Spawn a single thread that checks for PostgreSQL fallback.

lib/active_record/connection_adapters/postgresql_fallback_adapter.rb

@@ -6,23 +6,46 @@ class PostgreSQLFallbackHandler
   include Singleton
 
   def initialize
-    @master = {}
+    @masters_down = {}
-    @running = {}
+    @mutex = Mutex.new
-    @mutex = {}
-    @last_check = {}
-
-    setup!
   end
 
   def verify_master
-    @mutex[namespace].synchronize do
+    synchronize { return if @thread && @thread.alive? }
-      return if running || recently_checked?
+
-      @running[namespace] = true
+    @thread = Thread.new do
+      while true do
+        begin
+          thread = Thread.new { initiate_fallback_to_master }
+          thread.join
+          break if synchronize { @masters_down.empty? }
+          sleep 10
+        ensure
+          thread.kill
+        end
+      end
+    end
   end
 
-    current_namespace = namespace
+  def master_down?
-    Thread.new do
+    synchronize { @masters_down[namespace] }
-      RailsMultisite::ConnectionManagement.with_connection(current_namespace) do
+  end
+
+  def master_down=(args)
+    synchronize { @masters_down[namespace] = args }
+  end
+
+  def master_up(namespace)
+    synchronize { @masters_down.delete(namespace) }
+  end
+
+  def running?
+    synchronize { @thread.alive? }
+  end
+
+  def initiate_fallback_to_master
+    @masters_down.keys.each do |key|
+      RailsMultisite::ConnectionManagement.with_connection(key) do
         begin
           logger.warn "#{log_prefix}: Checking master server..."
           connection = ActiveRecord::Base.postgresql_connection(config)
@@ -32,54 +55,20 @@ class PostgreSQLFallbackHandler
             ActiveRecord::Base.clear_all_connections!
             logger.warn "#{log_prefix}: Master server is active. Reconnecting..."
 
-            if namespace == RailsMultisite::ConnectionManagement::DEFAULT
+            self.master_up(key)
-              ActiveRecord::Base.establish_connection(config)
-            else
-              RailsMultisite::ConnectionManagement.establish_connection(db: namespace)
-            end
-
             Discourse.disable_readonly_mode
-            self.master = true
           end
         rescue => e
-          if e.message.include?("could not connect to server")
+          byebug
           logger.warn "#{log_prefix}: Connection to master PostgreSQL server failed with '#{e.message}'"
-          else
-            raise e
-          end
-        ensure
-          @mutex[namespace].synchronize do
-            @last_check[namespace] = Time.zone.now
-            @running[namespace] = false
-          end
         end
       end
     end
   end
 
-  def master
+  # Use for testing
-    @master[namespace]
-  end
-
-  def master=(args)
-    @master[namespace] = args
-  end
-
-  def running
-    @running[namespace]
-  end
-
   def setup!
-    RailsMultisite::ConnectionManagement.all_dbs.each do |db|
+    @masters_down = {}
-      @master[db] = true
-      @running[db] = false
-      @mutex[db] = Mutex.new
-      @last_check[db] = nil
-    end
-  end
-
-  def verify?
-    !master && !running
   end
 
   private
@@ -96,17 +85,13 @@ class PostgreSQLFallbackHandler
     "#{self.class} [#{namespace}]"
   end
 
-  def recently_checked?
-    if @last_check[namespace]
-      Time.zone.now <= (@last_check[namespace] + 5.seconds)
-    else
-      false
-    end
-  end
-
   def namespace
     RailsMultisite::ConnectionManagement.current_db
   end
+
+  def synchronize
+    @mutex.synchronize { yield }
+  end
 end
 
 module ActiveRecord
@@ -115,7 +100,7 @@ module ActiveRecord
       fallback_handler = ::PostgreSQLFallbackHandler.instance
       config = config.symbolize_keys
 
