Add a entity revision parameter converter & route enhancer

Looks good, but I'd feel better if somebody with better routing system knowledge would review/RTBC this.

Why not use this for the node revision routes?

1. +++ b/core/lib/Drupal/Core/ParamConverter/EntityRevisionParamConverter.php
   @@ -0,0 +1,51 @@
   + * Parameter converter for single revisions.
   

   Nit: Shouldn't it be singular revision here?

2. +++ b/core/tests/Drupal/Tests/Core/ParamConverter/EntityRevisionParamConverterTest.php
   @@ -0,0 +1,89 @@
   +  /**
   +   * The entity manager.
   +   *
   +   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   +   */
   +  protected $entityTypeManager;
   

   Nit: can be removed, this is never used.

What if someone replaced the service and extended the existing class?

How can we then ever let core use improved APIs?

The code in general is solid.

The only nitpick I have is that the param converter could have more docs. Look at the Entity param converter:

/**
 * Parameter converter for upcasting entity IDs to full objects.
 *
 * This is useful in cases where the dynamic elements of the path can't be
 * auto-determined; for example, if your path refers to multiple of the same
 * type of entity ("example/{node1}/foo/{node2}") or if the path can act on any
 * entity type ("example/{entity_type}/{entity}/foo").
 *
 * In order to use it you should specify some additional options in your route:
 * @code
 * example.route:
 *   path: foo/{example}
 *   options:
 *     parameters:
 *       example:
 *         type: entity:node
 * @endcode
 *
 * If you want to have the entity type itself dynamic in the url you can
 * specify it like the following:
 * @code
 * example.route:
 *   path: foo/{entity_type}/{example}
 *   options:
 *     parameters:
 *       example:
 *         type: entity:{entity_type}
 * @endcode
 */

That provides a nice example to follow.

Looks good to me. Agreed on not converting node here. We can still do that later on if we want to and keep the existing class as @deprecated if anyone extends from it or something.

+1. But can we file a follow-up to update node.module to use this new parameter converter & route enhancer?

Thanks!

None of this should hold up commit.

1. +++ b/core/lib/Drupal/Core/ParamConverter/EntityRevisionParamConverter.php
   @@ -0,0 +1,65 @@
   + *   path: foo/{entity_example_revision}
   

   Missing leading slash path: /foo/{entity_example_revision}

2. +++ b/core/lib/Drupal/Core/ParamConverter/EntityRevisionParamConverter.php
   @@ -0,0 +1,65 @@
   +    list (, $entity_type_id) = explode(':', $definition['type']);
   

   Should probably have , 2 just 'cause

3. +++ b/core/lib/Drupal/Core/Routing/Enhancer/EntityRevisionRouteEnhancer.php
   @@ -0,0 +1,52 @@
   + * Contains \Drupal\Core\Routing\EntityRevisionRouteEnhancer.
   ...
   +namespace Drupal\Core\Routing\Enhancer;
   ...
   +class EntityRevisionRouteEnhancer implements RouteEnhancerInterface {
   

   Contains doesn't match the namespace. (though aren't we getting rid of that?)

4. +++ b/core/tests/Drupal/Tests/Core/Enhancer/EntityRevisionRouteEnhancerTest.php
   @@ -0,0 +1,95 @@
   +  public function testAppliesWithNoParameters() {
   ...
   +  public function testAppliesWithNoneRevisionParameters() {
   ...
   +  public function testAppliesWithRevisionParameters() {
   

   I would have put these three in a dataProvider

Super minor point that can be fixed on commit:

+++ b/core/lib/Drupal/Core/ParamConverter/EntityRevisionParamConverter.php
@@ -0,0 +1,65 @@
+ * "entity_example/{entity_example}/revision/{entity_example_revision}".

For consistency with the code example below, this should have a leading slash as well :)

Otherwise the patch looks great to me as well.

Committed/pushed to 8.2.x and cherry-picked to 8.1.x. Thanks! Let's do the follow-up for 8.2.x.

Added review credit post-commit.

Uh, I think we missed a tiny little detail here? :)

Actually registering this new service in core.services.yml. Found while trying to make https://github.com/fago/entity/pull/38 work.

Also, looking at this again, not sure why this was added in the Routing namespace and not in Entity/Routing, like the standard entity routing enhancer?

Opened #2688668: Register the new entity revision param enhancer/converter as services.