[tor-bugs] #15937 [Tor]: Clients fail on the 7th rapid SOCKS request to the same HS

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#15937: Clients fail on the 7th rapid SOCKS request to the same HS
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 Reporter:  teor      |          Owner:
     Type:  defect    |         Status:  new
 Priority:  normal    |      Milestone:  Tor: 0.2.8.x-final
Component:  Tor       |        Version:
 Keywords:  SponsorR  |  Actual Points:
Parent ID:            |         Points:
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 If a tor client gets 7 SOCKS connection requests to a hidden service with
 an uncached descriptor in rapid succession, it launches a HSDir request
 for each connection. It doesn't wait for the circuit to any of the HSDirs
 to be built, or wait for a response from any of the HSDirs.

 After 6 connections, it fails on the 7th attempted fetch, because it has
 tried to (rapidly) fetch the descriptor 6 times, and hasn't got it yet. It
 then fails each of the outstanding SOCKS requests 1-7.

 It can do this all in the same second, before any circuits have a chance
 to be built.

 The client then backs off after the descriptor fetch failure, the circuit
 build attempts succeed, and then the 8th and subsequent requests succeed.

 This behaviour is unlikely to be triggered by HTML-based hidden services.
 There is typiclly 1 connection with an uncached descriptor to load the
 initial HTML page, then further connections with a cached descriptor load
 any page resources.

 Observed in 0.2.7.0-dev.

 tor logs attached for 3 HSDirs, 1 client, and 1 HS in a chutney network
 hs-100-clients (branch with these changes in chutney upcoming in #15936)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15937>
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