Crash in mail module during SMTP setup

[Rob N ★]

On Wed, 31 Jul 2019, at 2:11 AM, Maxim Dounin wrote:
> > I think I see the problem - when using SMTP with SSL and resolver, 
> > read events might be enabled during address resolving, leading to 
> > duplicate ngx_mail_ssl_handshake_handler() calls if something 
> > arrives from the client, and duplicate session initialization - 
> > including starting another resolving.

That neatly explains why the problem became more noticeable as number of connections went up. With the load a little higher, DNS resolution could conceivable take a little longer, making it more likely that the bug would be triggered.

> > The following patch should resolve this:

I've been running the second patch you posted for ~22hrs with no crashes, compared to one every 10-20mins previously. So I think you got it! Thank you so much!

Cheers,
Rob N.
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