Keep some buffered pages, that won't be disposed. Fix #1939 (#2592)

* Keep some buffered pages, that won't be disposed. Fix #1939

* With reworked buffer

server/on-demand-entry-handler.js

@@ -14,7 +14,8 @@ export default function onDemandEntryHandler (devMiddleware, compiler, {
   dir,
   dev,
   reload,
-  maxInactiveAge = 1000 * 25
+  maxInactiveAge = 1000 * 25,
+  pagesBufferLength = 2
 }) {
   let entries = {}
   let lastAccessPages = ['']
@@ -202,8 +203,12 @@ export default function onDemandEntryHandler (devMiddleware, compiler, {
           if (entryInfo.status !== BUILT) return
 
           // If there's an entryInfo
-          lastAccessPages.pop()
-          lastAccessPages.unshift(page)
+          if (!lastAccessPages.includes(page)) {
+            lastAccessPages.unshift(page)
+
+            // Maintain the buffer max length
+            if (lastAccessPages.length > pagesBufferLength) lastAccessPages.pop()
+          }
           entryInfo.lastActiveTime = Date.now()
         }
       }
@@ -234,7 +239,7 @@ function disposeInactiveEntries (devMiddleware, entries, lastAccessPages, maxIna
     // We should not build the last accessed page even we didn't get any pings
     // Sometimes, it's possible our XHR ping to wait before completing other requests.
     // In that case, we should not dispose the current viewing page
-    if (lastAccessPages[0] === page) return
+    if (lastAccessPages.includes(page)) return
 
     if (Date.now() - lastActiveTime > maxInactiveAge) {
       disposingPages.push(page)

test/integration/ondemand/pages/third.js

@@ -0,0 +1,5 @@
+export default () => (
+  <div>
+    <p>Third Page</p>
+  </div>
+)

test/integration/ondemand/test/index.test.js

@@ -22,7 +22,10 @@ describe('On Demand Entries', () => {
   afterAll(() => killApp(context.server))
 
   it('should compile pages for SSR', async () => {
+    // The buffer of built page uses the on-demand-entries-ping to know which pages should be
+    // buffered. Therefore, we need to double each render call with a ping.
     const pageContent = await renderViaHTTP(context.appPort, '/')
+    await renderViaHTTP(context.appPort, '/_next/on-demand-entries-ping', {page: '/'})
     expect(pageContent.includes('Index Page')).toBeTruthy()
   })
 
@@ -32,15 +35,26 @@ describe('On Demand Entries', () => {
   })
 
   it('should dispose inactive pages', async () => {
+    const indexPagePath = resolve(__dirname, '../.next/bundles/pages/index.js')
+    expect(existsSync(indexPagePath)).toBeTruthy()
+
+    // Render two pages after the index, since the server keeps at least two pages
     await renderViaHTTP(context.appPort, '/_next/-/page/about')
+    await renderViaHTTP(context.appPort, '/_next/on-demand-entries-ping', {page: '/about'})
     const aboutPagePath = resolve(__dirname, '../.next/bundles/pages/about.js')
-    expect(existsSync(aboutPagePath)).toBeTruthy()
+
+    await renderViaHTTP(context.appPort, '/_next/-/page/third')
+    await renderViaHTTP(context.appPort, '/_next/on-demand-entries-ping', {page: '/third'})
+    const thirdPagePath = resolve(__dirname, '../.next/bundles/pages/third.js')
 
     // Wait maximum of jasmine.DEFAULT_TIMEOUT_INTERVAL checking
     // for disposing /about
     while (true) {
       await waitFor(1000 * 1)
-      if (!existsSync(aboutPagePath)) return
+      // Assert that the two lastly demanded page are not disposed
+      expect(existsSync(aboutPagePath)).toBeTruthy()
+      expect(existsSync(thirdPagePath)).toBeTruthy()
+      if (!existsSync(indexPagePath)) return
     }
   })
 })