MySQL 5.7.29 includes the following fix:
Bug #30287668 INNODB: A LONG SEMAPHORE WAIT
mysql/mysql-server@5cdbb22b51cf2b35dbdf5666a251ffbec2f84dec

There is no test case. It seems that the problem could occur when
a spatial index is large and peculiar enough so that multiple R-tree
leaf pages will have the exactly same maximum bounding rectangle (MBR).

The commit message suggests that the hang can occur when R-tree
non-leaf pages are being merged, which should only be possible
during transaction rollback or the purge of transaction history,
when the R-tree index is at least 2 levels high and very many records
are being deleted. The message says that a comparison result that two
spatial index node pointer records are equal will cause an infinite loop
in rtr_page_copy_rec_list_end_no_locks(). Hence, we must include the
child page number in the comparison to be consistent with
mysql/mysql-server@2e11fe0e152e34d73579e1a9ec19aedc3f6010f6.

We fix this bug in a simpler way, involving fewer code changes.

cmp_rec_rec(): Renamed from cmp_rec_rec_with_match().
Assert that rec2 always resides in an index page.
Treat non-leaf spatial index pages specially.