client: Allow absolute paths in WAYLAND_DISPLAY

In order to support system compositor instances, it is necessary to
allow clients' wl_display_connect() to find the compositor's listening
socket somewhere outside of XDG_RUNTIME_DIR. For a full account, see
the discussion beginning here:

https://lists.freedesktop.org/archives/wayland-devel/2017-November/035664.html

This change adjusts the client-side connection logic so that, if
WAYLAND_DISPLAY is formatted as an absolute pathname, the socket
connection attempt is made to just $WAYLAND_DISPLAY rather than
usual user-private location $XDG_RUNTIME_DIR/$WAYLAND_DISPLAY.

This change is based on Davide Bettio's submission of the same concept
at:

https://lists.freedesktop.org/archives/wayland-devel/2015-August/023838.html.

v4 changes:

* Improved internal comments and some boundary-condition
  error checks in test case.
* Refer to compositor as "Wayland server" rather than "Wayland
  display" in wl_display_connect() doxygen comments.
* Remove redundant descriptions of parameter-interpretation
  mechanics from wl_display_connect() manpage. Reworked things
  to make it clear that 'name' and $WAYLAND_DISLAY are each
  capable of encoding absolute server socket paths.
* Remove callout to reference implementation behavior in protocol
  documented. In its place there is now a simple statement that
  implementations can optionally support absolute socket paths.

v3 changes:

* Added test case.
* Clarified documentation to note that 'name' parameter to wl_display_connect()
  can also be an absolute path.

v2 changes:

* Added backward incompatibility note to wl_display_connect() manpage.
* Rephased wl_display_connect() manpage changes to precisely match actual
  changed behavior.
* Added mention of new absolute path behavior in wl_display_connect()
  doxygen comments.
* Mentioned new absolute path interpretation of WAYLAND_DISPLAY in
  protocol documentation.

Signed-off-by: Matt Hoosier <matt.hoosier@gmail.com>
Acked-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

doc/man/wl_display_connect.xml

@@ -55,15 +55,39 @@
     <title>Description</title>
     <para><function>wl_display_connect</function> connects to a Wayland socket
           that was previously opened by a Wayland server. The server socket must
-          be placed in <envar>XDG_RUNTIME_DIR</envar> for this function to
-          find it. The <varname>name</varname> argument specifies the name of
+          be placed in <envar>XDG_RUNTIME_DIR</envar> when <envar>WAYLAND_DISPLAY</envar>
+	  (or <varname>name</varname>, see below) is a simple name, for this
+	  function to find it. The server socket is also allowed to exist at an
+	  arbitrary path; usage details follow. See below for compatibility issue
+	  details.</para>
+
+    <para>The <varname>name</varname> argument specifies the name of
           the socket or <constant>NULL</constant> to use the default (which is
           <constant>"wayland-0"</constant>). The environment variable
-          <envar>WAYLAND_DISPLAY</envar> replaces the default value. If
-          <envar>WAYLAND_SOCKET</envar> is set, this function behaves like
+          <envar>WAYLAND_DISPLAY</envar> replaces the default value.
+
+	  If <varname>name</varname> is an absolute path, then that path is used
+	  as the Wayland socket to which the connection is attempted. Note that
+	  in combination with the default-value behavior described above, this
+	  implies that setting <envar>WAYLAND_DISPLAY</envar> to an absolute
+	  path will implicitly cause <varname>name</varname> to take on that
+	  absolute path if <varname>name</varname> is <constant>NULL</constant>.
+
+          If <envar>WAYLAND_SOCKET</envar> is set, this function behaves like
           <function>wl_display_connect_to_fd</function> with the file-descriptor
           number taken from the environment variable.</para>
 
