zfs: read an unwritten ledger instance as state null and sequence 0, pass every error up (review finding 2, A-40)

Builds: design 42 zfs-owns-zfs A-40

modules/zfs/cmd/zfs-tools/bus.go

@@ -162,15 +162,22 @@ func (busLedger) Apply(instance, operation string, body any, call string) error
 	return err
 }
 
-// absentInstance says whether a ledger get said the instance is not there, which an instance never written is.
-func absentInstance(err error) bool {
-	s := strings.ToLower(err.Error())
-	for _, w := range []string{"not found", "no instance", "no such", "does not exist", "unknown instance", "not kept"} {
-		if strings.Contains(s, w) {
-			return true
-		}
+// takenOf reads a ledger get's answer: an instance never written answers state null and sequence 0, which is empty (A-40).
+func takenOf(raw []byte) (struct {
+	Taken map[string]LedgerEntry `json:"taken"`
+}, error) {
+	var body struct {
+		Taken map[string]LedgerEntry `json:"taken"`
 	}
-	return false
+	var st buscalls.InstanceState
+	if err := json.Unmarshal(raw, &st); err != nil {
+		return body, err
+	}
+	if st.Sequence == 0 && (len(st.State) == 0 || string(st.State) == "null") {
+		return body, nil
+	}
+	err := json.Unmarshal(st.State, &body)
+	return body, err
 }
 
 func (busLedger) Entries(datasets []string) ([]LedgerEntry, error) {
@@ -179,19 +186,10 @@ func (busLedger) Entries(datasets []string) ([]LedgerEntry, error) {
 		for _, kind := range keepKinds {
 			raw, err := stdio.BusState(buscalls.LedgerGet, buscalls.InstanceRequest{Ledger: LedgerName, Instance: LedgerInstance(kind, ds)})
 			if err != nil {
-				if absentInstance(err) {
-					continue
-				}
 				return nil, err
 			}
-			var st buscalls.InstanceState
-			if err := json.Unmarshal(raw, &st); err != nil {
-				return nil, err
-			}
-			var body struct {
-				Taken map[string]LedgerEntry `json:"taken"`
-			}
-			if err := json.Unmarshal(st.State, &body); err != nil {
+			body, err := takenOf(raw)
+			if err != nil {
 				return nil, err
 			}
 			for _, name := range sortedKeys(body.Taken) {

modules/zfs/cmd/zfs-tools/ledger_absent_test.go

@@ -0,0 +1,21 @@
+package main
+
+import "testing"
+
+func TestTakenOfLedgerAnswers(t *testing.T) {
+	for _, c := range []struct {
+		name, raw string
+		want      int
+		err       bool
+	}{
+		{"never written", `{"instance":"x","state":null,"sequence":0}`, 0, false},
+		{"written", `{"instance":"x","state":{"taken":{"a":{}}},"sequence":3}`, 1, false},
+		{"bad state", `{"instance":"x","state":"oops","sequence":3}`, 0, true},
+		{"bad answer", `nope`, 0, true},
+	} {
+		b, err := takenOf([]byte(c.raw))
+		if (err != nil) != c.err || len(b.Taken) != c.want {
+			t.Errorf("%s: %v %d", c.name, err, len(b.Taken))
+		}
+	}
+}