Fix elapsed time calculations

I've found a few places in Rails code base where I think it makes sense
to calculate elapsed time more precisely by using
`Concurrent.monotonic_time`:

- Fix calculation of elapsed time in `ActiveSupport::Cache::MemoryStore#prune`
- Fix calculation of elapsed time in
  `ActiveRecord::ConnectionAdapters::ConnectionPool::Queue#wait_poll`
- Fix calculation of elapsed time in
  `ActiveRecord::ConnectionAdapters::ConnectionPool#attempt_to_checkout_all_existing_connections`
- Fix calculation of elapsed time in `ActiveRecord::ConnectionAdapters::Mysql2Adapter#explain`

See
https://docs.ruby-lang.org/en/2.5.0/Process.html#method-c-clock_gettime
https://blog.dnsimple.com/2018/03/elapsed-time-with-ruby-the-right-way

Related to 7c4542146f0dde962205e5a90839349631ae60fb

2 files changed, 3 insertions, 3 deletions

diff --git a/activesupport/lib/active_support/cache/memory_store.rb b/activesupport/lib/active_support/cache/memory_store.rb
index 106b616529..629eb2dd70 100644
--- a/activesupport/lib/active_support/cache/memory_store.rb
+++ b/activesupport/lib/active_support/cache/memory_store.rb
@@ -62,13 +62,13 @@ module ActiveSupport
         return if pruning?
         @pruning = true
         begin
-          start_time = Time.now
+          start_time = Concurrent.monotonic_time
           cleanup
           instrument(:prune, target_size, from: @cache_size) do
             keys = synchronize { @key_access.keys.sort { |a, b| @key_access[a].to_f <=> @key_access[b].to_f } }
             keys.each do |key|
               delete_entry(key, options)
-              return if @cache_size <= target_size || (max_time && Time.now - start_time > max_time)
+              return if @cache_size <= target_size || (max_time && Concurrent.monotonic_time - start_time > max_time)
             end
           end
         ensure
diff --git a/activesupport/lib/active_support/notifications/instrumenter.rb b/activesupport/lib/active_support/notifications/instrumenter.rb
index 125c06f37a..00a57c38c9 100644
--- a/activesupport/lib/active_support/notifications/instrumenter.rb
+++ b/activesupport/lib/active_support/notifications/instrumenter.rb
@@ -137,7 +137,7 @@ module ActiveSupport
 
       private
         def now
-          Process.clock_gettime(Process::CLOCK_MONOTONIC)
+          Concurrent.monotonic_time
         end
 
         if clock_gettime_supported?