[HarfBuzz] glyph_func expected behavior for variationSelector?

[Koji Ishii]

Sorry if this was asked before, I'm new to this list (and to harfbuzz
too), please accept my apologies if so.

When a variation selector is in the source, such as:
  U+0030 U+FE0E
glyph_func receives U+FE0E as variationSelector argument.

What is the expected behavior for the glyph_func if it cannot find the
variant glyph?

>From my observation, if glyph_func returns false, harfbuzz returns 2
glyphs for the above string. So I'm guessing that when the specified
variant does not exist, glyph_func should fallback to the base glyph
and returns true if the base glyph exists.

Could someone confirm whether my understanding above is correct or not?

/koji