backend/drm: introduce wlr_drm_backend_get_non_master_fd

backend/drm/drm.c

@@ -3,6 +3,7 @@
 #include <drm_fourcc.h>
 #include <drm_mode.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <inttypes.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -1479,6 +1480,31 @@ void destroy_drm_connector(struct wlr_drm_connector *conn) {
 	free(conn);
 }
 
+int wlr_drm_backend_get_non_master_fd(struct wlr_backend *backend) {
+	assert(backend);
+
+	struct wlr_drm_backend *drm = get_drm_backend_from_backend(backend);
+	char *path = drmGetDeviceNameFromFd2(drm->fd);
+	if (!path) {
+		wlr_log(WLR_ERROR, "Failed to get device name from DRM fd");
+		return -1;
+	}
+
+	int fd = open(path, O_RDWR | O_CLOEXEC);
+	if (fd < 0) {
+		wlr_log_errno(WLR_ERROR, "Unable to clone DRM fd for client fd");
+		free(path);
+		return -1;
+	}
+
+	if (drmIsMaster(fd) && drmDropMaster(fd) < 0) {
+		wlr_log_errno(WLR_ERROR, "Failed to drop master");
+		return -1;
+	}
+
+	return fd;
+}
+
 /* TODO: make the function return a `wlr_drm_lease` to provide a destroy event
  * that can be fired when the kernel notifies us through uevent that the lease
  * has been destroyed

include/wlr/backend/drm.h

@@ -33,6 +33,13 @@ bool wlr_output_is_drm(struct wlr_output *output);
  */
 uint32_t wlr_drm_connector_get_id(struct wlr_output *output);
 
+/**
+ * Tries to open non-master DRM FD. The compositor must not call `drmSetMaster`
+ * on the returned FD.
+ * Returns a valid opened DRM FD, or -1 on error.
+ */
+int wlr_drm_backend_get_non_master_fd(struct wlr_backend *backend);
+
 /**
  * Leases a given output to the caller. The output must be from the associated
  * DRM backend.