compute/metadata: add an error type for undefined metadata

Change-Id: I215d03cd6443903fd642c8558c1ecb8a32a369ca

compute/metadata/metadata.go

@@ -73,8 +73,23 @@ func dialer() *net.Dialer {
 	}
 }
 
+// NotDefinedError is returned when requested metadata is not defined.
+//
+// The underlying string is the suffix after "/computeMetadata/v1/".
+//
+// This error is not returned if the value is defined to be the empty
+// string.
+type NotDefinedError string
+
+func (suffix NotDefinedError) Error() string {
+	return fmt.Sprintf("metadata: GCE metadata %q not defined", string(suffix))
+}
+
 // Get returns a value from the metadata service.
 // The suffix is appended to "http://metadata/computeMetadata/v1/".
+//
+// If the requested metadata is not defined, the returned error will
+// be of type NotDefinedError.
 func Get(suffix string) (string, error) {
 	// Using 169.254.169.254 instead of "metadata" here because Go
 	// binaries built with the "netgo" tag and without cgo won't
@@ -89,6 +104,9 @@ func Get(suffix string) (string, error) {
 		return "", err
 	}
 	defer res.Body.Close()
+	if res.StatusCode == http.StatusNotFound {
+		return "", NotDefinedError(suffix)
+	}
 	if res.StatusCode != 200 {
 		return "", fmt.Errorf("status code %d trying to fetch %s", res.StatusCode, url)
 	}
@@ -216,12 +234,24 @@ func lines(suffix string) ([]string, error) {
 
 // InstanceAttributeValue returns the value of the provided VM
 // instance attribute.
+//
+// If the requested attribute is not defined, the returned error will
+// be of type NotDefinedError.
+//
+// InstanceAttributeValue may return ("", nil) if the attribute was
+// defined to be the empty string.
 func InstanceAttributeValue(attr string) (string, error) {
 	return Get("instance/attributes/" + attr)
 }
 
 // ProjectAttributeValue returns the value of the provided
 // project attribute.
+//
+// If the requested attribute is not defined, the returned error will
+// be of type NotDefinedError.
+//
+// ProjectAttributeValue may return ("", nil) if the attribute was
+// defined to be the empty string.
 func ProjectAttributeValue(attr string) (string, error) {
 	return Get("project/attributes/" + attr)
 }