fix: retry mysql connections on message DB pool

lib/postal/message_db/connection_pool.rb

@@ -12,9 +12,11 @@ module Postal
       end
 
       def use
-        connection = checkout
+        retried = false
         do_not_checkin = false
         begin
+          connection = checkout
+
           yield connection
         rescue Mysql2::Error => e
           if e.message =~ /(lost connection|gone away|not connected)/i
@@ -22,6 +24,12 @@ module Postal
             # we won't add it back in to the pool so that it'll reconnect
             # next time.
             do_not_checkin = true
+
+            # If we haven't retried yet, we'll retry the block once more.
+            if retried == false
+              retried = true
+              retry
+            end
           end
 
           raise

spec/lib/postal/message_db/connection_pool_spec.rb

@@ -41,5 +41,16 @@ describe Postal::MessageDB::ConnectionPool do
       end.to raise_error Mysql2::Error
       expect(pool.connections).to eq []
     end
+
+    it "retries the block once if there is a connection error" do
+      clients_seen = []
+      expect do
+        pool.use do |client|
+          clients_seen << client
+          raise Mysql2::Error, "lost connection to server"
+        end
+      end.to raise_error Mysql2::Error
+      expect(clients_seen.uniq.size).to eq 2
+    end
   end
 end