Does not support executable sourced credential configurations

[apvd]

The Application Default Credential system allows for cred config json files which specify a few types of credential sources. This file, URL, and executable sources for external workload and workforce identity credentials. The go implementation of oauth2 does not currently support executable sources, so cred configs which are valid for gcloud auth application-default print-access-token will produce an error with go applications requesting tokens from the same source.

For example, using the docker helper docker-credential-gcr

docker-credential-gcr/helper: could not retrieve GCR's access token: oauth2/google: unable to generate access token: Post "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/xxxx@xxxx.iam.gserviceaccount.com:generateAccessToken": oauth2/google: unable to parse credential source

https://cloud.google.com/sdk/gcloud/reference/iam/workload-identity-pools/create-cred-config#--executable-command
https://cloud.google.com/iam/docs/workforce-obtaining-short-lived-credentials#oidc-non-int-exec