feat: support relative URI resolution in $dynamicRef (#2928)

src/Microsoft.OpenApi/Reader/V31/OpenApiSchemaDeserializer.cs

@@ -455,7 +455,10 @@ public static IOpenApiSchema LoadSchema(JsonNode node, OpenApiDocument hostDocum
         {
             var nodeLocation = context.GetLocation();
             var anchorName = JsonNodeHelper.ExtractDynamicAnchorName(dynamicPointer);
-            var result = new OpenApiSchemaReference(!string.IsNullOrEmpty(anchorName) ? anchorName! : dynamicPointer, hostDocument);
+            string? externalResource = JsonNodeHelper.IsFragmentOnlyDynamicRef(dynamicPointer)
+                ? null
+                : JsonNodeHelper.ExtractDocumentUri(dynamicPointer);
+            var result = new OpenApiSchemaReference(!string.IsNullOrEmpty(anchorName) ? anchorName! : dynamicPointer, hostDocument, externalResource);
             var referenceMetadata = new OpenApiSchema();
             jsonObject.ParseMap(referenceMetadata, _openApiSchemaFixedFields, _openApiSchemaPatternFields, hostDocument, context,
                 static (schema, name, value) =>

src/Microsoft.OpenApi/Reader/V32/OpenApiSchemaDeserializer.cs

@@ -455,7 +455,10 @@ public static IOpenApiSchema LoadSchema(JsonNode node, OpenApiDocument hostDocum
         {
             var nodeLocation = context.GetLocation();
             var anchorName = JsonNodeHelper.ExtractDynamicAnchorName(dynamicPointer);
-            var result = new OpenApiSchemaReference(!string.IsNullOrEmpty(anchorName) ? anchorName! : dynamicPointer, hostDocument);
+            string? externalResource = JsonNodeHelper.IsFragmentOnlyDynamicRef(dynamicPointer)
+                ? null
+                : JsonNodeHelper.ExtractDocumentUri(dynamicPointer);
+            var result = new OpenApiSchemaReference(!string.IsNullOrEmpty(anchorName) ? anchorName! : dynamicPointer, hostDocument, externalResource);
             var referenceMetadata = new OpenApiSchema();
             jsonObject.ParseMap(referenceMetadata, _openApiSchemaFixedFields, _openApiSchemaPatternFields, hostDocument, context,
                 static (schema, name, value) =>

src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs

@@ -689,6 +689,12 @@ private void RegisterAnchor(OpenApiDocument document, string anchorName, IOpenAp
         /// For fragment-only refs (#node), resolves against the host document.
         /// For URI refs (absolute), finds the external document and resolves there.
         /// </summary>
+        /// <summary>
+        /// Resolves a $dynamicRef value against the workspace's anchor registries.
+        /// For fragment-only refs (#node), resolves against the host document.
+        /// For URI refs (absolute or relative), finds the external document and resolves there.
+        /// Relative URIs are resolved against the host document's BaseUri.
+        /// </summary>
         internal IOpenApiSchema? ResolveDynamicRef(OpenApiDocument hostDocument, string dynamicRef)
         {
             var anchorName = JsonNodeHelper.ExtractDynamicAnchorName(dynamicRef);
@@ -698,7 +704,16 @@ private void RegisterAnchor(OpenApiDocument document, string anchorName, IOpenAp
             if (!JsonNodeHelper.IsFragmentOnlyDynamicRef(dynamicRef))
             {
                 var docUri = JsonNodeHelper.ExtractDocumentUri(dynamicRef);
-                targetDoc = docUri is not null ? FindDocumentByBaseUri(docUri) : null;
+                if (docUri is not null)
+                {
+                    if (!Uri.IsWellFormedUriString(docUri, UriKind.Absolute) && hostDocument.BaseUri is not null)
+                        docUri = new Uri(hostDocument.BaseUri, docUri).AbsoluteUri;
+                    targetDoc = FindDocumentByBaseUri(docUri);
+                }
+                else
+                {
+                    targetDoc = null;
+                }
             }
 
             if (targetDoc is null) return null;

