fix: update Content-Type header when body is re-extracted on redirect

lib/web/fetch/index.js

@@ -1322,7 +1322,11 @@ function httpRedirectFetch (fetchParams, response) {
   // value of safely extracting request’s body’s source.
   if (request.body != null) {
     assert(request.body.source != null)
-    request.body = safelyExtractBody(request.body.source)[0]
+    const [body, contentType] = safelyExtractBody(request.body.source)
+    request.body = body
+    if (contentType) {
+      request.headersList.set('content-type', contentType, true)
+    }
   }
 
   // 15. Let timingInfo be fetchParams’s timing info.

test/fetch/redirect.js

@@ -3,7 +3,7 @@
 const { test } = require('node:test')
 const { createServer } = require('node:http')
 const { once } = require('node:events')
-const { fetch } = require('../..')
+const { fetch, FormData } = require('../..')
 const { closeServerAsPromise } = require('../utils/node-http')
 
 // https://github.com/nodejs/undici/issues/1776
@@ -75,3 +75,47 @@ test('Redirecting with a body does not fail to write body - #2543', async (t) =>
   t.assert.strictEqual(await resp.text(), 'ok')
   t.assert.ok(resp.redirected)
 })
+
+// https://github.com/nodejs/undici/issues/4065
+test('Redirecting with FormData updates Content-Type header boundary - #4065', async (t) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
+    if (req.url === '/redirect') {
+      res.writeHead(307, { location: '/target' })
+      res.end()
+      return
+    }
+
+    // Collect the request body and verify Content-Type boundary matches body boundary
+    const contentType = req.headers['content-type']
+    const boundaryMatch = contentType?.match(/boundary=(.+)$/)
+    const headerBoundary = boundaryMatch?.[1]
+
+    let body = ''
+    req.on('data', (chunk) => { body += chunk.toString() })
+    req.on('end', () => {
+      // Extract boundary from the body (first line is --boundary)
+      const bodyBoundaryMatch = body.match(/^--(.+)\r\n/)
+      const bodyBoundary = bodyBoundaryMatch?.[1]
+
+      // The header boundary must match the body boundary
+      t.assert.ok(headerBoundary, 'Content-Type header should have boundary')
+      t.assert.ok(bodyBoundary, 'Body should have boundary')
+      t.assert.strictEqual(headerBoundary, bodyBoundary, 'Content-Type boundary must match body boundary')
+
+      res.end('ok')
+    })
+  }).listen(0)
+
+  t.after(closeServerAsPromise(server))
+  await once(server, 'listening')
+
+  const formData = new FormData()
+  formData.append('field', 'value')
+
+  const resp = await fetch(`http://localhost:${server.address().port}/redirect`, {
+    method: 'POST',
+    body: formData
+  })
+  t.assert.strictEqual(await resp.text(), 'ok')
+  t.assert.ok(resp.redirected)
+})