Move DatabaseAlreadyExists detection to DB adapter

Previously it was the responsibility of the database tasks to translate
the invalid statement from creating a duplicate database into an
ActiveRecord::Tasks::DatabaseAlreadyExists error.

It's actually easier for us to do this detection inside of the adapter,
where we already do a case statement on the return code to translate the
error.

This commit introduces ActiveRecord::DatabaseAlreadyExists, a subclass
of StatementInvalid, and updates both AbstractMysqlAdapter and
PostgresqlAdapter to return this more specific exception in that case.

Because this is a subclass of the old exception, StatementInvalid, it
should be backwards compatible with any code expecting that from
create_database.

This works for both create_database and exectute("CREATE DATABASE")

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -573,6 +573,7 @@ def extract_precision(sql_type)
         end
 
         # See https://dev.mysql.com/doc/refman/5.7/en/server-error-reference.html
+        ER_DB_CREATE_EXISTS     = 1007
         ER_DUP_ENTRY            = 1062
         ER_NOT_NULL_VIOLATION   = 1048
         ER_NO_REFERENCED_ROW    = 1216
@@ -592,6 +593,8 @@ def extract_precision(sql_type)
 
         def translate_exception(exception, message:, sql:, binds:)
           case error_number(exception)
+          when ER_DB_CREATE_EXISTS
+            DatabaseAlreadyExists.new(message, sql: sql, binds: binds)
           when ER_DUP_ENTRY
             RecordNotUnique.new(message, sql: sql, binds: binds)
           when ER_NO_REFERENCED_ROW, ER_ROW_IS_REFERENCED, ER_ROW_IS_REFERENCED_2, ER_NO_REFERENCED_ROW_2

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -467,6 +467,7 @@ def check_version # :nodoc:
         UNIQUE_VIOLATION      = "23505"
         SERIALIZATION_FAILURE = "40001"
         DEADLOCK_DETECTED     = "40P01"
+        DUPLICATE_DATABASE    = "42P04"
         LOCK_NOT_AVAILABLE    = "55P03"
         QUERY_CANCELED        = "57014"
 
@@ -488,6 +489,8 @@ def translate_exception(exception, message:, sql:, binds:)
             SerializationFailure.new(message, sql: sql, binds: binds)
           when DEADLOCK_DETECTED
             Deadlocked.new(message, sql: sql, binds: binds)
+          when DUPLICATE_DATABASE
+            DatabaseAlreadyExists.new(message, sql: sql, binds: binds)
           when LOCK_NOT_AVAILABLE
             LockWaitTimeout.new(message, sql: sql, binds: binds)
           when QUERY_CANCELED

activerecord/lib/active_record/errors.rb

@@ -187,6 +187,10 @@ class PreparedStatementInvalid < ActiveRecordError
   class NoDatabaseError < StatementInvalid
   end
 
+  # Raised when creating a database if it exists.
+  class DatabaseAlreadyExists < StatementInvalid
+  end
+
   # Raised when PostgreSQL returns 'cached plan must not change result type' and
   # we cannot retry gracefully (e.g. inside a transaction)
   class PreparedStatementCacheExpired < StatementInvalid

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -4,7 +4,6 @@
 
 module ActiveRecord
   module Tasks # :nodoc:
-    class DatabaseAlreadyExists < StandardError; end # :nodoc:
     class DatabaseNotSupported < StandardError; end # :nodoc:
 
     # ActiveRecord::Tasks::DatabaseTasks is a utility class, which encapsulates

activerecord/lib/active_record/tasks/mysql_database_tasks.rb

@@ -15,12 +15,6 @@ def create
         establish_connection configuration_without_database
         connection.create_database configuration["database"], creation_options
         establish_connection configuration
-      rescue ActiveRecord::StatementInvalid => error
-        if connection.error_number(error.cause) == ER_DB_CREATE_EXISTS
-          raise DatabaseAlreadyExists
-        else
-          raise
-        end
       end
 
       def drop

activerecord/lib/active_record/tasks/postgresql_database_tasks.rb

@@ -21,12 +21,6 @@ def create(master_established = false)
         connection.create_database configuration["database"],
           configuration.merge("encoding" => encoding)
         establish_connection configuration
-      rescue ActiveRecord::StatementInvalid => error
-        if error.cause.is_a?(PG::DuplicateDatabase)
-          raise DatabaseAlreadyExists
-        else
-          raise
-        end
       end
 
       def drop

activerecord/test/cases/tasks/mysql_rake_test.rb

@@ -93,11 +93,9 @@ def test_create_when_database_exists_outputs_info_to_stderr
         with_stubbed_connection_establish_connection do
           ActiveRecord::Base.connection.stub(
             :create_database,
-            proc { raise ActiveRecord::StatementInvalid }
+            proc { raise ActiveRecord::DatabaseAlreadyExists }
           ) do
-            ActiveRecord::Base.connection.stub(:error_number, 1007) do
-              ActiveRecord::Tasks::DatabaseTasks.create @configuration
-            end
+            ActiveRecord::Tasks::DatabaseTasks.create @configuration
 
             assert_equal "Database 'my-app-db' already exists\n", $stderr.string
           end

activerecord/test/cases/tasks/postgresql_rake_test.rb

@@ -129,7 +129,7 @@ def test_create_when_database_exists_outputs_info_to_stderr
         with_stubbed_connection_establish_connection do
           ActiveRecord::Base.connection.stub(
             :create_database,
-            proc { raise ActiveRecord::Tasks::DatabaseAlreadyExists }
+            proc { raise ActiveRecord::DatabaseAlreadyExists }
           ) do
             ActiveRecord::Tasks::DatabaseTasks.create @configuration