Simplify OpenApiDocumentReferencesAreValid

Remove reparsing of the document and just validate the in-memory `OpenApiDocument` instead.

Author: martincostello

src/Microsoft.OpenApi/Validations/Rules/OpenApiDocumentRules.cs

@@ -2,8 +2,6 @@
 // Licensed under the MIT license.
 
 using System;
-using System.Text.Json.Nodes;
-using Microsoft.OpenApi.Reader;
 
 namespace Microsoft.OpenApi
 {
@@ -39,101 +37,54 @@ public static class OpenApiDocumentRules
                 {
                     const string RuleName = nameof(OpenApiDocumentReferencesAreValid);
 
-                    JsonNode document;
-
-                    using (var textWriter = new System.IO.StringWriter(System.Globalization.CultureInfo.InvariantCulture))
-                    {
-                        var writer = new OpenApiJsonWriter(textWriter);
-
-                        item.SerializeAsV31(writer);
-
-                        var json = textWriter.ToString();
-
-                        document = JsonNode.Parse(json)!;
-                    }
-
-                    var visitor = new OpenApiSchemaReferenceVisitor(RuleName, context, document);
+                    var visitor = new OpenApiSchemaReferenceVisitor(RuleName, context);
                     var walker = new OpenApiWalker(visitor);
 
                     walker.Walk(item);
                 });
 
         private sealed class OpenApiSchemaReferenceVisitor(
             string ruleName,
-            IValidationContext context,
-            JsonNode document) : OpenApiVisitorBase
+            IValidationContext context) : OpenApiVisitorBase
         {
             public override void Visit(IOpenApiReferenceHolder referenceHolder)
             {
-                if (referenceHolder is OpenApiSchemaReference { Reference.IsLocal: true } reference)
+                if (referenceHolder is OpenApiSchemaReference reference)
                 {
                     ValidateSchemaReference(reference);
                 }
             }
 
             public override void Visit(IOpenApiSchema schema)
             {
-                if (schema is OpenApiSchemaReference { Reference.IsLocal: true } reference)
+                if (schema is OpenApiSchemaReference reference)
                 {
                     ValidateSchemaReference(reference);
                 }
             }
 
             private void ValidateSchemaReference(OpenApiSchemaReference reference)
             {
-                try
+                if (!reference.Reference.IsLocal)
                 {
-                    if (reference.RecursiveTarget is not null)
-                    {
-                        // The reference was followed to a valid schema somewhere in the document
-                        return;
-                    }
-                }
-                catch (InvalidOperationException ex)
-                {
-                    context.Enter(GetSegment());
-                    context.CreateWarning(ruleName, ex.Message);
-                    context.Exit();
-
                     return;
                 }
 
-                var id = reference.Reference.ReferenceV3;
-
-                if (id is { Length: > 0 } && !IsValidSchemaReference(id, document))
+                try
                 {
-                    var isValid = false;
-
-                    // Sometimes ReferenceV3 is not a JSON valid JSON pointer, but the $ref
-                    // associated with it still points to a valid location in the document.
-                    // In these cases, we need to find it manually to verify that fact before
-                    // generating a warning that the schema reference is indeed invalid.
-                    // TODO Why is this, and can it be avoided?
-                    var parent = Find(PathString, document);
-
-                    if (parent?["$ref"] is { } @ref &&
-                        @ref.GetValueKind() is System.Text.Json.JsonValueKind.String &&
-                        @ref.GetValue<string>() is { Length: > 0 } refId)
-                    {
-                        id = refId;
-                        isValid = IsValidSchemaReference(id, document);
-                    }
-
-                    if (!isValid)
+                    if (reference.RecursiveTarget is null)
                     {
+                        // The reference was not followed to a valid schema somewhere in the document
                         context.Enter(GetSegment());
-                        context.CreateWarning(ruleName, string.Format(SRResource.Validation_SchemaReferenceDoesNotExist, id));
+                        context.CreateWarning(ruleName, string.Format(SRResource.Validation_SchemaReferenceDoesNotExist, reference.Reference.ReferenceV3));
                         context.Exit();
                     }
                 }
-
-                static bool IsValidSchemaReference(string id, JsonNode baseNode)
-                    => Find(id, baseNode) is not null;
-
-                static JsonNode? Find(string id, JsonNode baseNode)
+                catch (InvalidOperationException ex)
                 {
-                    var pointer = new JsonPointer(id.Replace("#/", "/"));
-                    return pointer.Find(baseNode);
+                    context.Enter(GetSegment());
+                    context.CreateWarning(ruleName, ex.Message);
+                    context.Exit();
                 }
 
                 string GetSegment()