Symfony is suddenly failing with “The service ”session.storage.metadata_bag“ has a dependency on a non-existent parameter”

Question

I upgraded from Symfony 3.1.5 to 3.2.0 yesterday morning and everything was fine up until an hour or so ago.

Now Symfony throws the following exception upon any type of access, including console commands:

[Symfony\\Component\\DependencyInjection\\Exception\\ParameterNotFoundException]
The service "session.storage.metadata_bag" has a dependency on a non-existent parameter "session.metadata.storage_key".

I have tried the following:

• Backed out my most recent changes
• Deleted var/cache/*

Here is the exception trace:

Exception trace:
 () at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php:100
 Symfony\Component\DependencyInjection\ParameterBag\ParameterBag->get() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/ParameterBag/EnvPlaceholderParameterBag.php:56
 Symfony\Component\DependencyInjection\ParameterBag\EnvPlaceholderParameterBag->get() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php:217
 Symfony\Component\DependencyInjection\ParameterBag\ParameterBag->resolveString() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php:187
 Symfony\Component\DependencyInjection\ParameterBag\ParameterBag->resolveValue() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php:177
 Symfony\Component\DependencyInjection\ParameterBag\ParameterBag->resolveValue() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Compiler/ResolveParameterPlaceHoldersPass.php:39
 Symfony\Component\DependencyInjection\Compiler\ResolveParameterPlaceHoldersPass->process() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Compiler/Compiler.php:120
 Symfony\Component\DependencyInjection\Compiler\Compiler->compile() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/ContainerBuilder.php:565
 Symfony\Component\DependencyInjection\ContainerBuilder->compile() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/Kernel.php:484
 Symfony\Component\HttpKernel\Kernel->initializeContainer() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/Kernel.php:116
 Symfony\Component\HttpKernel\Kernel->boot() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Console/Application.php:68
 Symfony\Bundle\FrameworkBundle\Console\Application->doRun() at /Volumes/Data01/Projects/Beck/WWUI/site/vendor/symfony/symfony/src/Symfony/Component/Console/Application.php:122
 Symfony\Component\Console\Application->run() at /Volumes/Data01/Projects/Beck/WWUI/site/bin/console:29

As you can see, it's part of Symfony, and not my code.

I am at a loss.
Has anybody else seen this?
Suggestions?

Edit 1:
Based on Dagon's suggestion that I check session.xml, and seeing that it appears correct, I decided to replace the

<argument>%session.metadata.storage_key%</argument>

tag with

<argument>_sf2_meta</argument>

in session.xml. This resulted in a new, similar error:

[Symfony\\Component\\DependencyInjection\\Exception\\ParameterNotFoundException]
The service "64e3c91c625381e3c2c0184cf9ea0c6d5353986d77aa111949ca6fb8b17a8f07_2" has a dependency on a non-existent parameter "validator.mapping.
cache.prefix".

Answer 1

This isn't really an answer per-se, but is is a fix.
I renamed vendor to vendor-HOLD and then ran composer install. Problem gone.

I did a diff on the old and new session.xml files and found that the definition of the parameter was different. The version causing the error contained:

<parameter string="session.metadata.storage_key">_sf2_meta</parameter>

The newly installed version contained:

<parameter key="session.metadata.storage_key">_sf2_meta</parameter>

instead. Obviously other files were incorrect, as well, based on the experimental edit I did to session.xml. No clue as to what happened, but it's nice that composer install fixed it. Oh, and yes, I deleted vendor-HOLD afterwards.