Reject "data" as the name of an extension attribute

This isn't currently in the CloudEvents spec, but was agreed at
the meeting of 2022-02-03, on the grounds that the most prominent
event formt - JSON - already *implicitly* prohibits it.

We should wait until the spec is updated before merging this.

Signed-off-by: Jon Skeet <[email protected]>

Author: jskeet

src/CloudNative.CloudEvents/CloudEventAttribute.cs

@@ -4,6 +4,7 @@
 
 using CloudNative.CloudEvents.Core;
 using System;
+using System.Collections.Generic;
 
 namespace CloudNative.CloudEvents
 {
@@ -15,6 +16,12 @@ namespace CloudNative.CloudEvents
     /// </summary>
     public class CloudEventAttribute
     {
+        private static readonly IList<string> ReservedNames = new List<string>
+        {
+            CloudEventsSpecVersion.SpecVersionAttributeName,
+            "data"
+        };
+
         /// <summary>
         /// The type of the attribute. All values provided must be compatible with this.
         /// </summary>
@@ -60,7 +67,7 @@ public static CloudEventAttribute CreateExtension(string name, CloudEventAttribu
         {
             Validation.CheckNotNull(name, nameof(name));
             Validation.CheckNotNull(type, nameof(type));
-            if (name == CloudEventsSpecVersion.SpecVersionAttributeName)
+            if (ReservedNames.Contains(name))
             {
                 throw new ArgumentException($"The attribute name '{name}' is reserved and cannot be used for an extension attribute.");
             }

test/CloudNative.CloudEvents.UnitTests/CloudEventAttributeTest.cs

@@ -78,6 +78,11 @@ public void CreateExtension_SpecVersionName() =>
             Assert.Throws<ArgumentException>(() =>
                 CloudEventAttribute.CreateExtension(CloudEventsSpecVersion.SpecVersionAttributeName, CloudEventAttributeType.String));
 
+        [Fact]
+        public void CreateExtension_DataName() =>
+            Assert.Throws<ArgumentException>(() =>
+                CloudEventAttribute.CreateExtension("data", CloudEventAttributeType.String));
+
         [Fact]
         public void Validate_NoValidator_Valid()
         {