Reap connections based on owning-thread death

.. not a general timeout.

Now, if a thread checks out a connection then dies, we can immediately
recover that connection and re-use it.

This should alleviate the pool exhaustion discussed in #12867. More
importantly, it entirely avoids the potential issues of the reaper
attempting to check whether connections are still active: as long as the
owning thread is alive, the connection is its business alone.

As a no-op reap is now trivial (only entails checking a thread status
per connection), we can also perform one in-line any time we decide to
sleep for a connection.

3 files changed, 24 insertions, 22 deletions

diff --git a/activerecord/test/cases/connection_adapters/abstract_adapter_test.rb b/activerecord/test/cases/connection_adapters/abstract_adapter_test.rb
index eb2fe5639b..deed226eab 100644
--- a/activerecord/test/cases/connection_adapters/abstract_adapter_test.rb
+++ b/activerecord/test/cases/connection_adapters/abstract_adapter_test.rb
@@ -29,12 +29,6 @@ module ActiveRecord
         assert_not adapter.lease, 'should not lease adapter'
       end
 
-      def test_last_use
-        assert_not adapter.last_use
-        adapter.lease
-        assert adapter.last_use
-      end
-
       def test_expire_mutates_in_use
         assert adapter.lease, 'lease adapter'
         assert adapter.in_use?, 'adapter is in use'
diff --git a/activerecord/test/cases/connection_pool_test.rb b/activerecord/test/cases/connection_pool_test.rb
index c4108ad7f6..6300011f4a 100644
--- a/activerecord/test/cases/connection_pool_test.rb
+++ b/activerecord/test/cases/connection_pool_test.rb
@@ -124,7 +124,6 @@ module ActiveRecord
         @pool.checkout
         @pool.checkout
         @pool.checkout
-        @pool.dead_connection_timeout = 0
 
         connections = @pool.connections.dup
 
@@ -134,21 +133,25 @@ module ActiveRecord
       end
 
       def test_reap_inactive
+        ready = false
         @pool.checkout
-        @pool.checkout
-        @pool.checkout
-        @pool.dead_connection_timeout = 0
-
-        connections = @pool.connections.dup
-        connections.each do |conn|
-          conn.extend(Module.new { def active_threadsafe?; false; end; })
+        child = Thread.new do
+          @pool.checkout
+          @pool.checkout
+          ready = true
+          Thread.stop
         end
+        Thread.pass until ready
+
+        assert_equal 3, active_connections(@pool).size
 
+        child.terminate
+        child.join
         @pool.reap
 
-        assert_equal 0, @pool.connections.length
+        assert_equal 1, active_connections(@pool).size
       ensure
-        connections.each(&:close)
+        @pool.connections.each(&:close)
       end
 
       def test_remove_connection
diff --git a/activerecord/test/cases/reaper_test.rb b/activerecord/test/cases/reaper_test.rb
index 015c37cce8..f52fd22489 100644
--- a/activerecord/test/cases/reaper_test.rb
+++ b/activerecord/test/cases/reaper_test.rb
@@ -63,17 +63,22 @@ module ActiveRecord
         spec.config[:reaping_frequency] = 0.0001
 
         pool = ConnectionPool.new spec
-        pool.dead_connection_timeout = 0
 
-        conn = pool.checkout
-        count = pool.connections.length
+        conn = nil
+        child = Thread.new do
+          conn = pool.checkout
+          Thread.stop
+        end
+        Thread.pass while conn.nil?
+
+        assert conn.in_use?
 
-        conn.extend(Module.new { def active_threadsafe?; false; end; })
+        child.terminate
 
-        while count == pool.connections.length
+        while conn.in_use?
           Thread.pass
         end
-        assert_equal(count - 1, pool.connections.length)
+        assert !conn.in_use?
       end
     end
   end