[Patch] Fix hang in safe_sendfile on SmartOS

[Sebastian Wiedenroth]

> Am 07.09.2015 um 21:34 schrieb Timo Sirainen <tss at iki.fi>:
> 
> On 07/16/2015 06:03 PM, Sebastian Wiedenroth wrote:
>> Fix hang in safe_sendfile on SmartOS
>> 
>> The call to sendfile on SmartOS can fail with EOPNOTSUPP. This is a valid error
>> code and documented in the man page. This error code needs to be handled or
>> else dovecot will retry the sendfile call endlessly and hang.
> 
> Committed .. However, I think a more important bug is that it hangs.
> It's definitely not supposed to hang. Which process was it that was
> hanging How can I reproduce that? I can only get it to disconnect the
> IMAP client.


Thanks!

It was the managesieve process that was hanging.
To trigger it we used sieve-connect [1] like this:
	sieve-connect -u demo at example.com mailbox.example.com

To find the issue we used a dtrace script [2].
With an unpatched version it would show:

	CPU     ID                    FUNCTION:NAME
	  0   4623             safe_sendfile:return 7 4 fffffd7fffdff8b8 154 -> 77
	  0   4623             safe_sendfile:return 7 4 fffffd7fffdff8b8 9223372036854775807 -> 77
	  0   4623             safe_sendfile:return 7 4 fffffd7fffdff8b8 9223372036854775807 -> 77

and then just repeat the last line as it retried the call forever. This is where it hangs, spinning on the cpu.

After looking at the code I confirmed the issue with a dtrace one-liner that tracks the sendfilev syscall:

	dtrace -n 'syscall::sendfilev:return {printf("%d %x\n", arg0, errno)}'

This showed that the call returned with EOPNOTSUPP (0x7a):
 	  0   6155                 sendfilev:return -1 7a

The man page lists this as a valid error code and handling it the same way as EAFNOSUPPORT fixed the issue for us.
There are a few more error codes in the man page that currently are not handled by dovecot.
This might be something to look into in the future.

I hope this answer provides the details you’re looking for.

Best regards,
Sebastian 

[1] https://github.com/philpennock/sieve-connect
[2] https://gist.github.com/wiedi/4b4ebe5f92ac5b54951b