This PR was merged into the 4.3-dev branch.

Discussion
----------

[Messenger] Add a Doctrine transport

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | symfony/symfony-docs#10616
| DoctrineBundle PR | doctrine/DoctrineBundle#868

As discussed with @sroze at PHPForum in Paris I've worked on adding a Doctrine transport to the Messenger component.

Actually `AMQP` is the only supported transport and it could be a good thing to support multiple transports. Having a Doctrine transport could help users to start using the component IMHO (Almost all projects use a database).

# How it works

The code is splitted betwwen this PR and the one on the DoctrineBundle : doctrine/DoctrineBundle#868

## Configuration

To configure a Doctrine transport the dsn MUST have the format `doctrine://<entity_manager_name>` where `<entity_manager_name>` is the name of the entity manager (usually `default`)
```yml
        # config/packages/messenger.yaml
        framework:
            messenger:
                transports:
                    my_transport: "doctrine://default?queue=important"
```

## Table schema

Dispatched messages are stored into a database table with the following schema:

| Column       | Type     | Options                  | Description                                                       |
|--------------|----------|--------------------------|-------------------------------------------------------------------|
| id           | bigint   | AUTO_INCREMENT, NOT NULL | Primary key                                                       |
| body         | text     | NOT NULL                 | Body of the message                                               |
| headers      | text     | NOT NULL                 | Headers of the message                                            |
| queue      | varchar(32)     | NOT NULL                 | Headers of the message                                            |
| created_at   | datetime | NOT NULL                 | When the message was inserted onto the table. (automatically set) |
| available_at       | datetime   | NOT NULL                 | When the message is available to be handled                      |
| delivered_at | datetime | NULL                     | When the message was delivered to a worker                        |

## Message dispatching

When dispatching a message a new row is inserted into the table. See `Symfony\Component\Messenger\Transport\Doctrine::publish`

## Message consuming

The message is retrieved by the `Symfony\Component\Messenger\Transport\Doctrine\DoctrineReceiver`. It calls the `Symfony\Component\Messenger\Transport\Doctrine::get` method to get the next message to handle.

### Getting the next message

* Start a transaction
* Lock the table to get the first message to handle (The lock is done with the `SELECT ... FOR UPDATE` query)
* Update the message in database to update the delivered_at columns
* Commit the transaction

### Handling the message

The retrieved message is then passed to the handler. If the message is correctly handled the receiver call the `Symfony\Component\Messenger\Transport\Doctrine::ack` which delete the message from the table.

If an error occured the receiver call the `Symfony\Component\Messenger\Transport\Doctrine::nack` method which update the message to set the delivered_at column to `null`.

## Message requeueing

It may happen that a message is stuck in `delivered` state but the handler does not really handle the message (Database connection error, server crash, ...). To requeue messages the `DoctrineReceiver` call the `Symfony\Component\Messenger\Transport\Doctrine::requeueMessages`. This method update all the message with a  `delivered_at` not null since more than the "redeliver timeout" (default to 3600 seconds)

# TODO

- [x] Add tests
- [x] Create DOC PR
- [x] PR on doctrine-bundle for transport factory
- [x] Add a `available_at` column
- [x] Add a `queue` column
- [x] Implement the retry functionnality : See #30557
- [x] Rebase after #29476

Commits
-------

88d008c [Messenger] Add a Doctrine transport

…terface (sroze)

This PR was merged into the 4.3-dev branch.

Discussion
----------

[Messenger] Fix the Doctrine transport to use the new interface

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #29007
| License       | MIT
| Doc PR        | ø

Commits
-------

75e3355 Fix the Doctrine transport to use the new interface

…uzet)

This PR was submitted for the master branch but it was merged into the 4.3 branch instead (closes #10616).

Discussion
----------

[Messenger] Describe the doctrine transport

Add documentation relative to the PR symfony/symfony#29007

TODO

- [ ] Document the max granularity in seconds for delay (doctrine/dbal#2873)

Commits
-------

27c0f9d [Messenger] Describe the doctrine tranport