"Connection refused" when try connect to Cloud SQL from a Symfony application deployed on App Engine Standard - symfony

Question

I have changed the Symfony demo application configuration from SQLite to MySQL. Locally it works fine for me, but if I try to deploy it in the Google App Engine Standard environment it doesn't work.
It is not clear to me how I should put the connection string if the database is in Google Cloud SQL. Currently the project's .env file has this:
DATABASE_URL="mysql://dbname=THEDATABASENAME;unix_socket=PROJECTNAME:ZONE:INSTANCENAME"
I'm not sure if it's correct, or if something is missing, or if I need to configure something else in Symfony or in the Google Cloud Console.
When I deploy and browse to the site, I'm having Error 500.
The error log shows this:
2022-11-02 14:41:00 default[20221102t151407] "GET / HTTP/1.1" 302
2022-11-02 14:41:01 default[20221102t151407] nginx: [warn] the "user" directive makes sense only if the master process runs with super-user privileges, ignored in /tmp/google-config/nginx.conf:3
2022-11-02 14:41:03 default[20221102t151407] "GET /es HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/app.c5bd2611.js HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/runtime.725dd7e0.js HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/755.5a8586e9.js HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/95.1ed6b32f.js HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/app.17316efd.css HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/fonts/fa-solid-900.1551f4f6.woff2 HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/fonts/fa-regular-400.7a333762.woff2 HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/95.22c66ae4.css HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/823.168b723b.js HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/31.46aa6824.js HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/fonts/lato-bold.21b3848a.woff2 HTTP/1.1" 200
2022-11-02 14:41:03 default[20221102t151407] "GET /build/fonts/lato-normal.75614cfc.woff2 HTTP/1.1" 200
2022-11-02 14:41:08 default[20221102t151407] "GET /es/blog/ HTTP/1.1" 500
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Matched route \"blog_index\".","context":{"route":"blog_index","route_parameters":{"_route":"blog_index","page":"1","_format":"html","_controller":"App\\Controller\\BlogController::index","_locale":"es"},"request_uri":"https://testMYPROJECT.uc.r.appspot.com/es/blog/","method":"GET"},"level":200,"level_name":"INFO","channel":"request","datetime":"2022-11-02T14:41:08.499913+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Checking for authenticator support.","context":{"firewall_name":"main","authenticators":1},"level":100,"level_name":"DEBUG","channel":"security","datetime":"2022-11-02T14:41:08.500353+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Checking support on authenticator.","context":{"firewall_name":"main","authenticator":"Symfony\\Component\\Security\\Http\\Authenticator\\FormLoginAuthenticator"},"level":100,"level_name":"DEBUG","channel":"security","datetime":"2022-11-02T14:41:08.500361+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Authenticator does not support the request.","context":{"firewall_name":"main","authenticator":"Symfony\\Component\\Security\\Http\\Authenticator\\FormLoginAuthenticator"},"level":100,"level_name":"DEBUG","channel":"security","datetime":"2022-11-02T14:41:08.500367+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Failed to save key \"App%5CController%5CBlogController\" of type array: fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","context":{"key":"App%5CController%5CBlogController","exception":{"class":"ErrorException","message":"fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","code":0,"file":"/workspace/vendor/symfony/cache/Traits/FilesystemCommonTrait.php:99"},"cache-adapter":"Symfony\\Component\\Cache\\Adapter\\PhpFilesAdapter"},"level":300,"level_name":"WARNING","channel":"cache","datetime":"2022-11-02T14:41:08.528050+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Failed to save key \"ea12799f***********\" of type Doctrine\\ORM\\Query\\ParserResult: fopen(/workspace/var/cache/prod/pools/system/16ctGUzn1Q/774b86986a15): Failed to open stream: No such file or directory","context":{"key":"ea12799f**********","exception":{"class":"ErrorException","message":"fopen(/workspace/var/cache/prod/pools/system/16ctGU**********): Failed to open stream: No such file or directory","code":0,"file":"/workspace/vendor/symfony/cache/Traits/FilesystemCommonTrait.php:99"},"cache-adapter":"Symfony\\Component\\Cache\\Adapter\\PhpFilesAdapter"},"level":300,"level_name":"WARNING","channel":"cache","datetime":"2022-11-02T14:41:09.092063+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Uncaught PHP Exception Doctrine\\DBAL\\Exception\\ConnectionException: \"An exception occurred in the driver: SQLSTATE[HY000] [2002] Connection refused\" at /workspace/vendor/doctrine/dbal/src/Driver/API/MySQL/ExceptionConverter.php line 101","context":{"exception":{"class":"Doctrine\\DBAL\\Exception\\ConnectionException","message":"An exception occurred in the driver: SQLSTATE[HY000] [2002] Connection refused","code":2002,"file":"/workspace/vendor/doctrine/dbal/src/Driver/API/MySQL/ExceptionConverter.php:101","previous":{"class":"Doctrine\\DBAL\\Driver\\PDO\\Exception","message":"SQLSTATE[HY000] [2002] Connection refused","code":2002,"file":"/workspace/vendor/doctrine/dbal/src/Driver/PDO/Exception.php:28","previous":{"class":"PDOException","message":"SQLSTATE[HY000] [2002] Connection refused","code":2002,"file":"/workspace/vendor/doctrine/dbal/src/Driver/PDO/MySQL/Driver.php:28"}}}},"level":500,"level_name":"CRITICAL","chann"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "el":"request","datetime":"2022-11-02T14:41:09.173672+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Failed to save key \"Symfony%5CComponent%5CHttpKernel%5CController%5CErrorController\" of type array: fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","context":{"key":"Symfony%5CComponent%5CHttpKernel%5CController%5CErrorController","exception":{"class":"ErrorException","message":"fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","code":0,"file":"/workspace/vendor/symfony/cache/Traits/FilesystemCommonTrait.php:99"},"cache-adapter":"Symfony\\Component\\Cache\\Adapter\\PhpFilesAdapter"},"level":300,"level_name":"WARNING","channel":"cache","datetime":"2022-11-02T14:41:09.191792+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Failed to save key \"Symfony%5CComponent%5CHttpKernel%5CController%5CErrorController%23__invoke\" of type array: fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","context":{"key":"Symfony%5CComponent%5CHttpKernel%5CController%5CErrorController%23__invoke","exception":{"class":"ErrorException","message":"fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","code":0,"file":"/workspace/vendor/symfony/cache/Traits/FilesystemCommonTrait.php:99"},"cache-adapter":"Symfony\\Component\\Cache\\Adapter\\PhpFilesAdapter"},"level":300,"level_name":"WARNING","channel":"cache","datetime":"2022-11-02T14:41:09.192185+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Failed to save key \"Symfony%5CBundle%5CFrameworkBundle%5CController%5CTemplateController\" of type array: fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","context":{"key":"Symfony%5CBundle%5CFrameworkBundle%5CController%5CTemplateController","exception":{"class":"ErrorException","message":"fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","code":0,"file":"/workspace/vendor/symfony/cache/Traits/FilesystemCommonTrait.php:99"},"cache-adapter":"Symfony\\Component\\Cache\\Adapter\\PhpFilesAdapter"},"level":300,"level_name":"WARNING","channel":"cache","datetime":"2022-11-02T14:41:09.700565+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] [02-Nov-2022 14:41:09] WARNING: [pool app] child 20 said into stderr: "{"message":"Failed to save key \"Symfony%5CBundle%5CFrameworkBundle%5CController%5CTemplateController%23templateAction\" of type array: fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","context":{"key":"Symfony%5CBundle%5CFrameworkBundle%5CController%5CTemplateController%23templateAction","exception":{"class":"ErrorException","message":"fopen(/workspace/var/cache/prod/pools/system/qGYD7CGfrN/d7b0f5b96ec8): Failed to open stream: Read-only file system","code":0,"file":"/workspace/vendor/symfony/cache/Traits/FilesystemCommonTrait.php:99"},"cache-adapter":"Symfony\\Component\\Cache\\Adapter\\PhpFilesAdapter"},"level":300,"level_name":"WARNING","channel":"cache","datetime":"2022-11-02T14:41:09.701047+00:00","extra":{}}"
2022-11-02 14:41:09 default[20221102t151407] "GET /build/fonts/fa-brands-400.d878b0a6.woff2 HTTP/1.1" 200
2022-11-02 14:41:09 default[20221102t151407] "GET /build/css/app.4aa95248.css HTTP/1.1" 404

Answer 1

The Google App Engine (Standard) docs for connecting to Cloud SQL note that:
Once correctly configured, you can connect your service to your Cloud SQL instance's Unix domain socket accessed on the environment's filesystem at the following path: /cloudsql/INSTANCE_CONNECTION_NAME
Your unix_socket value above should be:
/cloudsql/project:region:instance-id
rather than just
project:region:instance-id
Also note that it should be the region, not the zone, i.e., northamerica-northeast1 rather than northamerica-northeast1-a. When in doubt, you can verify the connection name either:
on the overview page in the console for the target SQL instance
by running gcloud sql instances describe INSTANCE_NAME
Finally, you may also need to verify that:
the service account for your App Engine service has the Cloud SQL Client role in the project hosting the SQL instance and the project hosting the App Engine service
the Cloud SQL Admin API (sqladmin.googleapis.com) is enabled on both the project hosting the SQL instance and the project hosting the App Engine service