Use correct procpid or pid in unlock, based on postgres version.

Closes QueueClassic#261.

In postgres 9.2, `procpid` was changed to `pid`.

Earlier versions should use `procpid`, this change fixes the issue
where this query would return an error in versions prior to postgres
9.2.

changelog

@@ -2,6 +2,7 @@ Unreleased
 - Fixed a bug in the offset calculation of `.enqueue_at`.
 - Use the jsonb type for the args column from now on. If not available, fall back to json or text.
 - `enqueue`, `enqueue_at`, `enqueue_in` return job hash with id.
+- Fixed unlock query for versions below Postgres 9.2
 
 Version 3.0.0rc
 - Improved signal handling

lib/queue_classic.rb

@@ -100,7 +100,8 @@ def self.measure(data)
   # This will unlock all jobs any postgres' PID that is not existing anymore
   # to prevent any infinitely locked jobs
   def self.unlock_jobs_of_dead_workers
-    default_conn_adapter.execute("UPDATE #{QC.table_name} SET locked_at = NULL, locked_by = NULL WHERE locked_by NOT IN (SELECT pid FROM pg_stat_activity);")
+    pid_column = default_conn_adapter.server_version < 90200 ? "procpid" : "pid"
+    default_conn_adapter.execute("UPDATE #{QC.table_name} SET locked_at = NULL, locked_by = NULL WHERE locked_by NOT IN (SELECT #{pid_column} FROM pg_stat_activity);")
   end
 
   # private class methods

lib/queue_classic/conn_adapter.rb

@@ -48,6 +48,13 @@ def disconnect
       end
     end
 
+    def server_version
+      @server_version ||= begin
+                            version = execute("SHOW server_version_num;")["server_version_num"]
+                            version && version.to_i
+                          end
+    end
+
     private
 
     def wait_for_notify(t)