Failed to retrieve function source code when deploying a cloud function from a repository on a different project - google-cloud-functions

Question

I am trying to deploy a Cloud Function from a Cloud Source Repository placed in a different project, but getting the following error: Failed to retrieve function source code (see full proto below).
Project-A contains the cloud function and service accounts listed below.
Project-B contains the source repository.
I have successfully deployed the function on Project-B.
I've tried giving the following service accounts the Source Repository Administrator role on the cloud source repository, but that did not help.
{project_A_number}#cloudservices.gserviceaccount.com
{project_A_number}-compute#developer.gserviceaccount.com
{project_A_number}#cloudbuild.gserviceaccount.com
Project-A#appspot.gserviceaccount.com
I have also tried disabling the Cloud Functions API on Project-A and turning it back on again.
I am not sure what is going wrong - if anyone has a clue as to where to further look, I would appreciate it - thanks in advance!
The deployment creates two entries in monitoring - a NOTICE followed by an ERROR:
The ERROR log:
{
"protoPayload": {
"#type": "type.googleapis.com/google.cloud.audit.AuditLog",
"status": {
"code": 5,
"message": "Failed to retrieve function source code"
},
"authenticationInfo": {
"principalEmail": "***#***.**"
},
"serviceName": "cloudfunctions.googleapis.com",
"methodName": "google.cloud.functions.v1.CloudFunctionsService.UpdateFunction",
"resourceName": "projects/Project-A/locations/europe-west1/functions/pubsub-to-gcs"
},
"insertId": "-vmfbt4cd54",
"resource": {
"type": "cloud_function",
"labels": {
"function_name": "pubsub-to-gcs",
"region": "europe-west1",
"project_id": "Project-A"
}
},
"timestamp": "2021-10-20T12:21:45.352043Z",
"severity": "ERROR",
"logName": "projects/Project-A/logs/cloudaudit.googleapis.com%2Factivity",
"operation": {
"id": "operations/cm9ldHotbGlmZS1kYXRhLXRlc3QvZXVyb3BlLXdlc3QxL3B1YnN1Yi10by1nY3MvVEhFbUQtLTZITWM",
"producer": "cloudfunctions.googleapis.com",
"last": true
},
"receiveTimestamp": "2021-10-20T12:21:45.781856467Z"
}
The NOTICE log (logged right before the ERROR):
{
"protoPayload": {
"#type": "type.googleapis.com/google.cloud.audit.AuditLog",
"authenticationInfo": {
"principalEmail": "***#****.**"
},
"requestMetadata": {
"callerIp": "35.205.252.75",
"callerSuppliedUserAgent": "google-cloud-sdk gcloud/360.0.0 command/gcloud.functions.deploy invocation-id/917d697431e84b91bfa2bd9f9cc4f302 environment/devshell environment-version/None interactive/True from-script/False python/3.7.3 term/screen (Linux 5.4.144+),gzip(gfe),gzip(gfe)",
"requestAttributes": {
"time": "2021-10-20T12:21:44.909430Z",
"auth": {}
},
"destinationAttributes": {}
},
"serviceName": "cloudfunctions.googleapis.com",
"methodName": "google.cloud.functions.v1.CloudFunctionsService.UpdateFunction",
"authorizationInfo": [
{
"resource": "projects/Project-A/locations/europe-west1/functions/pubsub-to-gcs",
"permission": "cloudfunctions.functions.update",
"granted": true,
"resourceAttributes": {}
}
],
"resourceName": "projects/Project-A/locations/europe-west1/functions/pubsub-to-gcs",
"request": {
"#type": "type.googleapis.com/google.cloud.functions.v1.UpdateFunctionRequest",
"function": {
"timeout": "60s",
"status": "UNKNOWN",
"serviceAccountEmail": "Project-A#appspot.gserviceaccount.com",
"availableMemoryMb": 256,
"name": "projects/Project-A/locations/europe-west1/functions/pubsub-to-gcs",
"runtime": "python39",
"labels": {
"deployment-tool": "cli-gcloud"
},
"entryPoint": "pubsub-to-gcs",
"updateTime": "2021-10-20T12:21:40.149Z",
"sourceRepository": {
"url": "https://source.developers.google.com/projects/Project-B/repos/my-repo/moveable-aliases/master/paths/my-folder"
},
"httpsTrigger": {},
"ingressSettings": "ALLOW_ALL",
"versionId": "1"
},
"updateMask": "eventTrigger,httpsTrigger,runtime,sourceRepository"
},
"resourceLocation": {
"currentLocations": [
"europe-west1"
]
}
},
"insertId": "1xdbim3e16pgu",
"resource": {
"type": "cloud_function",
"labels": {
"function_name": "pubsub-to-gcs",
"region": "europe-west1",
"project_id": "Project-A"
}
},
"timestamp": "2021-10-20T12:21:44.650257Z",
"severity": "NOTICE",
"logName": "projects/Project-A/logs/cloudaudit.googleapis.com%2Factivity",
"operation": {
"id": "operations/cm9ldHotbGlmZS1kYXRhLXRlc3QvZXVyb3BlLXdlc3QxL3B1YnN1Yi10by1nY3MvVEhFbUQtLTZITWM",
"producer": "cloudfunctions.googleapis.com",
"first": true
},
"receiveTimestamp": "2021-10-20T12:21:45.832588036Z"
}

Answer 1

Turns out it wasn't an IAM issue: I've tried deploying the function from the UI, but that's not possible when deploying from a source repo in a different project.
Deploying using gcloud function deploy solved the issue.