GMappedFile: return an error when trying to map a device

Previously, we were returning an empty buffer for all filenames
where fstat() gives a size of 0. But this is only appropriate
for regular files.

Also improve the documentation around this issue. Based on a
patch by Ryan Lortie.

Conflicts:

	glib/tests/mappedfile.c

https://bugzilla.gnome.org/show_bug.cgi?id=659212

docs/reference/glib/tmpl/fileutils.sgml

@@ -80,8 +80,8 @@ differences in when a system will report a given error, etc.
      couldn't find the device.  This can mean that the device file was
      installed incorrectly, or that the physical device is missing or
      not correctly attached to the computer.
-@G_FILE_ERROR_NODEV: This file is of a type that doesn't support
-     mapping.
+@G_FILE_ERROR_NODEV: The  underlying  file system of the specified file
+     does not support memory mapping.
 @G_FILE_ERROR_ROFS: The directory containing the new link can't be
           modified because it's on a read-only file system.
 @G_FILE_ERROR_TXTBSY: Text file busy.

glib/gmappedfile.c

@@ -112,14 +112,19 @@ g_mapped_file_destroy (GMappedFile *file)
  * Maps a file into memory. On UNIX, this is using the mmap() function.
  *
  * If @writable is %TRUE, the mapped buffer may be modified, otherwise
- * it is an error to modify the mapped buffer. Modifications to the buffer 
- * are not visible to other processes mapping the same file, and are not 
+ * it is an error to modify the mapped buffer. Modifications to the buffer
+ * are not visible to other processes mapping the same file, and are not
  * written back to the file.
  *
  * Note that modifications of the underlying file might affect the contents
- * of the #GMappedFile. Therefore, mapping should only be used if the file 
+ * of the #GMappedFile. Therefore, mapping should only be used if the file
  * will not be modified, or if all modifications of the file are done
- * atomically (e.g. using g_file_set_contents()). 
+ * atomically (e.g. using g_file_set_contents()).
+ *
+ * If @filename is the name of an empty, regular file, the function
+ * will successfully return an empty #GMappedFile. In other cases of
+ * size 0 (e.g. device files such as /dev/null), @error will be set
+ * to the #GFileError value #G_FILE_ERROR_INVAL.
  *
  * Return value: a newly allocated #GMappedFile which must be unref'd
  *    with g_mapped_file_unref(), or %NULL if the mapping failed.

glib/tests/mappedfile.c

@@ -36,6 +36,18 @@ test_empty (void)
   g_mapped_file_free (file);
 }
 
+static void
+test_device (void)
+{
+  GError *error = NULL;
+  GMappedFile *file;
+
+  file = g_mapped_file_new ("/dev/null", FALSE, &error);
+  g_assert_error (error, G_FILE_ERROR, G_FILE_ERROR_INVAL);
+  g_assert (file == NULL);
+  g_error_free (error);
+}
+
 static void
 test_nonexisting (void)
 {