Fix stackoverflow in recursive definitions

core/src/main/java/org/openapitools/openapidiff/core/output/MarkdownRender.java

@@ -11,8 +11,10 @@
 import io.swagger.v3.oas.models.media.Schema;
 import io.swagger.v3.oas.models.parameters.Parameter;
 import io.swagger.v3.oas.models.responses.ApiResponse;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import org.apache.commons.lang3.StringUtils;
 import org.openapitools.openapidiff.core.model.*;
 import org.openapitools.openapidiff.core.utils.RefPointer;
@@ -35,6 +37,7 @@ public class MarkdownRender implements Render {
 
   protected RefPointer<Schema<?>> refPointer = new RefPointer<>(RefType.SCHEMAS);
   protected ChangedOpenApi diff;
+  protected Set<Schema<?>> handledSchemas = new HashSet<>();
   /**
    * A parameter which indicates whether or not metadata (summary and metadata) changes should be
    * logged in the changelog file.
@@ -43,6 +46,7 @@ public class MarkdownRender implements Render {
 
   public String render(ChangedOpenApi diff) {
     this.diff = diff;
+    this.handledSchemas.clear();
     return listEndpoints("What's New", diff.getNewEndpoints())
         + listEndpoints("What's Deleted", diff.getMissingEndpoints())
         + listEndpoints("What's Deprecated", diff.getDeprecatedEndpoints())
@@ -335,6 +339,8 @@ protected String schema(int deepness, ComposedSchema schema, DiffContext context
   }
 
   protected String schema(int deepness, Schema schema, DiffContext context) {
+    if (handledSchemas.contains(schema)) return "";
+    handledSchemas.add(schema);
     StringBuilder sb = new StringBuilder();
     sb.append(listItem(deepness, "Enum", schema.getEnum()));
     sb.append(properties(deepness, "Property", schema.getProperties(), true, context));

core/src/test/java/org/openapitools/openapidiff/core/MarkdownRenderTest.java

@@ -14,4 +14,11 @@ public void renderDoesNotFailWhenPropertyHasBeenRemoved() {
         OpenApiCompare.fromLocations("missing_property_1.yaml", "missing_property_2.yaml");
     assertThat(render.render(diff)).isNotBlank();
   }
+
+  @Test
+  public void renderDoesNotCauseStackOverflowWithRecursiveDefinitions() {
+    MarkdownRender render = new MarkdownRender();
+    ChangedOpenApi diff = OpenApiCompare.fromLocations("recursive_old.yaml", "recursive_new.yaml");
+    assertThat(render.render(diff)).isNotBlank();
+  }
 }

core/src/test/resources/recursive_new.yaml

@@ -0,0 +1,29 @@
+openapi: 3.0.1
+info:
+  title: recursive test
+  version: "1.0"
+servers:
+  - url: "http://localhost:8000/"
+paths:
+  /ping:
+    get:
+      operationId: ping
+      responses:
+        "200":
+          description: OK
+          content:
+            text/plain:
+              schema:
+                $ref: "#/components/schemas/A"
+components:
+  schemas:
+    A:
+      type: object
+      properties:
+        propname2:
+          $ref: "#/components/schemas/B"
+    B:
+      type: object
+      properties:
+        propname2:
+          $ref: "#/components/schemas/A"

core/src/test/resources/recursive_old.yaml

@@ -0,0 +1,29 @@
+openapi: 3.0.1
+info:
+  title: recursive test
+  version: "1.0"
+servers:
+  - url: "http://localhost:8000/"
+paths:
+  /ping:
+    get:
+      operationId: ping
+      responses:
+        "200":
+          description: OK
+          content:
+            text/plain:
+              schema:
+                $ref: "#/components/schemas/A"
+components:
+  schemas:
+    A:
+      type: object
+      properties:
+        propname:
+          $ref: "#/components/schemas/B"
+    B:
+      type: object
+      properties:
+        propname:
+          $ref: "#/components/schemas/A"