metaCache generation is cut off by mysql setting

The metaCache field of segments is set to longtext. For the generation of the metaCache siblingdata the concat method is used. The length of concat is limited by the mysql setting group_concat_max_len (SHOW VARIABLES LIKE 'group_concat_max_len') which defaults to 1024.

This length is probably no problem, since this very long metacacheData was caused by TRANSLATE-1311.

This leads then for transunits with many mrks to a big data structure containing all the sibling data, which is to big for the concat then.

This results in an JS error "Uncaught SyntaxError: Unexpected end of JSON input" in the frontend then.