-      if fallback_handler.verify?
+      if fallback_handler.master_down?
         connection = postgresql_connection(config.dup.merge({
           host: config[:replica_host], port: config[:replica_port]
         }))
@@ -126,7 +111,8 @@ module ActiveRecord
         begin
           connection = postgresql_connection(config)
         rescue PG::ConnectionBad => e
-          fallback_handler.master = false
+          fallback_handler.master_down = true
+          fallback_handler.verify_master
           raise e
         end
       end
@@ -141,20 +127,4 @@ module ActiveRecord
       raise "Replica database server is not in recovery mode." if value == 'f'
     end
   end
-
-  module ConnectionAdapters
-    class PostgreSQLAdapter
-      set_callback :checkout, :before, :switch_back?
-
-      private
-
-      def fallback_handler
-        @fallback_handler ||= ::PostgreSQLFallbackHandler.instance
-      end
-
-      def switch_back?
-        fallback_handler.verify_master if fallback_handler.verify?
-      end
-    end
-  end
 end

lib/discourse.rb

@@ -228,6 +228,7 @@ module Discourse
 
   def self.keep_readonly_mode
     # extend the expiry by 1 minute every 30 seconds
+    unless Rails.env.test?
       Thread.new do
         while readonly_mode?
           $redis.expire(READONLY_MODE_KEY, READONLY_MODE_KEY_TTL)
@@ -235,6 +236,7 @@ module Discourse
         end
       end
     end
+  end
 
   def self.disable_readonly_mode(user_enabled: false)
     key = user_enabled ? USER_READONLY_MODE_KEY : READONLY_MODE_KEY

spec/components/active_record/connection_adapters/postgresql_fallback_adapter_spec.rb

@@ -48,8 +48,9 @@ describe ActiveRecord::ConnectionHandling do
       end
 
       it 'should failover to a replica server' do
+        current_threads = Thread.list
+
         RailsMultisite::ConnectionManagement.stubs(:all_dbs).returns(['default', multisite_db])
-        ::PostgreSQLFallbackHandler.instance.setup!
 
         [config, multisite_config].each do |configuration|
           ActiveRecord::Base.expects(:postgresql_connection).with(configuration).raises(PG::ConnectionBad)
@@ -60,7 +61,7 @@ describe ActiveRecord::ConnectionHandling do
           })).returns(@replica_connection)
         end
 
-        expect(postgresql_fallback_handler.master).to eq(true)
+        expect(postgresql_fallback_handler.master_down?).to eq(nil)
 
         expect { ActiveRecord::Base.postgresql_fallback_connection(config) }
           .to raise_error(PG::ConnectionBad)
@@ -68,10 +69,10 @@ describe ActiveRecord::ConnectionHandling do
         expect{ ActiveRecord::Base.postgresql_fallback_connection(config) }
           .to change{ Discourse.readonly_mode? }.from(false).to(true)
 
-        expect(postgresql_fallback_handler.master).to eq(false)
+        expect(postgresql_fallback_handler.master_down?).to eq(true)
 
         with_multisite_db(multisite_db) do
-          expect(postgresql_fallback_handler.master).to eq(true)
+          expect(postgresql_fallback_handler.master_down?).to eq(nil)
 
           expect { ActiveRecord::Base.postgresql_fallback_connection(multisite_config) }
             .to raise_error(PG::ConnectionBad)
@@ -79,30 +80,18 @@ describe ActiveRecord::ConnectionHandling do
           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)
+          expect(postgresql_fallback_handler.master_down?).to eq(true)
         end
 
+        postgresql_fallback_handler.master_up(multisite_db)
+
         ActiveRecord::Base.unstub(:postgresql_connection)
 
-        current_threads = Thread.list
+        postgresql_fallback_handler.initiate_fallback_to_master
-
-        expect{ ActiveRecord::Base.connection_pool.checkout }
-          .to change{ Thread.list.size }.by(1)
-
-        # Ensure that we don't try to connect back to the replica when a thread
-        # is running
-        begin
-          ActiveRecord::Base.postgresql_fallback_connection(config)
-        rescue PG::ConnectionBad => e
-          # This is expected if the thread finishes before the above is called.
-        end
-
-        # Wait for the thread to finish execution
-        (Thread.list - current_threads).each(&:join)
 
         expect(Discourse.readonly_mode?).to eq(false)
 
-        expect(PostgreSQLFallbackHandler.instance.master).to eq(true)
+        expect(postgresql_fallback_handler.master_down?).to eq(nil)
 
         expect(ActiveRecord::Base.connection_pool.connections.count).to eq(0)