device-pool: Add way to open files read-only

Will be used by libinput's tablet device led device files.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1828>

src/backends/native/meta-device-pool.c

@@ -247,9 +247,17 @@ meta_device_pool_open (MetaDevicePool       *pool,
     }
   else
     {
+      int open_flags;
+
+      if (flags & META_DEVICE_FILE_FLAG_READ_ONLY)
+        open_flags = O_RDONLY;
+      else
+        open_flags = O_RDWR;
+      open_flags |= O_CLOEXEC;
+
       do
         {
-          fd = open (path, O_RDWR | O_CLOEXEC);
+          fd = open (path, open_flags);
         }
       while (fd == -1 && errno == EINTR);
 

src/backends/native/meta-device-pool.h

@@ -27,6 +27,7 @@ typedef enum _MetaDeviceFileFlags
 {
   META_DEVICE_FILE_FLAG_NONE = 0,
   META_DEVICE_FILE_FLAG_TAKE_CONTROL = 1 << 0,
+  META_DEVICE_FILE_FLAG_READ_ONLY = 1 << 1,
 } MetaDeviceFileFlags;
 
 typedef struct _MetaDeviceFile MetaDeviceFile;