[Varnish] #1156: first_byte_timeout is "doubled" on the client side when keep-alive is used

[Varnish]

#1156: first_byte_timeout is "doubled" on the client side when keep-alive is used
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 Reporter:  tnt    |        Type:  defect  
   Status:  new    |    Priority:  normal  
Milestone:         |   Component:  varnishd
  Version:  trunk  |    Severity:  normal  
 Keywords:         |  
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 I'm experiencing a problem with first_byte_timeout in keepalive
 connections. For the second request, if the backend doesn't respond within
 the timeout, the backend connection will be closed (normal), but the 503
 response will not be sent to the client immediately ... it's gonna take an
 additional first_byte_timeout sec for 503 error response to be sent to the
 client ...

 So if you have a 5s timeout, after 5s the backend connection is closed,
 but it's gonna take 10s for the 503 response to be sent out ...

 Confirmed on 3.0.3 rc1 and master

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1156>
Varnish <https://varnish-cache.org/>
The Varnish HTTP Accelerator