kms/impl-device: Add function to get a non-master fd

Add meta_kms_impl_device_open_non_privileged_fd() that returns a
non-master file descriptor for a MetaKmsImplDevice.

It'll be required to implement wp_drm_lease_device_v1_send_drm_fd() in a
future commit.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3746>

src/backends/native/meta-kms-impl-device.c

@@ -20,6 +20,7 @@
 #include "backends/native/meta-kms-impl-device.h"
 
 #include <errno.h>
+#include <fcntl.h>
 #include <glib/gstdio.h>
 #include <linux/dma-buf.h>
 #include <sys/ioctl.h>
@@ -1193,6 +1194,52 @@ meta_kms_impl_device_get_fd (MetaKmsImplDevice *impl_device)
   return meta_device_file_get_fd (priv->device_file);
 }
 
+/**
+ * meta_kms_impl_device_open_non_privileged_fd:
+ * @impl_device: a #MetaKmsImplDevice object
+ *
+ * Returns a non-master file descriptor for the given impl_device. The caller is
+ * responsable of closing the file descriptor.
+ *
+ * On error, returns a negative value.
+ */
+int
+meta_kms_impl_device_open_non_privileged_fd (MetaKmsImplDevice *impl_device)
+{
+  int fd;
+  const char *path;
+  MetaKmsImplDevicePrivate *priv =
+    meta_kms_impl_device_get_instance_private (impl_device);
+
+  if (!priv->device_file)
+    return -1;
+
+  path = meta_device_file_get_path (priv->device_file);
+
+  fd = open (path, O_RDWR | O_CLOEXEC);
+  if (fd < 0)
+    {
+      meta_topic (META_DEBUG_KMS,
+                  "Error getting non-master fd for device at '%s': %s",
+                  path,
+                  g_strerror (errno));
+      return -1;
+    }
+
+  if (drmIsMaster (fd))
+    {
+      if (drmDropMaster (fd) < 0)
+        {
+          meta_topic (META_DEBUG_KMS,
+                      "Error dropping master for device at '%s'",
+                      path);
+          return -1;
+        }
+    }
+
+  return fd;
+}
+
 /**
  * meta_kms_impl_device_get_signaled_sync_file:
  * @impl_device: a #MetaKmsImplDevice object

src/backends/native/meta-kms-impl-device.h

@@ -165,6 +165,8 @@ void meta_kms_impl_device_hold_fd (MetaKmsImplDevice *impl_device);
 
 void meta_kms_impl_device_unhold_fd (MetaKmsImplDevice *impl_device);
 
+int meta_kms_impl_device_open_non_privileged_fd (MetaKmsImplDevice *impl_device);
+
 int meta_kms_impl_device_get_signaled_sync_file (MetaKmsImplDevice *impl_device);
 
 MetaKmsResourceChanges meta_kms_impl_device_update_states (MetaKmsImplDevice *impl_device,