Field synchronization is brittle because field values used as array keys

The entity_translation_sync function creates a $change_map to track changes given by $columns.

I've made a compound field (cardinality -1) with subfields a (text), b (int) and c (int).

The problem is that some times subfields b and/or c is either empty or contains the same value. The $change_map uses the subfield values as array keys which means that when saving three rows with identical b and/or c, only the last row is synchronized with the other translations.

Change map structure

$change_map[$column][$value]['old'] = $delta;

(I also have to use the patch in #2628094: Field synchronization only works with a single column and won't keep non synced values that set $changed to TRUE if a single field is different, which is the expected behavior)

My initial idea was to concatenate the subfield value with its delta value and use that as array keys:

$change_map[$column][$delta . $value]['old'] = $delta;

It works, as a proof of concept, but this particular solution will have a collision when
[delta = 1][b] = 11
and
[delta = 11][b] = 1
and even
[delta = 111][b] = '' if empty values are allowed.

I think however we can ensure uniqueness across any combination of delta and value if we add a non-integer character between $delta and $value like this:

$change_map[$column][$delta . '-' . $value]['old'] = $delta;

Since this operation is transient and doesn't touch the data itself, I think the patch is very safe to use.