[Varnish] #1139: default_keep makes objects stay around too long

[Varnish]

#1139: default_keep makes objects stay around too long
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 Reporter:  martin    |       Owner:       
     Type:  defect    |      Status:  new  
 Priority:  normal    |   Milestone:       
Component:  varnishd  |     Version:  trunk
 Severity:  normal    |    Keywords:       
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 In some "forced expiry" situations (e.g. purge from vcl), the default_keep
 parameter will make the objects stay in cache longer than expected.

 One possible solution is to use EXP_Clr() always (the ban mechanism uses
 this), which resets also the entered time, causing the ttl to be very
 negative.

 Another solution is to not take these parameters into account at ttl
 calculation time, but set the keep and grace of an object to the values of
 these parameters at the time the object is created  (instead of -1).

 See attached varnishtest script.

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1139>
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