[PATCH 2/2] proto, server: Add internal server error message. (v2)

[Christopher James Halse Rogers]

Many languages such as C++ or Rust have an unwinding error-reporting
mechanism. Code in these languages can (and must!) wrap request handling
callbacks in unwind guards to avoid undefined behaviour.

As a consequence such code will detect internal server errors, but have
no way to communicate such failures to the client.

This adds a WL_DISPLAY_ERROR_IMPLEMENTATION error to wl_display so that
such code can notify (and disconnect) clients which hit internal bugs.
While servers can currently abuse other wl_display errors for the same
effect, adding an explicit error code allows clients to tell the
difference between errors which are their fault and errors which are the
server's fault. This is particularly interesting for automated bug
reporting.

v2: Rename error from "internal" to "implementation", in sympathy with
    X11's BadImplementation error.
    Add more justification in the commit message.

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers at canonical.com>
Acked-by: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
---
 protocol/wayland.xml      |  2 ++
 src/wayland-client.c      |  3 +++
 src/wayland-server-core.h |  4 ++++
 src/wayland-server.c      | 24 +++++++++++++++++++++++
 tests/display-test.c      | 40 +++++++++++++++++++++++++++++++++++++++
 5 files changed, 73 insertions(+)

diff --git a/protocol/wayland.xml b/protocol/wayland.xml
index 141038b..754b789 100644
--- a/protocol/wayland.xml
+++ b/protocol/wayland.xml
@@ -94,6 +94,8 @@
 	     summary="method doesn't exist on the specified interface"/>
       <entry name="no_memory" value="2"
 	     summary="server is out of memory"/>
+      <entry name="implementation" value="3"
+	     summary="implementation error in compositor"/>
     </enum>
 
     <event name="delete_id">
diff --git a/src/wayland-client.c b/src/wayland-client.c
index 0ccfc66..b0805f1 100644
--- a/src/wayland-client.c
+++ b/src/wayland-client.c
@@ -185,6 +185,9 @@ display_protocol_error(struct wl_display *display, uint32_t code,
 		case WL_DISPLAY_ERROR_NO_MEMORY:
 			err = ENOMEM;
 			break;
+		case WL_DISPLAY_ERROR_IMPLEMENTATION:
+			err = EPROTO;
+			break;
 		default:
 			err = EFAULT;
 		}
diff --git a/src/wayland-server-core.h b/src/wayland-server-core.h
index 2e725d9..3e0272b 100644
--- a/src/wayland-server-core.h
+++ b/src/wayland-server-core.h
@@ -324,6 +324,10 @@ wl_client_get_object(struct wl_client *client, uint32_t id);
 void
 wl_client_post_no_memory(struct wl_client *client);
 
+void
+wl_client_post_implementation_error(struct wl_client *client,
+                                    const char* msg, ...) WL_PRINTF(2,3);
+
 void
 wl_client_add_resource_created_listener(struct wl_client *client,
                                         struct wl_listener *listener);
diff --git a/src/wayland-server.c b/src/wayland-server.c
index c0ad229..19f6a76 100644
--- a/src/wayland-server.c
+++ b/src/wayland-server.c
@@ -650,6 +650,30 @@ wl_client_post_no_memory(struct wl_client *client)
 			       WL_DISPLAY_ERROR_NO_MEMORY, "no memory");
 }
 
+/** Report an internal server error
+ *
+ * \param client The client object
+ * \param msg A printf-style format string
+ * \param ... Format string arguments
+ *
+ * Report an unspecified internal implementation error and disconnect
+ * the client.
+ *
+ * \memberof wl_client
+ */
+WL_EXPORT void
+wl_client_post_implementation_error(struct wl_client *client,
+				    char const *msg, ...)
+{
+	va_list ap;
+
+	va_start(ap, msg);
+	wl_resource_post_error_vargs(client->display_resource,
+				     WL_DISPLAY_ERROR_IMPLEMENTATION,
+				     msg, ap);
+	va_end(ap);
+}
+
 WL_EXPORT void
 wl_resource_post_no_memory(struct wl_resource *resource)
 {
diff --git a/tests/display-test.c b/tests/display-test.c
index 9b49a0e..6d98cc7 100644
--- a/tests/display-test.c
+++ b/tests/display-test.c
@@ -419,6 +419,46 @@ TEST(post_nomem_tst)
 	display_destroy(d);
 }
 
+static void
+post_implementation_error_main(void)
+{
+	struct client *c = client_connect();
+	struct wl_seat *seat = client_get_seat(c);
+	uint32_t object_id, protocol_error;
+	const struct wl_interface *interface;
+
+	assert(stop_display(c, 1) == -1);
+	int err = wl_display_get_error(c->wl_display);
+	fprintf(stderr, "Err is %i\n", err);
+	assert(err == EPROTO);
+	protocol_error = wl_display_get_protocol_error(c->wl_display,
+						       &interface,
+						       &object_id);
+	assert(protocol_error == WL_DISPLAY_ERROR_IMPLEMENTATION);
+	assert(interface == &wl_display_interface);
+
+	wl_proxy_destroy((struct wl_proxy *) seat);
+	client_disconnect_nocheck(c);
+}
+
+TEST(post_internal_error_tst)
+{
+	struct display *d = display_create();
+	struct client_info *cl;
+
+	wl_global_create(d->wl_display, &wl_seat_interface,
+			 1, d, bind_seat);
+
+	cl = client_create_noarg(d, post_implementation_error_main);
+	display_run(d);
+
+	wl_client_post_implementation_error(cl->wl_client, "Error %i", 20);
+
+	display_resume(d);
+
+	display_destroy(d);
+}
+
 static void
 register_reading(struct wl_display *display)
 {
-- 
2.19.1