Increment @counter of prepared postgres statements prior to running the query.

If the next query or the prepared statement itself get interrupted, this prevents the database session getting stuck perpetually retrying to recreate the same prepared statement.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Increment postgres prepared statement counter before making a prepared statement, so if the statement is aborted 
+    without Rails knowledge (e.g., if app gets kill -9d during long-running query or due to Rack::Timeout), app won't end 
+    up in perpetual crash state for being inconsistent with Postgres.
+
+    *wbharding*, *Martin Tepper*
+
 *   Switch to database adapter return type for `ActiveRecord::Calculations.calculate`
     when called with `:average` (aliased as `ActiveRecord::Calculations.average`)
 

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -228,11 +228,7 @@ module ActiveRecord
         end
 
         def next_key
-          "a#{@counter + 1}"
-        end
-
-        def []=(sql, key)
-          super.tap { @counter += 1 }
+          "a#{@counter += 1}"
         end
 
         private

activerecord/test/cases/adapters/postgresql/statement_pool_test.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 require "cases/helper"
+require "models/computer"
+require "models/developer"
 
 module ActiveRecord
   module ConnectionAdapters
@@ -16,6 +18,8 @@ module ActiveRecord
       end
 
       class StatementPoolTest < ActiveRecord::PostgreSQLTestCase
+        fixtures :developers
+
         if Process.respond_to?(:fork)
           def test_cache_is_per_pid
             cache = StatementPool.new nil, 10
@@ -37,6 +41,20 @@ module ActiveRecord
           cache["foo"] = "bar"
           assert_nothing_raised { cache.clear }
         end
+
+        def test_prepared_statements_do_not_get_stuck_on_query_interruption
+          pg_connection = ActiveRecord::Base.connection.instance_variable_get(:@connection)
+          pg_connection.stub(:get_last_result, -> { raise "random error" }) do
+            assert_raises(RuntimeError) do
+              Developer.where(name: "David").last
+            end
+
+            # without fix, this raises PG::DuplicatePstatement: ERROR:  prepared statement "a3" already exists
+            assert_raises(RuntimeError) do
+              Developer.where(name: "David").last
+            end
+          end
+        end
       end
     end
   end