!!! PLEASE DO NOT CHANGE OBJECTIVE OF THIS BUG AND DON'T CLOSE IT UNTIL DESCRIBED PROBLEM IS SOLVED !!!
Apparently InnoDB doesn't copy blob data during online ALTER TABLE. In a meanwhile it calls innobase_rec_reset(), which may reallocate memory for blobs.
It wasn't a problem before column compression was introduced, since in most (if not all) cases memory reallocation was not needed. With compressed columns reallocation is always there.
To reproduce this issue grep for
MDEV-13359 in source code for relevant markers. Re-enable online ALTER TABLE and run main.column_compression test.
Objective of this bug is to enable online ALTER TABLE for compressed columns.