[Varnish] #1772: first_byte_timeout is ignored when re-using a backend connection (HTTP Keep Alive)

[Varnish]

#1772: first_byte_timeout is ignored when re-using a backend connection (HTTP Keep
Alive)
-----------------------+--------------------
 Reporter:  tnt        |       Type:  defect
   Status:  new        |   Priority:  normal
Milestone:             |  Component:  build
  Version:  4.1.0-TP1  |   Severity:  normal
 Keywords:             |
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 On a given backend TCP connection, only the first request will have
 first_byte_timeout properly applied, all subsequent requests will wait
 indefinitely long (AFAICT).

 What I traced so far is that inside vbe_dir_gethdrs, it blocks on VBT_Wait
 on the second requests because state is STOLEN. And whatever event loop
 should be processing the STOLEN -> USED transition will not be triggered
 until a byte is received on the fd, which defeats the first_byte_timeout.

 (tested on 4.1.0-tp1 only)

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1772>
Varnish <https://varnish-cache.org/>
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