Updating OpenAPI3 emitter to sort response header in Lexicographic order

.chronus/changes/azhang_fixResponseHeadersOrdering-2024-2-12-23-34-6.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Update OpenAPI3 emitter to emit response headers in lexicographic order

packages/openapi3/src/openapi.ts

@@ -983,7 +983,7 @@ function createOAPIEmitter(
         obj.headers ??= {};
         // OpenAPI can't represent different headers per content type.
         // So we merge headers here, and report any duplicates unless they are identical
-        for (const [key, value] of Object.entries(data.headers)) {
+        for (const [key, value] of Object.entries(data.headers).sort()) {
           const headerVal = getResponseHeader(value);
           const existing = obj.headers[key];
           if (existing) {

packages/openapi3/test/response-headers-order.test.ts

@@ -0,0 +1,40 @@
+import { strictEqual } from "assert";
+import { describe, it } from "vitest";
+import { openApiFor } from "./test-host.js";
+
+describe("openapi3: response headers order", () => {
+  const headerDefinitions = `
+      model A_Header { @header a : string };
+      model B_Header { @header b : string };
+      model C_Header { @header c : string };
+  `;
+  it("header already in lexical order", async () => {
+    const res = await openApiFor(
+      `
+      ${headerDefinitions}
+      model Headers { ...A_Header, ...B_Header, ...C_Header };
+
+      op read(): {@statusCode _: 200, content: string, headers: Headers};
+      `
+    );
+    strictEqual(Object.keys(res.paths["/"].get.responses["200"].headers).length, 3);
+    strictEqual(Object.keys(res.paths["/"].get.responses["200"].headers)[0], "a");
+    strictEqual(Object.keys(res.paths["/"].get.responses["200"].headers)[1], "b");
+    strictEqual(Object.keys(res.paths["/"].get.responses["200"].headers)[2], "c");
+  });
+
+  it("header not in lexical order", async () => {
+    const res = await openApiFor(
+      `
+      ${headerDefinitions}
+      model Headers { ...C_Header, ...A_Header, ...B_Header };
+
+      op read(): {@statusCode _: 200, content: string, headers: Headers};
+      `
+    );
+    strictEqual(Object.keys(res.paths["/"].get.responses["200"].headers).length, 3);
+    strictEqual(Object.keys(res.paths["/"].get.responses["200"].headers)[0], "a");
+    strictEqual(Object.keys(res.paths["/"].get.responses["200"].headers)[1], "b");
+    strictEqual(Object.keys(res.paths["/"].get.responses["200"].headers)[2], "c");
+  });
+});