feat: expand MQTT contract and reliability lessons

internal/contracts/decode_test.go

@@ -0,0 +1,76 @@
+package contracts
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+var testTopic = Topic{DeviceID: "device-001", Kind: KindTelemetry}
+var testNow = time.Date(2026, 10, 13, 12, 0, 0, 0, time.UTC)
+
+// Topic tests reject malformed routes and unsupported message kinds.
+func TestParseTopic(t *testing.T) {
+	topic, err := ParseTopic("lab/v1/devices/device-001/sales")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if topic.DeviceID != "device-001" || topic.Kind != KindSales {
+		t.Fatalf("unexpected topic: %#v", topic)
+	}
+	for _, invalid := range []string{"lab/v1/devices/+/sales", "lab/v1/devices/device-001/unknown", "lab/v1/devices/device-001/sales/extra"} {
+		if _, err := ParseTopic(invalid); err == nil {
+			t.Errorf("ParseTopic(%q) succeeded", invalid)
+		}
+	}
+}
+
+// These cases cover strict JSON shape and single-value decoding.
+func TestDecodeRejectsUnknownAndTrailingValues(t *testing.T) {
+	if _, err := Decode[Heartbeat]([]byte(`{"schema_version":1,"unknown":true}`), testTopic, testNow); err == nil {
+		t.Fatal("unknown field was accepted")
+	}
+	if _, err := Decode[Heartbeat]([]byte(`{"schema_version":1,"message_id":"m-1","device_id":"device-001","occurred_at":"2026-10-13T12:00:00Z"} {"message_id":"m-2"}`), testTopic, testNow); err == nil {
+		t.Fatal("trailing JSON value was accepted")
+	}
+}
+
+// This case ensures the application limit is applied before decoding.
+func TestDecodeRejectsOversizedPayload(t *testing.T) {
+	payload := []byte(`"` + strings.Repeat("x", MaxPayloadBytes) + `"`)
+	if _, err := Decode[Heartbeat](payload, testTopic, testNow); err == nil {
+		t.Fatal("oversized payload was accepted")
+	}
+}
+
+// Decode validates the message against both its parsed route and supplied clock.
+func TestDecodeValidatesContract(t *testing.T) {
+	payload := []byte(`{"schema_version":1,"message_id":"m-1","device_id":"device-002","occurred_at":"2026-10-13T12:00:00Z"}`)
+	if _, err := Decode[Heartbeat](payload, testTopic, testNow); err == nil {
+		t.Fatal("topic/body identity mismatch was accepted")
+	}
+}
+
+// The type parameter must implement MQTTMessage; successful decoding also validates it.
+func TestDecodeAcceptsOnlyValidatedMQTTMessages(t *testing.T) {
+	payload := []byte(`{"schema_version":1,"message_id":"m-1","device_id":"device-001","occurred_at":"2026-10-13T12:00:00Z"}`)
+	message, err := Decode[Heartbeat](payload, testTopic, testNow)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if message.DeviceID != testTopic.DeviceID {
+		t.Fatalf("device_id = %q, want %q", message.DeviceID, testTopic.DeviceID)
+	}
+}
+
+// A body cannot override the device selected by the topic.
+func TestCommandRejectsTopicIdentityMismatch(t *testing.T) {
+	topic, err := ParseTopic("lab/v1/devices/device-001/commands")
+	if err != nil {
+		t.Fatal(err)
+	}
+	command := Command{SchemaVersion: SchemaVersion, MessageID: "m-1", DeviceID: "device-002", OccurredAt: time.Now().UTC().Format(time.RFC3339), CommandID: "c-1", Command: "ping"}
+	if err := command.Validate(topic, time.Now().UTC()); err == nil {
+		t.Fatal("topic/body identity mismatch was accepted")
+	}
+}