[ruby-core:104130] [Ruby master Bug#17781] Resolv::DNS RequestID table allocations are never freed, causing DNS lookups to eventually hang

Issue #17781 has been updated by usa (Usaku NAKAMURA).

Backport changed from 2.5: DONTNEED, 2.6: REQUIRED, 2.7: REQUIRED, 3.0: DONE to 2.5: DONTNEED, 2.6: REQUIRED, 2.7: DONE, 3.0: DONE

ruby_2_7 87d02eacd26d0b2884016315baf2440d100f177e merged revision(s) 9edc162583a4f685332239f6249745ad9b518cbe.

----------------------------------------
Bug #17781: Resolv::DNS RequestID table allocations are never freed, causing DNS lookups to eventually hang
https://bugs.ruby-lang.org/issues/17781#change-92302

* Author: supermathie (Michael Brown)
* Status: Closed
* Priority: Normal
* Assignee: akr (Akira Tanaka)
* ruby -v: ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-linux]
* Backport: 2.5: DONTNEED, 2.6: REQUIRED, 2.7: DONE, 3.0: DONE
----------------------------------------
In between ruby 2.7.2 and 2.7.3/3.0.0 releases a bug was introduced into `Resolv::DNS` functionality that eventually will break long-running processes that do DNS lookups.

`Resolv::DNS.free_request_id` is no longer called once requests are complete. The implication of this is that **a maximum of 65536 DNS lookups can be performed against a single DNS server**; any further requests will cause ruby to hang in an infinite loop in `Resolv::DNS.allocate_request_id` (all allocations are taken, however the code will loop forever trying to find one that is unallocated).

Reproduction of the problem is possible with the following code:

```ruby
require 'resolv'
65536.times { Resolv::DNS.new.getresource('www.example.net', Resolv::DNS::Resource::IN::A) }
puts "Ran 65536 times"
Resolv::DNS.new.getresource('www.example.net', Resolv::DNS::Resource::IN::A)
puts "Ran 65537 times" # never printed
```

The following script can be used to evaluate if a given ruby version has this problem:
```ruby
#!/usr/bin/env ruby

require 'resolv'

puts "ruby version: #{RUBY_VERSION}"

r = Resolv::DNS.new

100.times { r.getresource('www.example.net', Resolv::DNS::Resource::IN::A).address }
puts "RequestID table size: #{Resolv::DNS::RequestID.values.first&.count or "(empty)"}"
```

A properly behaving version should report an empty `RequestID` table. For example:

```
○ → rvm use ruby-2.7.2; ./test_dns.rb
Using /home/michael/.rvm/gems/ruby-2.7.2
ruby version: 2.7.2
RequestID table size: (empty)

○ → rvm use ruby-2.7.3; ./test_dns.rb
Using /home/michael/.rvm/gems/ruby-2.7.3
ruby version: 2.7.3
RequestID table size: 100

○ → rvm use ruby-3.0.0; ./test_dns.rb
Using /home/michael/.rvm/gems/ruby-3.0.0
ruby version: 3.0.0
RequestID table size: 100

○ → rvm use ruby-3.0.1; ./test_dns.rb
Using /home/michael/.rvm/gems/ruby-3.0.1
ruby version: 3.0.1
RequestID table size: 100
```

Example output from rbtrace on 2.7.2:
```
$ rbtrace -p 42 -m 'allocate_request_id(RequestID)' 'free_request_id(RequestID)'
*** attached to process 42
Resolv::DNS.allocate_request_id(RequestID={}) <0.000094>
Resolv::DNS.free_request_id(RequestID={["192.168.1.51", 53]=>{6377=>true}}) <0.000092>
Resolv::DNS.allocate_request_id(RequestID={}) <0.000086>
Resolv::DNS.free_request_id(RequestID={["192.168.1.51", 53]=>{4537=>true}}) <0.000107>
Resolv::DNS.allocate_request_id(RequestID={}) <0.000086>
Resolv::DNS.free_request_id(RequestID={["192.168.1.51", 53]=>{64587=>true}}) <0.000100>
Resolv::DNS.allocate_request_id(RequestID={}) <0.000097>
Resolv::DNS.free_request_id(RequestID={["192.168.1.51", 53]=>{37627=>true}}) <0.000051>
Resolv::DNS.allocate_request_id(RequestID={}) <0.000045>
Resolv::DNS.free_request_id(RequestID={["192.168.1.51", 53]=>{37265=>true}}) <0.000044>
Resolv::DNS.allocate_request_id(RequestID={}) <0.000047>
Resolv::DNS.free_request_id(RequestID={["192.168.1.51", 53]=>{39586=>true}}) <0.000050>
Resolv::DNS.allocate_request_id(RequestID={}) <0.000040>
Resolv::DNS.free_request_id(RequestID={["192.168.1.51", 53]=>{50696=>true}}) <0.000051>
Resolv::DNS.allocate_request_id(RequestID={}) <0.000040>
Resolv::DNS.free_request_id(RequestID={["192.168.1.51", 53]=>{35266=>true}}) <0.000044>
Resolv::DNS.allocate_request_id(RequestID={}) <0.000041>
Resolv::DNS.free_request_id(RequestID={["192.168.1.51", 53]=>{42458=>true}}) <0.000043>
```

Example output from rbtrace on 2.7.3:
```
$ rbtrace -p 42 -m 'allocate_request_id(RequestID.values.first.count)' 'free_request_id(RequestID.values.first.count)'
*** attached to process 42
Resolv::DNS.allocate_request_id(RequestID.values.first.count=544) <0.000161>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=545) <0.000162>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=546) <0.000136>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=547) <0.000111>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=548) <0.000166>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=549) <0.000140>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=550) <0.000118>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=551) <0.000149>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=552) <0.000113>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=553) <0.000141>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=554) <0.000180>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=555) <0.000174>
Resolv::DNS.allocate_request_id(RequestID.values.first.count=556) <0.000155>
…
```

not reproducible on:
* ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux-musl] (ruby:2.7.2-alpine docker image)
* ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux]

reproducible on:
* ruby 2.7.3p183 (2021-04-05 revision 6847ee089d) [x86_64-linux-musl] (ruby:2.7.3-alpine docker image)
* ruby 2.7.3p183 (2021-04-05 revision 6847ee089d) [x86_64-linux]
* ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux]
* ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-linux]




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