FIX: Race-condition in fallback handlers (#8005)

Calling `verify_master` in multiple threads simultaneously would cause
multiple threads to be spawned.

lib/active_record/connection_adapters/postgresql_fallback_adapter.rb

@@ -32,7 +32,8 @@ class PostgreSQLFallbackHandler
   end
 
   def verify_master
-    synchronize { return if @thread && @thread.alive? }
+    synchronize do
+      return if @thread && @thread.alive?
 
       @thread = Thread.new do
         while true do
@@ -46,6 +47,7 @@ class PostgreSQLFallbackHandler
 
       @thread.abort_on_exception = true
     end
+  end
 
   def master_down?
     synchronize { @masters_down[namespace] }

lib/discourse_redis.rb

@@ -23,7 +23,8 @@ class DiscourseRedis
     end
 
     def verify_master
-      synchronize { return if @thread && @thread.alive? }
+      synchronize do
+        return if @thread && @thread.alive?
 
         @thread = Thread.new do
           loop do
@@ -38,6 +39,7 @@ class DiscourseRedis
           end
         end
       end
+    end
 
     def initiate_fallback_to_master
       success = false