Using flexParentDatabaseRow in userFunc or itemsProcFunc does not work with compatibility6

[sypets]

Problem description

The breaking change no 70132 ("FormEngine custom functions") in the TYPO3 core 7.6 suggests using 'flexParentDatabaseRow' instead of 'row' for retrieving the full parent database row in a userFunc / itemsProcFunc.

https://wiki.typo3.org/TYPO3.CMS/Releases/7.6/Breaking#Breaking:_.2370132_-_FormEngine_custom_functions

This will not work, if compatibility6 is installed.

Reproduce

1. Use current 7.6 installation (e.g. introduction package)
2. Install compatibility6
3. Install current tt_address version 4.0.0 as an example (any other extension which uses this functionality will reproduce the same results)
4. Create an address plugin content
5. Add tt_address static includes and for example the following typoscript

plugin.tx_ttaddress_pi1 {
    templatePath = fileadmin/template/ext/tt_address/
    defaultTemplateFileName = default_hcard.htm
 }

6. Create several template files to choose from, e.g.

mkdir -p fileadmin/template/ext/tt_address/
cp typo3conf/ext/tt_address/res/default_hcard.htm fileadmin/template/ext/tt_address/
cp typo3conf/ext/tt_address/res/default_hcard.htm  fileadmin/template/ext/tt_address/default_hcard2.htm

7. Edit the address plugin and click on "Plugin option" : "Display" : "Template"

Expected result

A dropdown list should appear with the 2 available files.

Actual result

Only default item in list displayed.

Confirm

Deinstall compatibility6 and try again

Reason for problem

https://github.com/FriendsOfTYPO3/tt_address/blob/60eed630ee66663e63422f9f89949bb5896e4731/Classes/Hooks/Tca/AddFilesToSelector.php#L41

$params['flexParentDatabaseRow']['pid'] will be empty. The same problem appears in other extensions if they use the same mechanism.