Support switching the user context

Yeah, this is important and i'd say critical to contextual processing.
I was hoping to find a patch here... :-)

Sounds like an appropriate way to handle certain things.

Note that account switching can have unwanted effects. For example, with Feeds, we had a problem with automatically setting a text field because anonymous did not have access to the necessary filter format.

Could this be applicable also by the current bounce analyzers? They now do BounceAnalyzerResult::setRecipient (where "recipient" means "recipient of the prior message", thus = sender of the current message). Or is that scope separate from how account switching is intended to be used? Do we have example use cases?

+++ b/src/MessageProcessor.php
@@ -114,6 +124,11 @@ class MessageProcessor implements MessageProcessorInterface {
+    $this->accountSwitcher->switchTo(User::getAnonymousUser());

@@ -124,6 +139,9 @@ class MessageProcessor implements MessageProcessorInterface {
+    $this->accountSwitcher->switchBack();

IMHO the code should be clear in that being anonymous is the default behaviour. We only want to switch away if the target user is non-anonymous. Similarly, switching back is optional only.

Agree that committing this as is, is useless as it doesn't do anything. At the same time we need the result with a user context.
The only way to generalise it on this level is by introducing an account switcher result interface and checking for that.
And indeed, the bounce doesn't need to implement that.

if ($result implements MessageResultAccountSwitcherInterface && $result->getAccount->id() != User::getAnonymousUser()->id()) {

Functional code will follow once the standard result is in.

Some suggestions, but nothing serious. I'm fine with committing as is.

1. +++ b/src/MessageProcessor.php
   @@ -117,6 +127,11 @@ class MessageProcessor implements MessageProcessorInterface {
   +    // Conditionally switch to the account identified by analyzers.
   +    if ($default_result->getAccount() != $anonymous_user) {
   +      $this->accountSwitcher->switchTo($default_result->getAccount());
   +    }
   
   @@ -127,6 +142,11 @@ class MessageProcessor implements MessageProcessorInterface {
   +    if ($default_result->getAccount() != $anonymous_user) {
   +      // Switch back to a previous account.
   +      $this->accountSwitcher->switchBack();
   +    }
   

   This could be a place to use $default_result->isUserAuthenticated(). We wouldn't have to use $anonymous_user then.

   Vaguely related: DefaultResult could lazily call getAnonymousUser() in getAccount(), then MessageProcessor doesn't have to setAccount().

2. +++ b/tests/src/Kernel/HandlerTest.php
   @@ -0,0 +1,63 @@
   +/**
   + * Tests handlers.
   + *
   + * @group inmail
   + */
   +class HandlerTest extends KernelTestBase {
   

   This test doesn't really test any handler. It's just handler-related. Maybe consider moving the method to ProcessorTest?

It's very important that we guarantee that if we switch the account, we call switch back.
This should not call twice an API. (Handlers could accidentally alter values...)
Instead, we should determine a local variable for it in MessageProcessor.php and check it.

Very nice. :-)