'gcloud sql connect' command allows to connect to a CloudSQL instance
from a local machine. In order to do so, it starts local
'cloud_sdk_proxy' instance. google-cloud-sdk expects to find one in SDK
root (installed by 'gcloud components') or on the PATH, if SDK is not
correctly installed ('.install' directory is missing).
Since google-cloud-sdk on NixOS is properly installed 'gcloud sql
connect' never looks for 'cloud_sql_proxy' on the PATH and simply
doesn't work at all.
The patch slightly modifies the check by looking not only for
'.install' directory, but for actual 'cloud_sql_proxy' binary before
falling back to the search on the PATH.
With this patch it's now possible to use 'gcloud sql connect' on NixOS,
provided that 'cloud_sql_proxy' is available either in user or system
enviroment (available in nixpkgs).
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