Add a visitor that walks through an OpenApi document and sets the host document value for proxy reference objects

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/OpenApiYamlDocumentReader.cs

@@ -59,6 +59,8 @@ public OpenApiDocument Read(JsonNode input, out OpenApiDiagnostic diagnostic)
                 {
                     throw new InvalidOperationException("Cannot load external refs using the synchronous Read, use ReadAsync instead.");
                 }
+
+                SetHostDocument(document);
             }
             catch (OpenApiException ex)
             {
@@ -107,6 +109,8 @@ public async Task<ReadResult> ReadAsync(JsonNode input, CancellationToken cancel
                         diagnostic.Warnings.AddRange(diagnosticExternalRefs.Warnings);
                     }
                 }
+
+                SetHostDocument(document);
             }
             catch (OpenApiException ex)
             {
@@ -145,6 +149,11 @@ private Task<OpenApiDiagnostic> LoadExternalRefs(OpenApiDocument document, Cance
             return workspaceLoader.LoadAsync(new() { ExternalResource = "/" }, document, null, cancellationToken);
         }
 
+        private void SetHostDocument(OpenApiDocument document)
+        {
+            document.SetHostDocument();
+        }
+
         /// <summary>
         /// Reads the stream input and parses the fragment of an OpenAPI description into an Open API Element.
         /// </summary>

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -452,6 +452,16 @@ public IEnumerable<OpenApiError> ResolveReferences()
             return resolver.Errors;
         }
 
+        /// <summary>
+        /// Walks the OpenApiDocument and sets the host document for all referenceable objects
+        /// </summary>
+        public void SetHostDocument()
+        {
+            var resolver = new HostDocumentResolver(this);
+            var walker = new OpenApiWalker(resolver);
+            walker.Walk(this);
+        }
+
         /// <summary>
         /// Load the referenced <see cref="IOpenApiReferenceable"/> object from a <see cref="OpenApiReference"/> object
         /// </summary>

src/Microsoft.OpenApi/Services/HostDocumentResolver.cs

@@ -0,0 +1,30 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+using Microsoft.OpenApi.Interfaces;
+using Microsoft.OpenApi.Models;
+
+namespace Microsoft.OpenApi.Services
+{
+    internal class HostDocumentResolver : OpenApiVisitorBase
+    {
+        private OpenApiDocument _currentDocument;
+
+        public HostDocumentResolver(OpenApiDocument currentDocument)
+        {
+            _currentDocument = currentDocument;
+        }
+
+        /// <summary>
+        /// Visits the referenceable element in the host document
+        /// </summary>
+        /// <param name="referenceable">The referenceable element in the doc.</param>
+        public override void Visit(IOpenApiReferenceable referenceable)
+        {
+            if (referenceable.Reference != null)
+            {
+                referenceable.Reference.HostDocument = _currentDocument;
+            }
+        }
+    }
+}