+    <para>Support for interpreting <envar>WAYLAND_DISPLAY</envar> as an
+          absolute path is a change in behavior compared to
+          <function>wl_display_connect</function>'s behavior in versions
+          1.14 and older of Wayland. It is no longer guaranteed in versions
+          1.15 and higher that the Wayland socket chosen is equivalent to
+          manually constructing a socket pathname by concatenating
+          <envar>XDG_RUNTIME_DIR</envar> and <envar>WAYLAND_DISPLAY</envar>.
+          Manual construction of the socket path must account for the
+          possibility that <envar>WAYLAND_DISPLAY</envar> contains an absolute
+          path.</para>
+
     <para><function>wl_display_connect_to_fd</function> connects to a Wayland
           socket with an explicit file-descriptor. The file-descriptor is passed
           as argument <varname>fd</varname>.</para>

doc/publican/sources/Protocol.xml

@@ -94,7 +94,10 @@
       The protocol is sent over a UNIX domain stream socket, where the endpoint
       usually is named <systemitem class="service">wayland-0</systemitem>
       (although it can be changed via <emphasis>WAYLAND_DISPLAY</emphasis>
-      in the environment).
+      in the environment). Beginning in Wayland 1.15, implementations can
+      optionally support server socket endpoints located at arbitrary
+      locations in the filesystem by setting <emphasis>WAYLAND_DISPLAY</emphasis>
+      to the absolute path at which the server endpoint listens.
     </para>
     <para>
       Every message is structured as 32-bit words; values are represented in the

src/wayland-client.c

@@ -857,9 +857,17 @@ connect_to_socket(const char *name)
 	socklen_t size;
 	const char *runtime_dir;
 	int name_size, fd;
+	bool path_is_absolute;
+
+	if (name == NULL)
+		name = getenv("WAYLAND_DISPLAY");
+	if (name == NULL)
+		name = "wayland-0";
+
+	path_is_absolute = name[0] == '/';
 
 	runtime_dir = getenv("XDG_RUNTIME_DIR");
-	if (!runtime_dir) {
+	if (!runtime_dir && !path_is_absolute) {
 		wl_log("error: XDG_RUNTIME_DIR not set in the environment.\n");
 		/* to prevent programs reporting
 		 * "failed to create display: Success" */
@@ -867,25 +875,32 @@ connect_to_socket(const char *name)
 		return -1;
 	}
 
-	if (name == NULL)
-		name = getenv("WAYLAND_DISPLAY");
-	if (name == NULL)
-		name = "wayland-0";
-
 	fd = wl_os_socket_cloexec(PF_LOCAL, SOCK_STREAM, 0);
 	if (fd < 0)
 		return -1;
 
 	memset(&addr, 0, sizeof addr);
 	addr.sun_family = AF_LOCAL;
-	name_size =
-		snprintf(addr.sun_path, sizeof addr.sun_path,
-			 "%s/%s", runtime_dir, name) + 1;
+	if (!path_is_absolute) {
+		name_size =
+			snprintf(addr.sun_path, sizeof addr.sun_path,
+			         "%s/%s", runtime_dir, name) + 1;
+	} else {
+		/* absolute path */
+		name_size =
+			snprintf(addr.sun_path, sizeof addr.sun_path,
+			         "%s", name) + 1;
+	}
 
 	assert(name_size > 0);
 	if (name_size > (int)sizeof addr.sun_path) {
-		wl_log("error: socket path \"%s/%s\" plus null terminator"
-		       " exceeds 108 bytes\n", runtime_dir, name);
+		if (!path_is_absolute) {
+			wl_log("error: socket path \"%s/%s\" plus null terminator"
+			       " exceeds %i bytes\n", runtime_dir, name, (int) sizeof(addr.sun_path));
+		} else {
+			wl_log("error: socket path \"%s\" plus null terminator"
+			       " exceeds %i bytes\n", name, (int) sizeof(addr.sun_path));
+		}
 		close(fd);
 		/* to prevent programs reporting
 		 * "failed to add socket: Success" */
@@ -994,6 +1009,16 @@ wl_display_connect_to_fd(int fd)
  * its value will be replaced with the WAYLAND_DISPLAY environment
  * variable if it is set, otherwise display "wayland-0" will be used.
  *
+ * If \c name is an absolute path, then that path is used as-is for
+ * the location of the socket at which the Wayland server is listening;
+ * no qualification inside XDG_RUNTIME_DIR is attempted.
+ *
+ * If \c name is \c NULL and the WAYLAND_DISPLAY environment variable
+ * is set to an absolute pathname, then that pathname is used as-is
+ * for the socket in the same manner as if \c name held an absolute
+ * path. Support for absolute paths in \c name and WAYLAND_DISPLAY
+ * is present since Wayland version 1.15.
+ *
  * \memberof wl_display
  */
 WL_EXPORT struct wl_display *

tests/socket-test.c

@@ -26,10 +26,14 @@
 #include <errno.h>
 #include <string.h>
 #include <stdio.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <sys/wait.h>
 #include <sys/un.h>
 #include <unistd.h>
 
