[tor-bugs] #29617 [- Select a component]: OOM manger wipes entire DNS cache

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#29617: OOM manger wipes entire DNS cache
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 Reporter:  pulls   |          Owner:  (none)
     Type:  defect  |         Status:  new
 Priority:  Medium  |      Component:  - Select a component
  Version:          |       Severity:  Normal
 Keywords:          |  Actual Points:
Parent ID:          |         Points:
 Reviewer:          |        Sponsor:
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 In relay.c, function cell_queues_check_size, the OOM manager attempts to
 clear one tenth of MaxMemInQueues bytes from the DNS cache by calling
 dns_cache_handle_oom. The function dns_cache_handle_oom, in dns.c, runs in
 a loop removing cached entries that are now+n*time_inc old, until at least
 the requested number of bytes have been freed. The first iteration of the
 loop has n=0, and likely will not remove enough bytes. The second
 iteration is way too aggressive, because:

 {{{
 time_inc += 3600; /* Increase time_inc by 1 hour. */
 }}}

 This is guaranteed to wipe the entire DNS cache, because in dns_clip_ttl
 the maximum time to cache is MAX_DNS_TTL_AT_EXIT, which is set in dns.h
 to:

 {{{
 /** Lowest value for DNS ttl that a server will give. */
 #define MIN_DNS_TTL_AT_EXIT (5*60)
 /** Highest value for DNS ttl that a server will give. */
 #define MAX_DNS_TTL_AT_EXIT (60*60)
 }}}

 One possible and reasonable fix would be to instead increment time_inc by
 MIN_DNS_TTL_AT_EXIT.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29617>
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