Regression: can no longer access previously stored values for block component instances

Also credited @penyaskito for his original work in identifying the bug and figuring out a viable work-around: https://git.drupalcode.org/project/canvas/-/merge_requests/142/diffs#not....

P.S.: I suspect this goes all the way back to how support for block forms was introduced in #3491978: Implement saving block settings forms.

Bump.

Looks like we actually don't need that, and it passes tests? 🤔

#3558448: UI must only set `value` on `StaticPropSource`s, not on `DynamicPropSource`s nor `HostEntityUrlPropSource`s landed, which seems to have touched this.

#3470422: Handle components provided by ComponentSources IMPLICITLY disappearing: gracefulness when developing SDCs: SDCs may appear/disappear from one request to the next significantly changed the lines this was updating; hope I got the update right.

EDIT: apparently I was the one who removed the block-specific bits over at https://git.drupalcode.org/project/canvas/-/merge_requests/220/diffs?com... 🤯 No recollection of this.

AFAICT #3577219: Add Multi-Value Image/Video Prop Support (UI)'s https://git.drupalcode.org/project/canvas/-/merge_requests/755 addressed this:

-  resolved[propName] = isRequired
-    ? propSourceData.default_values.resolved
-    : undefined;
+  resolved[propName] = isRequired ? propSourceData.default_values.resolved : [];

Merged in upstream. Re-testing.

Improved the clarity of the else branch (meaning "component powering this component instance is broken").

This actually brought us to failing tests that fail for the very reason this issue was opened: the client is sending nothing, whereas it should send the client-side data model it does have. IOW: \Drupal\Tests\canvas\Kernel\ComponentInstanceFormTest::testBlockComponentThatHasGoneAway() is replicating the client-side bug in a PHPUnit test;

$response = $this->getCrawlerForFormRequest('/canvas/api/v0/form/component-instance/canvas_page/' . $page->id(), $component, json_decode('undefined'));

That json_decode('undefined') bit at the end is the problem.

Reproducibility in current UI

So I set out to reproduce this.

1. I placed/instantiated core's "branding" block.
2. Then deleted core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
3. drush cr
4. When editing again:

That means the answer to Determine the root cause of the bug on the client side and fix it. in the issue summary is: "cannot reproduce at all", so the server-side work-around by definition is not necessary right now 😅

Meaning you can no longer even get to the component instance form to recover values (which was the very purpose of #3519168: Handle components provided by ComponentSources EXPLICITLY disappearing — enables deleting JS components that are in use).

It also means \Drupal\Tests\canvas\Kernel\ComponentInstanceFormTest::testBlockComponentThatHasGoneAway() is testing a client reality that is no longer a reality — an inherent risk of replicating client behavior in a PHPUnit test.

Updated IS.

Proposing 2 MRs:

1. The current one, which only brings more clarity to the server-side code by adding comments.
2. A new one, to fix the client-side regression and which should allow us to simplify the server-side code.

Bumping to critical because this will lead to:

• perceived data loss for Canvas content authors/site owners
• perceived poor DX when developing block plugins — related: yesterday's #3578142: [upstream] New Views blocks are not available to be placed until cache clear
• perceived instability/lack of robustness overall — just last 6 days ago we did #3578865: Hydrating a component tree with an instance containing garbage values OutOfRangeException when component trees have inputs for non-existent props to harden it

NR for the first MR. That can land and improve the clarity of what needs to happen.

Landed MR #1. MR #2 updated with pointers.