TYPO3 Core

According to the blog extension (created by the TYPO3 GmbH), this should already work. See here: https://bitbucket.typo3.com/projects/EXT/repos/blog/browse/Configuration/Routes/Default.yaml

I'm able to confirm this is working for TYPO3 9.5.3.

I just managed to do it by tweaking the load() function of YAML loader class(TYPO3\CMS\Core\Configuration\Loader\YamlFileLoader) in a way it don't affect the configuration writing or reading of yaml files other than site configuration yaml file(config.yaml).see the changes:

   public function load(string $fileName, int $flags = self::PROCESS_PLACEHOLDERS | self::PROCESS_IMPORTS): array
   {
         $content = $this->getFileContents($fileName);
         $content = Yaml::parse($content);
         if (!is_array($content)) {
             throw new \RuntimeException('YAML file "' . $fileName . '" could not be parsed into valid syntax, probably empty?', 1497332874);
         }
+        else{
+           //checking whether it is a routing configuration file of a specific domain.
+           $siteConfiguration = isset($content['rootPageId'])?1:0;
+        }
         if (($flags & self::PROCESS_IMPORTS) === self::PROCESS_IMPORTS) {
-            $content = $this->processImports($content);
+            //patch to avoid writing import file configuration into config.yaml of site configuration
+            if(TYPO3_MODE == 'BE'){
+                //Ensures it's not a routing configuration file.
+                if($siteConfiguration == 0){
+                  $content = $this->processImports($content);
+                }
+            }
+            else{
+               $content = $this->processImports($content);
+            }
         }
         if (($flags & self::PROCESS_PLACEHOLDERS) === self::PROCESS_PLACEHOLDERS) {
              // Check for "%" placeholders
            $content = $this->processPlaceholders($content, $content);
        }

        return $content;
    }

Before this patch i was able to import files into config.yaml as follows:

imports:
  -
    resource: fileadmin/pluginconfig/news.yaml

But the problem is , on saving the configuration via 'sites' module in backend for a specific domain, the configurations in news.yaml file got directly written into config.yaml instead of preserving 'import' directinve in config.yaml since reading and writing of yaml files are dealt with the same load() functionality in YamlFileLoader.

After applying this patch i achieved what i intended.ie preserving 'import' directive in the config.yaml itself(by checking whether the context of loading yaml is front end or backend) even after saving the configuration via sites module.

If this finds helpful and sensible,can anyone push this to typo3 gerrit review?

I think the Problem is not the YamlFileLoader but the SiteConfiguration. I find for me a temporary solution. I'm override the SiteConfiguration with XCLASS

localconf.php

$GLOBALS['TYPO3_CONF_VARS']['SYS']['Objects'][\TYPO3\CMS\Core\Configuration\SiteConfiguration::class] = array(
    'className' => \Vendor\Ext\Configuration\SiteConfiguration::class
);

Extend SiteConfiguration

<?php
declare(strict_types = 1);

namespace Vendor\Ext\Configuration;

use Symfony\Component\Finder\Finder;
use TYPO3\CMS\Core\Configuration\Loader\YamlFileLoader;
use TYPO3\CMS\Core\Utility\GeneralUtility;

class SiteConfiguration extends \TYPO3\CMS\Core\Configuration\SiteConfiguration
{

    /**
     * Read the site configuration from config files.
     *
     * @return array
     * @throws \TYPO3\CMS\Core\Cache\Exception\NoSuchCacheException
     */
    protected function getAllSiteConfigurationFromFiles(): array
    {
        // Check if the data is already cached
        if (TYPO3_MODE != 'BE' && $siteConfiguration = $this->getCache()->get($this->cacheIdentifier)) {
            // Due to the nature of PhpFrontend, the `<?php` and `#` wraps have to be removed
            $siteConfiguration = preg_replace('/^<\?php\s*|\s*#$/', '', $siteConfiguration);
            $siteConfiguration = json_decode($siteConfiguration, true);
        }
        // Nothing in the cache (or no site found)
        if (empty($siteConfiguration)) {
            $finder = new Finder();
            try {
                $finder->files()->depth(0)->name($this->configFileName)->in($this->configPath . '/*');
            } catch (\InvalidArgumentException $e) {
                // Directory $this->configPath does not exist yet
                $finder = [];
            }
            /** @var YamlFileLoader $loader */
            $loader = GeneralUtility::makeInstance(YamlFileLoader::class);
            $siteConfiguration = [];
            $siteConfigurationCache = [];
            foreach ($finder as $fileInfo) {
                $configuration = $loader->load(GeneralUtility::fixWindowsFilePath((string)$fileInfo), 0);
                $configurationCache = $loader->load(GeneralUtility::fixWindowsFilePath((string)$fileInfo));
                $identifier = basename($fileInfo->getPath());

                $siteConfiguration[$identifier] = $this->handleDeprecated($configuration);
                $siteConfigurationCache[$identifier] = $this->handleDeprecated($configurationCache);;
            }
            $this->getCache()->set($this->cacheIdentifier, json_encode($siteConfigurationCache));
        }
        return $siteConfiguration ?? [];
    }

    /**
     * @param $configuration array
     * @return array
     */
    private function handleDeprecated($configuration)
    {
        if (isset($configuration['site'])) {
            trigger_error(
                'Site configuration with key \'site\' has been deprecated, remove indentation level and site key.',
                E_USER_DEPRECATED
            );
            $configuration = $configuration['site'];

        }
        return $configuration;
    }

}

I render the Yaml File twice. One time for the config.yaml without processing the imports. And the second time for the Cache file with processing the imports.

I would go as far as calling the current implementation for saving a SiteConfiguration buggy, because it basically deactivates the imports feature of config.yaml. This can't possibly be the intended behaviour...

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/61573

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/61573

Patch set 7 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/61573

Patch set 8 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/61573

Patch set 9 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/61573

Patch set 1 for branch 9.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/61603

Patch set 3 for branch 9.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/61603