svoj, could you please review the final fix? It's a set of commits, the complete diff is

git diff bb-5.5-serg^{/DEBUG_SYNC}..bb-5.5-serg^{/my_delete_with_symlink}

and you can look at individual commits with

gitk bb-5.5-serg^{/DEBUG_SYNC}..bb-5.5-serg^{/my_delete_with_symlink}

The main one is "MDEV-11902 mi_open race condition" and the four topmost ones (about my_delete).

Looks good.

There's a minor typo in "all simlinks are resolved by realpath()".

I'm not a big fan of CREATE_NOSYMLINK_FUNCTION. I'd better put NOSYMLINK_FUNCTION_BODY directly into my_delete/etc. Consider it less than a minor wish.

Table creation still looks badly vulnerable, FWICS: one may create files in data directory, one may truncate files in data directory, one may delete files in data directory. But we agreed to keep it separately.

About CREATE_NOSYMLINK_FUNCTION — yes, originally I had two functions directly in my_open.c and my_delete.c. They were mostly identical, just one line difference. Then I needed to fix a bug there, had to fix it in both places. Then again, and I thought it's rather fragile approach (and I'm generally not a big fan of duplicated code). So I extracted the common part. If there's a better solution — I'll happily use it.

I was mostly unhappy about function definition hidden behind a macro. Otherwise I'm more or less fine with this big NOSYMLINK_FUNCTION_BODY.