fix: Validate that source is non-empty

URI references can be empty - we have a test proving that. But the source attribute is required to be non-empty, so we should validate that.

This bug fix is a breaking change for anyone who was previously creating (and then validating) CloudEvent instances with a present-but-empty source.
However, their events were already broken, and we'd expect them to cause issues with other consumers (at least any performing comprehensive validation).

Signed-off-by: Jon Skeet <jonskeet@google.com>

src/CloudNative.CloudEvents/CloudEventsSpecVersion.cs

@@ -38,10 +38,10 @@ public sealed class CloudEventsSpecVersion
         public static CloudEventsSpecVersion V1_0 { get; } = new CloudEventsSpecVersion(
             "1.0",
             CreateRequired("id", CloudEventAttributeType.String, NonEmptyString),
-            CreateRequired("source", CloudEventAttributeType.UriReference, NonEmptyUri),
+            CreateRequired("source", CloudEventAttributeType.UriReference, NonEmptyUriReference),
             CreateRequired("type", CloudEventAttributeType.String, NonEmptyString),
-            CreateOptional("datacontenttype", CloudEventAttributeType.String, Rfc2046String), // TODO: Do we want some way of adding validation that this is per RFC 2046?
-            CreateOptional("dataschema", CloudEventAttributeType.Uri, NonEmptyUri),
+            CreateOptional("datacontenttype", CloudEventAttributeType.String, Rfc2046String),
+            CreateOptional("dataschema", CloudEventAttributeType.Uri, NonEmptyUriReference),
             CreateOptional("subject", CloudEventAttributeType.String, NonEmptyString),
             CreateOptional("time", CloudEventAttributeType.Timestamp, null));
 
@@ -166,9 +166,13 @@ private static void NonEmptyString(object value)
             }
         }
 
-        private static void NonEmptyUri(object value)
+        private static void NonEmptyUriReference(object value)
         {
-            // TODO: is this actually useful?
+            Uri uri = (Uri) value;
+            if (uri.OriginalString.Length == 0)
+            {
+                throw new ArgumentException("URI reference must be non-empty");
+            }
         }
 
         private static void Rfc2046String(object value)

test/CloudNative.CloudEvents.UnitTests/CloudEventsSpecVersionTest.cs

@@ -26,5 +26,13 @@ public void FromVersionId_Known(string versionId)
             Assert.NotNull(version);
             Assert.Equal(versionId, version!.VersionId);
         }
+
+        [Fact]
+        public void V1Source_MustBeNonEmpty()
+        {
+            var attribute = CloudEventsSpecVersion.V1_0.SourceAttribute;
+            var uri = new Uri("", UriKind.RelativeOrAbsolute);
+            Assert.Throws<ArgumentException>(() => attribute.Validate(uri));
+        }
     }
 }