1. Scan-on-write is a reasonable scanner policy. This is not unique feature
of Avira. Many other scanner have the same one. You want to wait for another
bugs of this sort with another AV? Want we or not but antivirus scanner can
delay or interrupt any file operation at any time, at least on Windows.

Apropos, Avira manual clearly stated that with this option on files after
write will be unavailable for a short period of time.

2. File is not locked forever. It's locked only for a short period of time.

3. I'm using Avira and mercurial together for about an year. This is first
time I'd faced this bug. Of course this is the first time I'm applying patch
with 6000 files. But several times there were patches with almost 3000 files
and all was ok. So absence of issues with another AV doesn't mean that they
will not appear.

4. I'm surprised why you do endless open-write-close on merge/state instead
of write-flush? May be this will be a little faster at the same time?

5. Correct me if I'm not right but I think that the main sense is to make
mercurial as reliable as possible. This workaround is not large price for AV
problems absence.