[Varnish] #1799: Backend request coalescing fail

[Varnish]

#1799: Backend request coalescing fail
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 Reporter:  martin  |      Owner:
     Type:  defect  |     Status:  new
 Priority:  normal  |  Milestone:
Component:  build   |    Version:  unknown
 Severity:  normal  |   Keywords:
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 See attached test case.

 If we have a busy (flag OC_F_BUSY set) OC on the object head during
 HSH_Lookup, and there is an expired candidate available, the request won't
 be put on waitinglist but return with the expired OC. If this OC is
 rejected by the VCL (return(miss) from vcl_hit), the request is turned
 into a pass to the backend. This is logged SLT_VCL_Error: "vcl_hit{}
 returns fetch without busy object. Doing pass").

 If e.g. Varnish is configured to not use grace but with keep (for IMS
 against the backend), this happens just after the TTL of the object has
 expired. There is an expired object available, but not within grace, and
 all requests except the one that inserted the OC_F_BUSY will become passes
 to the backend until the OC_F_BUSY flag is removed. The attached test case
 covers this.

 Another scenario that will exhibit this is when you have a large grace
 period, but differentiate on how big grace you accept in vcl_hit based on
 the backend health status. If your object was not refreshed during the
 healthy backend shorter grace window, you'll get one backend request for
 each client request until the backend answers (slow backends suffer more).

 The expected outcome in my opinion is that the requests should have been
 placed on the waitinglist of the object head until the time the OC_F_BUSY
 flag is gone.

 I've been pondering how to fix this. But all attempts at reentering the
 waitinglist on the OH after HSH_Lookup has completed seems to open race
 conditions. The only sane way to fix this that I can see is to reintroduce
 the req.grace attribute, and move the graceability of the expired object
 back into the HSH_Lookup object head mutex critical region.

 Martin

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1799>
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