Avoid mutating original response in connection management middleware

Clarify automatic creation/deletion of join models for has_many through

call local_cache 1 time instead of 3 times

use prepend instead of extending every instance

extending an instance with a module puts the methods on top of it,
prepend does the same but on the class level, so less work for us and more standard way of doing things

[ci-skip]

LocalCache read_entry previously called the local_cache method 3 times,
now it only calls it once and keeps the result in a local variable.

cache nil replies from backend cache so misses are fast too

Update association_basics.md [ci skip]

JavaScript is spelled with a capital J and S.

It was deprecated in https://github.com/rails/rails/pull/19135.

We're now favoring `public_file_server.headers`.

`test_binary_data_is_not_logged` is for prepared statements logging

No need `MysqlDouble` and `MysqlDouble.reset_column_information`

Fix `static_cache_control` deprecation warning

1. Fix the message by wrapping the value in missing quotes and adding
the missing backtick at the end. Finally, 💄 by adding a space
inside the curly braces.

Before:

`config.public_file_server.headers = {'Cache-Control' => public,
max-age=31536000}

Now:

`config.public_file_server.headers = { 'Cache-Control' => 'public,
max-age=31536000' }`

2. Display `static_cache_control` instead of static_cache_control. This
follows what the 2 neighboring methods are doing.

3. Use strip_heredoc to improve the code formatting and readability
like the 2 neighboring methods and wrap to 80 characters.

Require only necessary concurrent-ruby classes.

docrails is a private repository now to avoid pull requests to it.

This solves:

```
Expected /Completed 200 OK in [\d]ms/ to match "Completed 200 OK in 943ms".
```

Should use `public_file_server.enabled` instead.

Clarified that static files will be served from the public directory, where it made sense.

Also removed occurrence of the deprecated `static_cache_control`.

Forgot to do it in 748b2f9c, when deprecating `serve_static_files`.

Since `bin/test` would define `Rails.root` before loading AM but `bundle
exec rake` would define it after loading AM, this lead to the following
test failure when using `bin/test`:

```
--- expected
+++ actual
@@ -1 +1 @@
-{:location=>"/var/folders/_r/9kh50y4j0vn7zgg2p65z1lcm0000gn/T/mails"}
+{:location=>"/Users/senny/Projects/rails/actionmailer/tmp/mails"}
```

This patch ensures that `Rails.root` is defined when loading AM to
achieve consistent test results.

Add enabled flag to the public file server settings.

The configuration for `config.static_cache_control`, and its replacement
`config.public_file_server.headers` are implemented in Railties.

People would configure this in environment files, which is Railties domain too.

document `try!` in ActiveSupport core ext guide

https://twitter.com/avdi/status/660141673993777156

Don’t allow arbitrary data in back urls

Update docs/comments to use setup/teardown blocks vs. methods. [ci skip]

* Remove super for setup block. Not needed.

Columns are no longer stored in an attribute since b8a533d5.

[ci skip]

we can just reuse the parent process.  We should figure out what files
are commonly required among the test files and try to require them in
the parent so that the require time cost is amortized across the
processes.

`link_to :back` creates a link to whatever was
passed in via the referer header. If an attacker
can alter the referer header, that would create
a cross-site scripting vulnerability on every
page that uses `link_to :back`

This commit restricts the back URL to valid
non-javascript URLs.

https://github.com/rails/rails/issues/14444