FormData: match WebKit's multipart boundary exactly

src/bun.js/webcore/Blob.zig

@@ -585,7 +585,7 @@ pub fn fromDOMFormData(
     var hex_buf: [70]u8 = undefined;
     const boundary = brk: {
         var random = globalThis.bunVM().rareData().nextUUID().bytes;
-        break :brk std.fmt.bufPrint(&hex_buf, "-WebkitFormBoundary{x}", .{&random}) catch unreachable;
+        break :brk std.fmt.bufPrint(&hex_buf, "----WebKitFormBoundary{x}", .{&random}) catch unreachable;
     };
 
     var context = FormDataContext{

test/js/bun/http/form-data-set-append.test.js

@@ -1,7 +1,46 @@
 import { expect, test } from "bun:test";
 
-// https://github.com/oven-sh/bun/issues/12325
+// https://github.com/oven-sh/bun/issues/29630
+// FormData must produce a boundary matching WebKit exactly:
+//   declared: "----WebKitFormBoundary{hex}" (4 leading dashes, capital K)
+//   body markers: "------WebKitFormBoundary{hex}" (6 dashes)
+// Previously Bun emitted "-WebkitFormBoundary{hex}" (1 dash, lowercase k),
+// which produced 3-dash body markers — a format no other mainstream client
+// uses and which trips buggy downstream multipart parsers.
+test("FormData request body uses WebKit-compatible boundary (#29630)", async () => {
+  const fd = new FormData();
+  fd.append("file", new Blob([new Uint8Array([1, 2, 3])], { type: "application/octet-stream" }), "page-17.pdf");
+  fd.append("purpose", "user_data");
+
+  const req = new Request("http://localhost/", { method: "POST", body: fd });
+
+  const contentType = req.headers.get("content-type");
+  // Must match WebKit exactly: 4 leading dashes, capital K, followed by 32 hex chars
+  expect(contentType).toMatch(/^multipart\/form-data; boundary=----WebKitFormBoundary[0-9a-f]{32}$/);
+
+  const boundary = contentType.slice("multipart/form-data; boundary=".length);
+
+  const body = await req.text();
+  // Body markers are "--" + boundary, so 6 dashes before WebKitFormBoundary.
+  expect(body.startsWith(`--${boundary}\r\n`)).toBe(true);
+  expect(body.startsWith("------WebKitFormBoundary")).toBe(true);
+  // Terminator is "--" + boundary + "--\r\n".
+  expect(body.endsWith(`\r\n--${boundary}--\r\n`)).toBe(true);
+});
 
+test("FormData via Response uses WebKit-compatible boundary (#29630)", async () => {
+  const fd = new FormData();
+  fd.append("field", "value");
+
+  const res = new Response(fd);
+  const contentType = res.headers.get("content-type");
+  expect(contentType).toMatch(/^multipart\/form-data; boundary=----WebKitFormBoundary[0-9a-f]{32}$/);
+
+  const body = await res.text();
+  expect(body.startsWith("------WebKitFormBoundary")).toBe(true);
+});
+
+// https://github.com/oven-sh/bun/issues/12325
 test("formdata set with File works as expected", async () => {
   const expected = ["617580375", "text-notes1.txt"];