Don't forcefully clear temporarily disabled query caches

Followup: rails#50938

The behavior changed to always clear the query cache as soon
as it's disabled, on the assumption that once queries have been
performed without it, all bets are off.

However that isn't quite true, we can apply the same clearing
logic than when it's enabled. If you perform read only queries
without the cache, there is no reason to clear it.

activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb

@@ -29,7 +29,7 @@ def #{method_name}(...)
       end
 
       class Store # :nodoc:
-        attr_reader :enabled
+        attr_accessor :enabled
         alias_method :enabled?, :enabled
 
         def initialize(max_size)
@@ -38,11 +38,6 @@ def initialize(max_size)
           @enabled = false
         end
 
-        def enabled=(enabled)
-          clear if @enabled && !enabled
-          @enabled = enabled
-        end
-
         def size
           @map.size
         end

activerecord/lib/active_record/query_cache.rb

@@ -8,7 +8,13 @@ module ClassMethods
       # If it's not, it will execute the given block.
       def cache(&block)
         if connected? || !configurations.empty?
-          connection_pool.enable_query_cache(&block)
+          pool = connection_pool
+          was_enabled = pool.query_cache_enabled
+          begin
+            pool.enable_query_cache(&block)
+          ensure
+            pool.clear_query_cache unless was_enabled
+          end
         else
           yield
         end
@@ -30,7 +36,10 @@ def self.run
     end
 
     def self.complete(pools)
-      pools.each(&:disable_query_cache!)
+      pools.each do |pool|
+        pool.disable_query_cache!
+        pool.clear_query_cache
+      end
 
       ActiveRecord::Base.connection_handler.each_connection_pool do |pool|
         pool.release_connection if pool.active_connection? && !pool.connection.transaction_open?

activerecord/test/cases/query_cache_test.rb

@@ -91,6 +91,24 @@ def test_writes_should_always_clear_cache
     assert_cache :off
   end
 
+  def test_reads_dont_clear_disabled_cache
+    assert_cache :off
+
+    mw = middleware { |env|
+      Post.first
+      query_cache = ActiveRecord::Base.connection.query_cache
+      assert_equal 1, query_cache.size, query_cache.inspect
+      Post.connection.uncached do
+        Post.count # shouldn't clear the cache
+      end
+      query_cache = ActiveRecord::Base.connection.query_cache
+      assert_equal 1, query_cache.size, query_cache.inspect
+    }
+    mw.call({})
+
+    assert_cache :off
+  end
+
   def test_exceptional_middleware_clears_and_disables_cache_on_error
     assert_cache :off
 
@@ -808,7 +826,7 @@ def test_cache_gets_cleared_after_migration
   end
 
   def test_find
-    assert_called(Task.connection.query_cache, :clear, times: 2) do
+    assert_called(Task.connection.query_cache, :clear, times: 1) do
       assert_not Task.connection.query_cache_enabled
       Task.cache do
         assert Task.connection.query_cache_enabled