 #include "wayland-client.h"
+#include "wayland-os.h"
 #include "wayland-server.h"
 #include "test-runner.h"
 
@@ -173,3 +177,108 @@ TEST(add_socket_auto)
 
 	wl_display_destroy(d);
 }
+
+struct client_create_listener {
+	struct wl_listener listener;
+	struct wl_display *display;
+};
+
+struct client_destroy_listener {
+	struct wl_listener listener;
+	struct wl_display *display;
+};
+
+static void
+client_destroy_notify(struct wl_listener *l, void *data)
+{
+	struct client_destroy_listener *listener =
+		wl_container_of(l, listener, listener);
+	wl_display_terminate(listener->display);
+	free(listener);
+}
+
+static void
+client_create_notify(struct wl_listener *l, void *data)
+{
+	struct wl_client *client = data;
+	struct client_create_listener *listener =
+		wl_container_of(l, listener, listener);
+	struct client_destroy_listener *destroy_listener = (struct client_destroy_listener *)malloc(sizeof *destroy_listener);
+	assert(destroy_listener != NULL);
+	destroy_listener->display = listener->display;
+	destroy_listener->listener.notify = client_destroy_notify;
+	wl_client_add_destroy_listener(client, &destroy_listener->listener);
+}
+
+TEST(absolute_socket_path)
+{
+	struct wl_display *display;
+	struct client_create_listener client_create_listener;
+	struct sockaddr_un addr;
+	int fd;
+	socklen_t size;
+	const char *xdg_runtime_dir;
+	size_t len;
+	int ret;
+	pid_t pid;
+
+	/* It's a little weird that this test about absolute socket paths
+	 * uses XDG_RUNTIME_DIR, but that's the only location guaranteed
+	 * by test-runner to be both writable and unique. This isn't
+	 * really a problem; we'll just take care that the leaf-level
+	 * filename used for the socket isn't anything that would
+	 * accidentally be generated by a default usage of wl_display_connect(). */
+	xdg_runtime_dir = require_xdg_runtime_dir();
+	memset(&addr, 0, sizeof addr);
+	len = snprintf(addr.sun_path, sizeof addr.sun_path,
+		       "%s/%s", xdg_runtime_dir, "wayland-absolute-0");
+	assert(len < sizeof addr.sun_path
+	       && "Bug in test. Path too long");
+
+	/* The path must not exist prior to binding. */
+	assert(access(addr.sun_path, F_OK) == -1);
+
+	size = offsetof (struct sockaddr_un, sun_path) + strlen(addr.sun_path);
+	addr.sun_family = AF_LOCAL;
+	fd = wl_os_socket_cloexec(PF_LOCAL, SOCK_STREAM, 0);
+	assert(fd >= 0 );
+	ret = bind(fd, (struct sockaddr *) &addr, size);
+	assert(ret >= 0);
+	ret = listen(fd, 128);
+	assert(ret >= 0);
+
+	/* Start server display. Be careful (by avoiding wl_display_add_socket_auto()
+	 * to offer only the absolutely qualified socket made above. */
+	display = wl_display_create();
+	assert(display != NULL);
+	client_create_listener.listener.notify = client_create_notify;
+	client_create_listener.display = display;
+	wl_display_add_client_created_listener(display, &client_create_listener.listener);
+	ret = wl_display_add_socket_fd(display, fd);
+	assert(ret == 0);
+
+	/* Execute client that connects to the absolutely qualified server socket path. */
+	pid = fork();
+	assert(pid != -1);
+
+	if (pid == 0) {
+		ret = setenv("WAYLAND_DISPLAY", addr.sun_path, 1);
+		assert(ret == 0);
+		struct wl_display *client_display = wl_display_connect(NULL);
+		assert(client_display != NULL);
+		ret = wl_display_roundtrip(client_display);
+		assert(ret != -1);
+		wl_display_disconnect(client_display);
+		exit(0);
+		assert(false);
+	}
+
+	wl_display_run(display);
+	ret = waitpid(pid, NULL, 0);
+	assert(ret == pid);
+
+	wl_display_destroy(display);
+
+	ret = unlink(addr.sun_path);
+	assert(ret == 0);
+}