feat: expand MQTT contract and reliability lessons

internal/contracts/decode.go

@@ -0,0 +1,36 @@
+package contracts
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"time"
+)
+
+const MaxPayloadBytes = 16 * 1024
+
+// Decode bounds input, rejects unknown fields, requires exactly one JSON value,
+// and validates the decoded MQTT contract against its parsed topic.
+func Decode[T MQTTMessage](payload []byte, topic Topic, now time.Time) (T, error) {
+	var value T
+	if len(payload) > MaxPayloadBytes {
+		return value, fmt.Errorf("payload exceeds %d bytes", MaxPayloadBytes)
+	}
+	decoder := json.NewDecoder(bytes.NewReader(payload))
+	decoder.DisallowUnknownFields()
+	if err := decoder.Decode(&value); err != nil {
+		return value, fmt.Errorf("decode JSON: %w", err)
+	}
+	var trailing any
+	if err := decoder.Decode(&trailing); err != io.EOF {
+		if err == nil {
+			return value, fmt.Errorf("payload contains more than one JSON value")
+		}
+		return value, fmt.Errorf("trailing data after JSON value: %w", err)
+	}
+	if err := value.Validate(topic, now); err != nil {
+		return value, fmt.Errorf("validate message: %w", err)
+	}
+	return value, nil
+}