next.js/packages/next/server/on-demand-entry-handler.js

import DynamicEntryPlugin from 'webpack/lib/DynamicEntryPlugin'
import { EventEmitter } from 'events'
import { join } from 'path'
import fs from 'fs'
import promisify from '../lib/promisify'
import globModule from 'glob'
import {normalizePagePath, pageNotFoundError} from 'next-server/dist/server/require'
import {createEntry} from '../build/webpack/utils'
import { ROUTE_NAME_REGEX, IS_BUNDLED_PAGE_REGEX } from 'next-server/constants'

const ADDED = Symbol('added')
const BUILDING = Symbol('building')
const BUILT = Symbol('built')

const glob = promisify(globModule)
const access = promisify(fs.access)

// Based on https://github.com/webpack/webpack/blob/master/lib/DynamicEntryPlugin.js#L29-L37
function addEntry (compilation, context, name, entry) {
  return new Promise((resolve, reject) => {
    const dep = DynamicEntryPlugin.createDependency(entry, name)
    compilation.addEntry(context, dep, name, (err) => {
      if (err) return reject(err)
      resolve()
    })
  })
}

export default function onDemandEntryHandler (devMiddleware, multiCompiler, {
  buildId,
  dir,
  dev,
  reload,
  pageExtensions,
  maxInactiveAge = 1000 * 60,
  pagesBufferLength = 2,
  wsPort
}) {
  const {compilers} = multiCompiler
  const invalidator = new Invalidator(devMiddleware, multiCompiler)
  let entries = {}
  let lastAccessPages = ['']
  let doneCallbacks = new EventEmitter()
  let reloading = false
  let stopped = false
  let reloadCallbacks = new EventEmitter()

  for (const compiler of compilers) {
    compiler.hooks.make.tapPromise('NextJsOnDemandEntries', (compilation) => {
      invalidator.startBuilding()

      const allEntries = Object.keys(entries).map(async (page) => {
        const { name, entry } = entries[page]
        const files = Array.isArray(entry) ? entry : [entry]
        // Is just one item. But it's passed as an array.
        for (const file of files) {
          try {
            await access(join(dir, file), (fs.constants || fs).W_OK)
          } catch (err) {
            console.warn('Page was removed', page)
            delete entries[page]
            return
          }
        }
        entries[page].status = BUILDING
        return addEntry(compilation, compiler.context, name, entry)
      })

      return Promise.all(allEntries)
    })
  }

  multiCompiler.hooks.done.tap('NextJsOnDemandEntries', (multiStats) => {
    const clientStats = multiStats.stats[0]
    const { compilation } = clientStats
    const hardFailedPages = compilation.errors
      .filter(e => {
        // Make sure to only pick errors which marked with missing modules
        const hasNoModuleFoundError = /ENOENT/.test(e.message) || /Module not found/.test(e.message)
        if (!hasNoModuleFoundError) return false

        // The page itself is missing. So this is a failed page.
        if (IS_BUNDLED_PAGE_REGEX.test(e.module.name)) return true

        // No dependencies means this is a top level page.
        // So this is a failed page.
        return e.module.dependencies.length === 0
      })
      .map(e => e.module.chunks)
      .reduce((a, b) => [...a, ...b], [])
      .map(c => {
        const pageName = ROUTE_NAME_REGEX.exec(c.name)[1]
        return normalizePage(`/${pageName}`)
      })

    // compilation.entrypoints is a Map object, so iterating over it 0 is the key and 1 is the value
    for (const [, entrypoint] of compilation.entrypoints.entries()) {
      const result = ROUTE_NAME_REGEX.exec(entrypoint.name)
      if (!result) {
        continue
      }

      const pagePath = result[1]

      if (!pagePath) {
        continue
      }

      const page = normalizePage('/' + pagePath)

      const entry = entries[page]
      if (!entry) {
        continue
      }

      if (entry.status !== BUILDING) {
        continue
      }

      entry.status = BUILT
      entry.lastActiveTime = Date.now()
      doneCallbacks.emit(page)
    }

    invalidator.doneBuilding()

    if (hardFailedPages.length > 0 && !reloading) {
      console.log(`> Reloading webpack due to inconsistant state of pages(s): ${hardFailedPages.join(', ')}`)
      reloading = true
      reload()
        .then(() => {
          console.log('> Webpack reloaded.')
          reloadCallbacks.emit('done')
          stop()
        })
        .catch(err => {
          console.error(`> Webpack reloading failed: ${err.message}`)
          console.error(err.stack)
          process.exit(1)
        })
    }
  })

  const disposeHandler = setInterval(function () {
    if (stopped) return
    disposeInactiveEntries(devMiddleware, entries, lastAccessPages, maxInactiveAge)
  }, 5000)

  disposeHandler.unref()

  function stop () {
    clearInterval(disposeHandler)
    stopped = true
    doneCallbacks = null
    reloadCallbacks = null
  }

  return {
    waitUntilReloaded () {
      if (!reloading) return Promise.resolve(true)
      return new Promise((resolve) => {
        reloadCallbacks.once('done', function () {
          resolve()
        })
      })
    },

