Dedupe in-flight server data requests

packages/next/next-server/lib/router/router.ts

@@ -503,6 +503,9 @@ export default class Router implements BaseRouter {
   components: { [pathname: string]: PrivateRouteInfo }
   // Static Data Cache
   sdc: { [asPath: string]: object } = {}
+  // In-flight Server Data Requests, for deduping
+  sdr: { [asPath: string]: Promise<object> } = {}
+
   sub: Subscription
   clc: ComponentLoadCancel
   pageLoader: any
@@ -1598,7 +1601,19 @@ export default class Router implements BaseRouter {
   }
 
   _getServerData(dataHref: string): Promise<object> {
-    return fetchNextData(dataHref, this.isSsr)
+    const { href: resourceKey } = new URL(dataHref, window.location.href)
+    if (this.sdr[resourceKey]) {
+      return this.sdr[resourceKey]
+    }
+    return (this.sdr[resourceKey] = fetchNextData(dataHref, this.isSsr)
+      .then((data) => {
+        delete this.sdr[resourceKey]
+        return data
+      })
+      .catch((err) => {
+        delete this.sdr[resourceKey]
+        throw err
+      }))
   }
 
   getInitialProps(

test/integration/build-output/test/index.test.js

@@ -104,7 +104,7 @@ describe('Build Output', () => {
       expect(parseFloat(err404FirstLoad)).toBeCloseTo(67.1, 0)
       expect(err404FirstLoad.endsWith('kB')).toBe(true)
 
-      expect(parseFloat(sharedByAll)).toBeCloseTo(63.8, 1)
+      expect(parseFloat(sharedByAll)).toBeCloseTo(63.9, 1)
       expect(sharedByAll.endsWith('kB')).toBe(true)
 
       if (_appSize.endsWith('kB')) {

test/integration/getserversideprops/pages/index.js

@@ -37,6 +37,10 @@ const Page = ({ world, time, url }) => {
         <a id="normal">to normal</a>
       </Link>
       <br />
+      <Link href="/slow">
+        <a id="slow">to slow</a>
+      </Link>
+      <br />
       <Link href="/blog/[post]" as="/blog/post-1">
         <a id="post-1">to dynamic</a>
       </Link>

test/integration/getserversideprops/pages/slow/index.js

@@ -0,0 +1,26 @@
+import Link from 'next/link'
+
+let serverHitCount = 0
+
+export async function getServerSideProps() {
+  await new Promise((resolve) => setTimeout(resolve, 500))
+  return {
+    props: {
+      count: ++serverHitCount,
+    },
+  }
+}
+
+export default ({ count }) => (
+  <>
+    <p>a slow page</p>
+    <p id="hit">hit: {count}</p>
+    <Link href="/">
+      <a id="home">to home</a>
+    </Link>
+    <br />
+    <Link href="/something">
+      <a id="something">to something</a>
+    </Link>
+  </>
+)

test/integration/getserversideprops/test/index.test.js

@@ -142,6 +142,12 @@ const expectedManifestRoutes = () => [
     ),
     page: '/refresh',
   },
+  {
+    dataRouteRegex: normalizeRegEx(
+      `^\\/_next\\/data\\/${escapeRegex(buildId)}\\/slow.json$`
+    ),
+    page: '/slow',
+  },
   {
     dataRouteRegex: normalizeRegEx(
       `^\\/_next\\/data\\/${escapeRegex(buildId)}\\/something.json$`
@@ -616,6 +622,32 @@ const runTests = (dev = false) => {
     expect(curRandom).toBe(initialRandom + '')
   })
 
+  it('should dedupe server data requests', async () => {
+    const browser = await webdriver(appPort, '/')
+    await waitFor(2000)
+
+    // Keep clicking on the link
+    await browser.elementByCss('#slow').click()
+    await browser.elementByCss('#slow').click()
+    await browser.elementByCss('#slow').click()
+    await browser.elementByCss('#slow').click()
+
+    await check(() => getBrowserBodyText(browser), /a slow page/)
+
+    // Requests should be deduped
+    const hitCount = await browser.elementByCss('#hit').text()
+    expect(hitCount).toBe('hit: 1')
+
+    // Should send request again
+    await browser.elementByCss('#home').click()
+    await browser.waitForElementByCss('#slow')
+    await browser.elementByCss('#slow').click()
+    await check(() => getBrowserBodyText(browser), /a slow page/)
+
+    const newHitCount = await browser.elementByCss('#hit').text()
+    expect(newHitCount).toBe('hit: 2')
+  })
+
   if (dev) {
     it('should not show warning from url prop being returned', async () => {
       const urlPropPage = join(appDir, 'pages/url-prop.js')