It doesn't have to do anything, but it shouldn't fail.

Fixes #31766.

Postgresql bulk_change_table should flatten procs array

Build a multi-statement query when inserting fixtures

* Add backticks
* Expand tabs
* Fix indentation

* rDBMS -> RDBMS. There is only place using rDBMS.
* a SQL -> an SQL

* Update active_record_basics.md

I made a bit of clarification for people, who are not familiar with SQL (pretty much like me). However, I don't know what tutorial for MySQL is better, so I haven't inserted a link yet.

* [ci skip] For those who new to GitHub

Added more instructions for contributing guides. Without them, it was somewhat confusing for me to find what I should actually do.

* Update active_record_basics.md

Fixed grammar and text wrapping as requested.

* Update contributing_to_ruby_on_rails.md

Revised instructions.

* Update contributing_to_ruby_on_rails.md

Typos

* Update active_record_basics.md

* [ci skip] Update active_record_basics

Added a few links to SQL tutorials found on the net.
Also, changed MySQL to SQL (or one of its extensions) - I think that it's a good compromise.

* [ci skip] I think now it's more clear what to do.

* [ci skip] Fixed strings

[Rafael Mendonça França + loothunter1]

Allow for custom handling of exceptions that are discarded

We don't implement much custom marshalling logic for these objects, but
the proc default case needs to be handled separately. Unfortunately
there's no way to just say "do what you would have done but with this
value for one ivar", so we have to manually implement `marshal_load` as
well.

The test case is a little bit funky, but I'd really like an equality
test in there, and there's no easy way to add one now that this is out
of AR (since the `attributes` method isn't here)

Fixes #31216

Fix actionmailer lambda default

- mysql will add a 2 bytes margin to the statement, so given a `max_allowed_packet` set to 1024 bytes, a 1024 bytes fixtures will no be inserted (mysql will throw an error)
- Preventing this by decreasing the max_allowed_packet by 2 bytes when doing the comparison with the actual statement size

Fix locale_selector JS bug in ActionMailer Preview

Improve fault tolerance for redis cache store

remove text repetition

[ci skip]

Add support for connection pooling on RedisCacheStore

Update guide text not to contradict image

The bottom line in the image of `ActionController::UnknownFormat`is not
truncated. Now the text does not say it is.

[ci skip]

Support for PostgreSQL foreign tables

Fixes #31762.

- The `insert_fixtures` method can be optimized by making a single multi statement query for all fixtures having the same connection instead of doing a single query per table
  - The previous code was bulk inserting fixtures for a single table, making X query for X fixture files
  - This patch builds a single **multi statement query** for every tables. Given a set of 3 fixtures (authors, dogs, computers):
  ```ruby
    # before
    %w(authors dogs computers).each do |table|
      sql = build_sql(table)
      connection.query(sql)
    end

    # after

    sql = build_sql(authors, dogs, computers)
    connection.query(sql)
  ```
- `insert_fixtures` is now deprecated, `insert_fixtures_set` is the new way to go with performance improvement
- My tests were done with an app having more than 700 fixtures, the time it takes to insert all of them was around 15s. Using a single multi statement query, it took on average of 8 seconds
- In order for a multi statement to be executed, mysql needs to be connected with the `MULTI_STATEMENTS` [flag](https://dev.mysql.com/doc/refman/5.7/en/c-api-multiple-queries.html), which is done before inserting the fixtures by reconnecting to da the database with the flag declared. Reconnecting to the database creates some caveats:
  1. We loose all open transactions; Inside the original code, when inserting fixtures, a transaction is open. Multple delete statements are [executed](https://github.com/rails/rails/blob/a681eaf22955734c142609961a6d71746cfa0583/activerecord/lib/active_record/fixtures.rb#L566) and finally the fixtures are inserted. The problem with this patch is that we need to open the transaction only after we reconnect to the DB otherwise reconnecting drops the open transaction which doesn't commit all delete statements and inserting fixtures doesn't work since we duplicated them (Primary key duplicate exception)...
    - In order to fix this problem, the transaction is now open directly inside the `insert_fixtures` method, right after we reconnect to the db
    - As an effect, since the transaction is open inside the `insert_fixtures` method, the DELETE statements need to be executed here since the transaction is open later
  2. The same problem happens for the `disable_referential_integrity` since we reconnect, the `FOREIGN_KEY_CHECKS` is reset to the original value
    - Same solution as 1. , the disable_referential_integrity can be called after we reconnect to the transaction
  3. When the multi statement query is executed, no other queries can be performed until we paginate over the set of results, otherwise mysql throws a "Commands out of sync" [Ref](https://dev.mysql.com/doc/refman/5.7/en/commands-out-of-sync.html)
    - Iterating over the set of results until `mysql_client.next_result` is false. [Ref](https://github.com/brianmario/mysql2#multiple-result-sets)
- Removed the `active_record.sql "Fixture delete"` notification, the delete statements are now inside the INSERT's one
- On mysql the `max_allowed_packet` is looked up:
  1. Before executing the multi-statements query, we check the packet length of each statements, if the packet is bigger than the max_allowed_packet config, an `ActiveRecordError` is raised
  2. Otherwise we concatenate the current sql statement into the previous and so on until the packet is `< max_allowed_packet`

Ignores a default subclass when `becomes(Parent)`

Fixes issue described in #30399: A default value on the
inheritance column prevented `child.becomes(Parent)` to return
an instance of `Parent` as expected, instead it returns an instance
of the default subclass.

The change was introduced by #17169 and it was meant to affect
initialization, alone. Where `Parent.new` is expected to return
an instance of the default subclass.

Don't perform unnecessary check with false, just use true/false values

Don't update counter cache when through record was not destroyed

bin/yarn: Pass through arguments with spaces

Use singular define_attribute_method

`define_attribute_methods` splats the arguments,
then calls out to `define_attribute_method` for
each. When defining a singule attribute, using
the singular version of the method saves us an
array and an extra method call.

#30985 caused `object.save` performance regression since calling
`changes` in `changes_applied` is very slow.
We don't need to call the expensive method in `changes_applied` as long
as `@attributes` is tracked by mutation tracker.

https://gist.github.com/kamipo/1a9f4f3891803b914fc72ede98268aa2

Before:

```
Warming up --------------------------------------
create_string_columns
                        73.000  i/100ms
Calculating -------------------------------------
create_string_columns
                        722.256  (± 5.8%) i/s -      3.650k in   5.073031s
```

After:

```
Warming up --------------------------------------
create_string_columns
                        96.000  i/100ms
Calculating -------------------------------------
create_string_columns
                        950.224  (± 7.7%) i/s -      4.800k in   5.084837s
```

Previously, the `bin/yarn` wrapper would "unquote" arguments to yarn like this:
`yarn run add-copyright "(c) 2017, 2018 MyCompany"`
That results in an ARGV of ['run', 'add-copyright', '(c) 2017, 2018 MyCompany'] in the yarn wrapper,
but a ARGV in the yarn executable of ['run', 'add-copyright', '(c)',  '2017,', '2018', MyCompany']

Use dup'ed options hash