backend/remote-state/gcs: Add support for the GOOGLE_PROJECT environment variable.

This copies behavior from the Google Cloud provider.

backend/remote-state/gcs/backend.go

@@ -23,10 +23,11 @@ type gcsBackend struct {
 	storageClient  *storage.Client
 	storageContext context.Context
 
-	projectID        string
 	bucketName       string
 	prefix           string
 	defaultStateFile string
+
+	projectID string
 }
 
 func New() backend.Backend {
@@ -85,12 +86,16 @@ func (b *gcsBackend) configure(ctx context.Context) error {
 
 	data := schema.FromContextBackendConfig(b.storageContext)
 
-	b.projectID = data.Get("project").(string)
 	b.bucketName = data.Get("bucket").(string)
 	b.prefix = strings.TrimLeft(data.Get("prefix").(string), "/")
 
 	b.defaultStateFile = strings.TrimLeft(data.Get("path").(string), "/")
 
+	b.projectID = data.Get("project").(string)
+	if id := os.Getenv("GOOGLE_PROJECT"); b.projectID == "" && id != "" {
+		b.projectID = id
+	}
+
 	opts := []option.ClientOption{
 		option.WithScopes(storage.ScopeReadWrite),
 		option.WithUserAgent(terraform.UserAgentString()),

website/docs/backends/types/gcs.html.md

@@ -55,5 +55,5 @@ The following configuration options are supported:
     The provided credentials need to have the `devstorage.read_write` scope and `WRITER` permissions on the bucket.
  *  `prefix` - (Optional) GCS prefix inside the bucket. Named states are stored in an object called `<prefix>/<name>.tfstate`.
  *  `path` - (Deprecated) GCS path to the state file of the default state. For backwards compatibility only, use `prefix` instead.
- *  `project` - (Optional) The project ID to which the bucket belongs. This is only used when creating a new bucket during initialization.
+ *  `project` / `GOOGLE_PROJECT` - (Optional) The project ID to which the bucket belongs. This is only used when creating a new bucket during initialization.
     Since buckets have globally unique names, the project ID is not required to access the bucket during normal operation.