent

If the argument to --server is parsed as a number, consider it to be a
file descriptor, and use that as a socket.
This is necessary to be able to use socket activation with the server
mode of foot.

server.c

@@ -1,6 +1,7 @@
 #include "server.h"
 
 #include <string.h>
+#include <fcntl.h>
 #include <unistd.h>
 #include <errno.h>
 
@@ -19,6 +20,7 @@
 #include "client-protocol.h"
 #include "shm.h"
 #include "terminal.h"
+#include "util.h"
 #include "wayland.h"
 #include "xmalloc.h"
 
@@ -425,44 +427,107 @@ err:
     return ret;
 }
 
+static bool
+prepare_socket(int fd)
+{
+    int flags = fcntl(fd, F_GETFD);
+    if (flags < 0) {
+        LOG_ERRNO("failed to get file descriptors flag for passed socket");
+        return false;
+    }
+    if (fcntl(fd, F_SETFD, flags | FD_CLOEXEC) == -1) {
+        LOG_ERRNO("failed to set FD_CLOEXEC for passed socket");
+        return false;
+    }
+
+    flags = fcntl(fd, F_GETFL);
+    if (flags < 0) {
+        LOG_ERRNO("failed to get file status flags for passed socket");
+        return false;
+    }
+    if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) == -1) {
+        LOG_ERRNO("failed to set non-blocking mode on passed socket");
+        return false;
+    }
+
+    int const socket_options[] = { SO_DOMAIN, SO_ACCEPTCONN, SO_TYPE };
+    int const socket_options_values[] = { AF_UNIX, 1, SOCK_STREAM};
+    char const * const socket_options_names[] = { "SO_DOMAIN", "SO_ACCEPTCONN", "SO_TYPE" };
+
+    xassert(ALEN(socket_options) == ALEN(socket_options_values));
+    xassert(ALEN(socket_options) == ALEN(socket_options_names));
+
+    int socket_option = 0;
+    socklen_t len;
+    for (size_t i = 0; i < ALEN(socket_options) ; i++) {
+        len = sizeof(socket_option);
+        if (getsockopt(fd, SOL_SOCKET, socket_options[i], &socket_option, &len) == -1 ||
+                len != sizeof(socket_option)) {
+            LOG_ERRNO("failed to read socket option from passed file descriptor");
+            return false;
+        }
+        if (socket_option != socket_options_values[i]) {
+            LOG_ERR("wrong socket value for socket option '%s' on passed file descriptor",
+                    socket_options_names[i]);
+            return false;
+        }
+    }
+
+    return true;
+}
+
 struct server *
 server_init(const struct config *conf, struct fdm *fdm, struct reaper *reaper,
             struct wayland *wayl)
 {
-    int fd = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);
-    if (fd == -1) {
-        LOG_ERRNO("failed to create UNIX socket");
-        return NULL;
-    }
-
+    int fd;
     struct server *server = NULL;
     const char *sock_path = conf->server_socket_path;
+    char *end;
 
-    switch (try_connect(sock_path)) {
-    case CONNECT_FAIL:
-        break;
-
-    case CONNECT_SUCCESS:
-        LOG_ERR("%s is already accepting connections; is 'foot --server' already running", sock_path);
-        /* FALLTHROUGH */
-
-    case CONNECT_ERR:
-        goto err;
+    errno = 0;
+    fd = strtol(sock_path, &end, 10);
+    if (*end == '\0' && *sock_path != '\0')
+    {
+        if (!prepare_socket(fd))
+            goto err;
+        LOG_DBG("we've been started by socket activation, using passed socket");
+        sock_path = NULL;
     }
+    else {
+        LOG_DBG("no suitable pre-existing socket found, creating our own");
+        fd = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);
+        if (fd == -1) {
+            LOG_ERRNO("failed to create UNIX socket");
+            return NULL;
+        }
 
-    unlink(sock_path);
+        switch (try_connect(sock_path)) {
+        case CONNECT_FAIL:
+            break;
 
-    struct sockaddr_un addr = {.sun_family = AF_UNIX};
-    strncpy(addr.sun_path, sock_path, sizeof(addr.sun_path) - 1);
+        case CONNECT_SUCCESS:
+            LOG_ERR("%s is already accepting connections; is 'foot --server' already running", sock_path);
+            /* FALLTHROUGH */
 
-    if (bind(fd, (const struct sockaddr *)&addr, sizeof(addr)) < 0) {
-        LOG_ERRNO("%s: failed to bind", addr.sun_path);
-        goto err;
-    }
+        case CONNECT_ERR:
+            goto err;
+        }
 
-    if (listen(fd, 0) < 0) {
-        LOG_ERRNO("%s: failed to listen", addr.sun_path);
-        goto err;
+        unlink(sock_path);
+
+        struct sockaddr_un addr = {.sun_family = AF_UNIX};
+        strncpy(addr.sun_path, sock_path, sizeof(addr.sun_path) - 1);
+
+        if (bind(fd, (const struct sockaddr *)&addr, sizeof(addr)) < 0) {
+            LOG_ERRNO("%s: failed to bind", addr.sun_path);
+            goto err;
+        }
+
+        if (listen(fd, 0) < 0) {
+            LOG_ERRNO("%s: failed to listen", addr.sun_path);
+            goto err;
+        }
     }
 
     server = malloc(sizeof(*server));
@@ -487,7 +552,7 @@ server_init(const struct config *conf, struct fdm *fdm, struct reaper *reaper,
     if (!fdm_add(fdm, fd, EPOLLIN, &fdm_server, server))
         goto err;
 
-    LOG_INFO("accepting connections on %s", sock_path);
+    LOG_INFO("accepting connections on %s", sock_path != NULL ? sock_path : "socket provided through socket activation");
 
     return server;