Implement deep copy for OpenApiSchema instances (#57223)

* Implement deep copy for OpenApiSchema instances

* Add more tests, use new List(), and fix reference comparer

* Add comment and clean up null check

* Update benchmark method return values

Author: captainsafia

src/OpenApi/perf/Microbenchmarks/OpenApiSchemaComparerBenchmark.cs

@@ -16,7 +16,7 @@ public class OpenApiSchemaComparerBenchmark
 
     private OpenApiSchema _schema;
 
-    [GlobalSetup(Target = nameof(OpenApiSchema_GetHashCode))]
+    [GlobalSetup]
     public void OpenApiSchema_Setup()
     {
         _schema = new OpenApiSchema
@@ -56,8 +56,8 @@ public void OpenApiSchema_Setup()
     }
 
     [Benchmark]
-    public void OpenApiSchema_GetHashCode()
-    {
-        OpenApiSchemaComparer.Instance.GetHashCode(_schema);
-    }
+    public int OpenApiSchema_GetHashCode() => OpenApiSchemaComparer.Instance.GetHashCode(_schema);
+
+    [Benchmark]
+    public OpenApiSchema OpenApiSchema_Clone() => OpenApiSchemaExtensions.Clone(_schema);
 }

src/OpenApi/src/Comparers/OpenApiReferenceComparer.cs

@@ -24,8 +24,10 @@ public bool Equals(OpenApiReference? x, OpenApiReference? y)
             return true;
         }
 
+        // We avoid comparing the HostDocument that a reference is associated with
+        // so that the same schema in the OpenApiSchemaStore cache and embedded in
+        // an OpenAPI document is considered equal.
         return x.ExternalResource == y.ExternalResource &&
-            x.HostDocument?.HashCode == y.HostDocument?.HashCode &&
             x.Id == y.Id &&
             x.Type == y.Type;
     }

src/OpenApi/src/Extensions/OpenApiSchemaExtensions.cs

@@ -0,0 +1,69 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Linq;
+using Microsoft.OpenApi.Any;
+using Microsoft.OpenApi.Interfaces;
+using Microsoft.OpenApi.Models;
+
+namespace Microsoft.AspNetCore.OpenApi;
+
+internal static class OpenApiSchemaExtensions
+{
+    /// <summary>
+    /// Generates a deep copy of a given <see cref="OpenApiSchema"/> instance.
+    /// </summary>
+    /// <remarks>
+    /// The copy constructors of the <see cref="OpenApiSchema"/> class do not perform a deep
+    /// copy of the instance which presents a problem whe making modifications in deeply nested
+    /// subschemas. This extension implements a deep copy on <see cref="OpenApiSchema" /> to guarantee
+    /// that modifications on cloned subschemas do not affect the original subschema.
+    /// /// </remarks>
+    /// <param name="schema">The <see cref="OpenApiSchema"/> to generate a deep copy of.</param>
+    public static OpenApiSchema Clone(this OpenApiSchema schema)
+    {
+        return new OpenApiSchema
+        {
+            Title = schema.Title,
+            Type = schema.Type,
+            Format = schema.Format,
+            Description = schema.Description,
+            Maximum = schema.Maximum,
+            ExclusiveMaximum = schema.ExclusiveMaximum,
+            Minimum = schema.Minimum,
+            ExclusiveMinimum = schema.ExclusiveMinimum,
+            MaxLength = schema.MaxLength,
+            MinLength = schema.MinLength,
+            Pattern = schema.Pattern,
+            MultipleOf = schema.MultipleOf,
+            Default = OpenApiAnyCloneHelper.CloneFromCopyConstructor<IOpenApiAny>(schema.Default),
+            ReadOnly = schema.ReadOnly,
+            WriteOnly = schema.WriteOnly,
+            AllOf = schema.AllOf != null ? new List<OpenApiSchema>(schema.AllOf.Select(s => s.Clone())) : null,
+            OneOf = schema.OneOf != null ? new List<OpenApiSchema>(schema.OneOf.Select(s => s.Clone())) : null,
+            AnyOf = schema.AnyOf != null ? new List<OpenApiSchema>(schema.AnyOf.Select(s => s.Clone())) : null,
+            Not = schema.Not?.Clone(),
+            Required = schema.Required != null ? new HashSet<string>(schema.Required) : null,
+            Items = schema.Items?.Clone(),
+            MaxItems = schema.MaxItems,
+            MinItems = schema.MinItems,
+            UniqueItems = schema.UniqueItems,
+            Properties = schema.Properties?.ToDictionary(kvp => kvp.Key, kvp => kvp.Value.Clone()),
+            MaxProperties = schema.MaxProperties,
+            MinProperties = schema.MinProperties,
+            AdditionalPropertiesAllowed = schema.AdditionalPropertiesAllowed,
+            AdditionalProperties = schema.AdditionalProperties?.Clone(),
+            Discriminator = schema.Discriminator != null ? new(schema.Discriminator) : null,
+            Example = OpenApiAnyCloneHelper.CloneFromCopyConstructor<IOpenApiAny>(schema.Example),
+            Enum = schema.Enum != null ? new List<IOpenApiAny>(schema.Enum) : null,
+            Nullable = schema.Nullable,
+            ExternalDocs = schema.ExternalDocs != null ? new(schema.ExternalDocs) : null,
+            Deprecated = schema.Deprecated,
+            Xml = schema.Xml != null ? new(schema.Xml) : null,
+            Extensions = schema.Extensions != null ? new Dictionary<string, IOpenApiExtension>(schema.Extensions) : null,
+            UnresolvedReference = schema.UnresolvedReference,
+            Reference = schema.Reference != null ? new(schema.Reference) : null,
+            Annotations = schema.Annotations != null ? new Dictionary<string, object>(schema.Annotations) : null,
+        };
+    }
+}

src/OpenApi/src/Transformers/Implementations/OpenApiSchemaReferenceTransformer.cs

@@ -32,7 +32,7 @@ public Task TransformAsync(OpenApiDocument document, OpenApiDocumentTransformerC
                 // the transformation process are consistent.
                 document.Components.Schemas.Add(
                     referenceId,
-                    ResolveReferenceForSchema(new OpenApiSchema(schema), schemasByReference, isTopLevel: true));
+                    ResolveReferenceForSchema(schema.Clone(), schemasByReference, isTopLevel: true));
             }
         }
 

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Comparers/OpenApiSchemaComparerTests.cs

@@ -303,9 +303,10 @@ public void ValidatePropertiesOnOpenApiSchema()
         Assert.False(OpenApiSchemaComparer.Instance.Equals(originalSchema, modifiedSchema));
         Assert.True(propertyNames.Remove(nameof(OpenApiSchema.Xml)));
 
+        // Disregard annotations in comparison checks since they are in-memory constructs
         modifiedSchema = new(originalSchema);
         modifiedSchema.Annotations["key"] = "another value";
-        Assert.False(OpenApiSchemaComparer.Instance.Equals(originalSchema, modifiedSchema));
+        Assert.True(OpenApiSchemaComparer.Instance.Equals(originalSchema, modifiedSchema));
         Assert.True(propertyNames.Remove(nameof(OpenApiSchema.Annotations)));
 
         Assert.Empty(propertyNames);