Programmatically get current Service Account on GCP - google-cloud-platform

Question

Is there a way to programmatically access the email of the currently used Service Account on a GCP instance when no GOOGLE_APPLICATION_CREDENTIALS is set? (ie. when using the default Service Account)
I've looked through the GCP documentation, but the only resource I found can't be used with the default Service Account when no GOOGLE_APPLICATION_CREDENTIALS is set. I know that it is possible to do so using gcloud (see this SO question or documentation), however these solutions aren't applicable when running on a ContainerOptimisedOS. I've spent a couple of weeks back and forth with the GCP support team, but they concluded with not being able to help me and redirected me to Stack Overflow for help.

Answer 1

The solution of John works great, on any language without any external library. However, it works only on Google Cloud environment, when a metadata server is deployed. You can't perform this test on your computer.
I propose just bellow a piece of Python code (with Google OAuth library, but it works in other languages that have this library) to ask the library the current credential. If the credential is a service account (from GOOGLE_APPLICATION_CREDENTIALS on your computer, the ADC (Application Default Credential) or from the metadata server), you have the email printed, else, you have warning message because you use your user account credential
import google.auth
credentials, project_id = google.auth.default()
if hasattr(credentials, "service_account_email"):
print(credentials.service_account_email)
else:
print("WARNING: no service account credential. User account credential?")
Note that if the default service account is used this method will print default instead of the entire email address.
EDIT 1
ctx := context.Background()
credential,err := google.FindDefaultCredentials(ctx)
content := map[string]interface{}{}
json.Unmarshal(credential.JSON,&content)
if content["client_email"] != nil {
fmt.Println(content["client_email"])
} else {
fmt.Println("WARNING: no service account credential. User account credential?")
}

Answer 2

Just adding to the accepted answer. As stated in the answer this seems to return "default":
import google.auth
credentials, project_id = google.auth.default()
# returns "default"
print(credentials.service_account_email)
I found to get the email name of the GSA currently active (via the metadata api) I had to manually refresh first:
import google.auth
import google.auth.transport.requests
credentials, project_id = google.auth.default()
request = google.auth.transport.requests.Request()
credentials.refresh(request=request)
# returns "mygsa#myproject.iam.gserviceaccount.com"
print(credentials.service_account_email)
I'm using workload ID. I think maybe there is a race condition and I'm trying to read the service_account_email property before the creds get initialized for the first time when the pod starts?
The _retrieve_info() function is called when refresh() is called and it appears this is the function that grabs the email name.
If I had my script sleep for a few seconds on start up would service_account_email eventually be populated with the email name of the GSA or does that not happen until you manually refresh or initialize a client API or something?

Answer 3

If you are interested in getting the exact email and not just the "default" alias (when you are using the compute engine), you can fetch it using the credentials metadata. This was particularly helpful in determining which service account is being used by AI Platform jobs.
import google.auth
from google.auth.transport.requests import Request
from google.auth.compute_engine import _metadata
if hasattr(credentials, "service_account_email"):
print(credentials.service_account_email)
# edits made on top of the top answer's code
info = _metadata.get_service_account_info(Request(),
service_account=credentials.service_account_email)
print(f"Service account email: {info.email}")
print(f"Service account aliases: {info.aliases}")
print(f"Service account scopes: {info.scopes}")
else:
print("WARNING: no service account credentials available.")