Using the Yaml::PARSE_KEYS_AS_STRINGS flag is deprecated since Symfony 3.4 as it will be removed in 4.0

Relying on a rare PECL extension is not really a solution: many sites won't have it and will have to rely on a user-space implementation anyway.

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Let's see what breaks.

This deprecation message is thrown *a lot*, it actually broke chrome/selenium based tests for me because chrome died with an ERR_RESPONSE_HEADERS_TOO_BIG error.

So I guess we need to try and get rid of this asap.

@fgm: I think there is a misunderstanding. Nobody wants to make the yaml extension a requirement. We already optionally support it, I think the comment from @alexpott was that we need the option to have compatibility between the two different implementations.

Was about to upload the same patch as @alexpott and got a crosspost conflict :)

arg, stupid crosspost resolution.. reuploading.

Let's force drupalci to use Symfony YAML just so we can see.

+++ b/core/lib/Drupal/Component/Serialization/Yaml.php
@@ -50,7 +50,7 @@ protected static function getSerializer() {
+      if (FALSE != TRUE && extension_loaded('yaml')) {

I think you want == here :)

My only excuse is it was late... lol.

Yeah, didn't spot that either :) Also, "if (FALSE == TRUE)", for days when if (FALSE) is too easy ;)

Strange, I did run the config tests locally and they passed for me, I don't have the yaml extension enabled either.

It's because the \Drupal\Tests\config\Functional\ConfigSingleImportExportTest has specific code for when the PECl yaml extension is loaded. So actually we're good to go here.

Ah I see, and it was enabled but the we didn't use it, which confused the test. That makes sense.

Lets get this in then. There are hundreds of such deprecation messages on the module patch and others when parsing a lot of yml files, enough to break running tests.

+1 to get this in, really hard to by-pass it running contrib tests

Thanks @catch

This has to be backported to 8.5.x. I've described in #2979317: Duplicate deprecation header messages break Chromedriver that due to the flag SymfonyYaml::PARSE_KEYS_AS_STRINGS javascript functional tests might fail when being run with chrome in Drupal 8.5.x.
I've reopened the issue, but feel free to close it and probably commit this as part of the referenced issue, where I've provided a test that unfortunately has to be run manually as we don't yet have chrome tests on the CI.

Here's a version of the patch backported to 8.5.x

Setting back to fixed, only critical issues have a chance to be backported to 8.5 now.