FIX: `PostgreSQLFallbackHandler` was bouncing in and out of readonly.

lib/active_record/connection_adapters/postgresql_fallback_adapter.rb

@@ -39,7 +39,7 @@ class PostgreSQLFallbackHandler
             end
 
             Discourse.disable_readonly_mode
-            master = true
+            self.master = true
           end
         rescue => e
           if e.message.include?("could not connect to server")
@@ -77,6 +77,10 @@ class PostgreSQLFallbackHandler
     end
   end
 
+  def verify?
+    !master && !running && !recently_checked?
+  end
+
   private
 
   def config
@@ -110,7 +114,7 @@ module ActiveRecord
       fallback_handler = ::PostgreSQLFallbackHandler.instance
       config = config.symbolize_keys
 
-      if !fallback_handler.master && !fallback_handler.running
+      if fallback_handler.verify?
         connection = postgresql_connection(config.dup.merge({
           host: config[:replica_host], port: config[:replica_port]
         }))
@@ -148,9 +152,7 @@ module ActiveRecord
       end
 
       def switch_back?
-        if !fallback_handler.master && !fallback_handler.running
-          fallback_handler.verify_master
-        end
+        fallback_handler.verify_master if fallback_handler.verify?
       end
     end
   end

spec/components/active_record/connection_adapters/postgresql_fallback_adapter_spec.rb

@@ -13,8 +13,10 @@ describe ActiveRecord::ConnectionHandling do
     }).symbolize_keys!
   end
 
+  let(:postgresql_fallback_handler) { PostgreSQLFallbackHandler.instance }
+
   after do
-    ::PostgreSQLFallbackHandler.instance.setup!
+    postgresql_fallback_handler.setup!
   end
 
   describe "#postgresql_fallback_connection" do
@@ -58,18 +60,26 @@ describe ActiveRecord::ConnectionHandling do
           })).returns(@replica_connection)
         end
 
+        expect(postgresql_fallback_handler.master).to eq(true)
+
         expect { ActiveRecord::Base.postgresql_fallback_connection(config) }
           .to raise_error(PG::ConnectionBad)
 
         expect{ ActiveRecord::Base.postgresql_fallback_connection(config) }
           .to change{ Discourse.readonly_mode? }.from(false).to(true)
 
+        expect(postgresql_fallback_handler.master).to eq(false)
+
         with_multisite_db(multisite_db) do
+          expect(postgresql_fallback_handler.master).to eq(true)
+
           expect { ActiveRecord::Base.postgresql_fallback_connection(multisite_config) }
             .to raise_error(PG::ConnectionBad)
 
           expect{ ActiveRecord::Base.postgresql_fallback_connection(multisite_config) }
             .to change{ Discourse.readonly_mode? }.from(false).to(true)
+
+          expect(postgresql_fallback_handler.master).to eq(false)
         end
 
         ActiveRecord::Base.unstub(:postgresql_connection)
@@ -92,6 +102,8 @@ describe ActiveRecord::ConnectionHandling do
 
         expect(Discourse.readonly_mode?).to eq(false)
 
+        expect(PostgreSQLFallbackHandler.instance.master).to eq(true)
+
         expect(ActiveRecord::Base.connection_pool.connections.count).to eq(0)
 
         expect(ActiveRecord::Base.connection)