[Form] Make sure to collect child forms created on *_SET_DATA events

[yceruto]

Q	A
Branch?	3.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	Fix #29291
License	MIT
Doc PR	-

See reproducer provided by @WubbleWobble https://github.com/WubbleWobble/symfony-issue-29291.

[stof]

wouldn't this have a bad side-effect if the call to setData happens after creating the form rather than passing the data to the form factory directly ?

[yceruto]

@stof in that case $this->defaultDataSet is already true and setData() wouldn't be called again.

[yceruto]

#29291 (comment)
I think this is related to the fact that the fields are added in a PRE_SUBMIT listener. Maybe extractConfiguration is not called properly for dynamic fields, and so the data is incomplete.

I've also checked the case when dynamic child forms are added on PRE_SUBMIT event and they are correctly collected thanks to this code:

symfony/src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php

Lines 151 to 155 in 29cabf9

	if (!isset($this->dataByForm[$hash])) {
	// field was created by form event
	$this->collectConfiguration($form);
	$this->collectDefaultData($form);
	}

[yceruto]

I've changed the solution consistent with collectSubmittedData() approach when the form hash doesn't exist.

[OskarStark]

Is it possible to add a testcase?