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiDynamicRefTests.cs

@@ -38,6 +38,7 @@ public void BareDynamicRefDeserializesAsSchemaReference()
         var reference = Assert.IsType<OpenApiSchemaReference>(result);
         Assert.Equal("#category", reference.Reference.DynamicRef);
         Assert.True(reference.Reference.IsDynamicRefOnly);
+        Assert.Null(reference.Reference.ExternalResource);
     }
 
     [Fact]
@@ -634,7 +635,7 @@ public async Task AbsoluteDynamicRefResolvesAcrossDocuments()
         docA.BaseUri = new("https://example.com/main");
         docB.BaseUri = new("https://example.com/external");
 
-        var workspace = docA.Workspace ?? new OpenApiWorkspace();
+        var workspace = docA.Workspace!;
         workspace.RegisterComponents(docA);
         workspace.RegisterComponents(docB);
 
@@ -1493,14 +1494,75 @@ public async Task ExternalDynamicRefResolvesToPlainAnchorInExternalDocument()
         docA.BaseUri = new("https://example.com/main");
         docB.BaseUri = new("https://example.com/external");
 
-        var workspace = docA.Workspace ?? new OpenApiWorkspace();
+        var workspace = docA.Workspace!;
         workspace.RegisterComponents(docA);
         workspace.RegisterComponents(docB);
 
         var tree = docA.Components.Schemas["Tree"];
         var next = tree.Properties["next"];
         var reference = Assert.IsType<OpenApiSchemaReference>(next);
         Assert.True(reference.Reference.IsDynamicRefOnly);
+        Assert.Equal("https://example.com/external", reference.Reference.ExternalResource);
+        Assert.NotNull(reference.Target);
+        Assert.Same(docB.Components.Schemas["ExternalNode"], reference.Target);
+    }
+
+    [Fact]
+    public async Task ExternalDynamicRefWithRelativeUriResolvesAcrossDocuments()
+    {
+        var yamlA =
+        """
+        openapi: 3.1.0
+        info:
+          title: Doc A
+          version: 1.0.0
+        paths: {}
+        components:
+          schemas:
+            Tree:
+              type: object
+              properties:
+                next:
+                  $dynamicRef: 'external.yaml#node'
+        """;
+
+        var yamlB =
+        """
+        openapi: 3.1.0
+        info:
+          title: Doc B
+          version: 1.0.0
+        paths: {}
+        components:
+          schemas:
+            ExternalNode:
+              $dynamicAnchor: node
+              type: object
+              properties:
+                value:
+                  type: string
+        """;
+
+        using var streamA = new MemoryStream(Encoding.UTF8.GetBytes(yamlA));
+        using var streamB = new MemoryStream(Encoding.UTF8.GetBytes(yamlB));
+        var resultA = await OpenApiDocument.LoadAsync(streamA, "yaml", SettingsFixture.ReaderSettings);
+        var resultB = await OpenApiDocument.LoadAsync(streamB, "yaml", SettingsFixture.ReaderSettings);
+
+        var docA = resultA.Document;
+        var docB = resultB.Document;
+
+        docA.BaseUri = new("https://example.com/main.yaml");
+        docB.BaseUri = new("https://example.com/external.yaml");
+
+        var workspace = docA.Workspace!;
+        workspace.RegisterComponents(docA);
+        workspace.RegisterComponents(docB);
+
+        var tree = docA.Components.Schemas["Tree"];
+        var next = tree.Properties["next"];
+        var reference = Assert.IsType<OpenApiSchemaReference>(next);
+        Assert.True(reference.Reference.IsDynamicRefOnly);
+        Assert.Equal("external.yaml", reference.Reference.ExternalResource);
         Assert.NotNull(reference.Target);
         Assert.Same(docB.Components.Schemas["ExternalNode"], reference.Target);
     }