    async ensurePage (page) {
      await this.waitUntilReloaded()
      page = normalizePage(page)
      let normalizedPagePath
      try {
        normalizedPagePath = normalizePagePath(page)
      } catch (err) {
        console.error(err)
        throw pageNotFoundError(normalizedPagePath)
      }

      const extensions = pageExtensions.join('|')
      const paths = await glob(`pages/{${normalizedPagePath}/index,${normalizedPagePath}}.+(${extensions})`, {cwd: dir})

      if (paths.length === 0) {
        throw pageNotFoundError(normalizedPagePath)
      }

      const relativePathToPage = paths[0]

      const pathname = join(dir, relativePathToPage)

      const {name, files} = createEntry(relativePathToPage, {buildId, pageExtensions: extensions})

      await new Promise((resolve, reject) => {
        const entryInfo = entries[page]

        if (entryInfo) {
          if (entryInfo.status === BUILT) {
            resolve()
            return
          }

          if (entryInfo.status === BUILDING) {
            doneCallbacks.once(page, handleCallback)
            return
          }
        }

        console.log(`> Building page: ${page}`)

        entries[page] = { name, entry: files, pathname, status: ADDED }
        doneCallbacks.once(page, handleCallback)

        invalidator.invalidate()

        function handleCallback (err) {
          if (err) return reject(err)
          resolve()
        }
      })
    },

    wsConnection (ws) {
      ws.onmessage = ({ data }) => {
        const page = normalizePage(data)
        const entryInfo = entries[page]

        // If there's no entry.
        // Then it seems like an weird issue.
        if (!entryInfo) {
          const message = `Client pings, but there's no entry for page: ${page}`
          console.error(message)
          return sendJson(ws, { invalid: true })
        }

        sendJson(ws, { success: true })

        // We don't need to maintain active state of anything other than BUILT entries
        if (entryInfo.status !== BUILT) return

        // If there's an entryInfo
        if (!lastAccessPages.includes(page)) {
          lastAccessPages.unshift(page)

          // Maintain the buffer max length
          if (lastAccessPages.length > pagesBufferLength) {
            lastAccessPages.pop()
          }
        }
        entryInfo.lastActiveTime = Date.now()
      }
    },

    middleware () {
      return (req, res, next) => {
        if (stopped) {
          // If this handler is stopped, we need to reload the user's browser.
          // So the user could connect to the actually running handler.
          res.statusCode = 302
          res.setHeader('Location', req.url)
          res.end('302')
        } else if (reloading) {
          // Webpack config is reloading. So, we need to wait until it's done and
          // reload user's browser.
          // So the user could connect to the new handler and webpack setup.
          this.waitUntilReloaded()
            .then(() => {
              res.statusCode = 302
              res.setHeader('Location', req.url)
              res.end('302')
            })
        } else {
          if (!/^\/_next\/on-demand-entries-ping/.test(req.url)) return next()

          res.statusCode = 200
          res.setHeader('port', wsPort)
          res.end('200')
        }
      }
    }
  }
}

function disposeInactiveEntries (devMiddleware, entries, lastAccessPages, maxInactiveAge) {
  const disposingPages = []

  Object.keys(entries).forEach((page) => {
    const { lastActiveTime, status } = entries[page]

    // This means this entry is currently building or just added
    // We don't need to dispose those entries.
    if (status !== BUILT) return

    // 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.includes(page)) return

    if (Date.now() - lastActiveTime > maxInactiveAge) {
      disposingPages.push(page)
    }
  })

  if (disposingPages.length > 0) {
    disposingPages.forEach((page) => {
      delete entries[page]
    })
    console.log(`> Disposing inactive page(s): ${disposingPages.join(', ')}`)
    devMiddleware.invalidate()
  }
}

// /index and / is the same. So, we need to identify both pages as the same.
// This also applies to sub pages as well.
export function normalizePage (page) {
  const unixPagePath = page.replace(/\\/g, '/')
  if (unixPagePath === '/index' || unixPagePath === '/') {
    return '/'
  }
  return unixPagePath.replace(/\/index$/, '')
}

function sendJson (ws, data) {
  ws.send(JSON.stringify(data))
}

// Make sure only one invalidation happens at a time
// Otherwise, webpack hash gets changed and it'll force the client to reload.
class Invalidator {
  constructor (devMiddleware, multiCompiler) {
    this.multiCompiler = multiCompiler
    this.devMiddleware = devMiddleware
    // contains an array of types of compilers currently building
    this.building = false
    this.rebuildAgain = false
  }

  invalidate () {
    // If there's a current build is processing, we won't abort it by invalidating.
    // (If aborted, it'll cause a client side hard reload)
    // But let it to invalidate just after the completion.
    // So, it can re-build the queued pages at once.
    if (this.building) {
      this.rebuildAgain = true
      return
    }

    this.building = true
    // Work around a bug in webpack, calling `invalidate` on Watching.js
    // doesn't trigger the invalid call used to keep track of the `.done` hook on multiCompiler
    for (const compiler of this.multiCompiler.compilers) {
      compiler.hooks.invalid.call()
    }
    this.devMiddleware.invalidate()
  }

  startBuilding () {
    this.building = true
  }

  doneBuilding () {
    this.building = false

    if (this.rebuildAgain) {
      this.rebuildAgain = false
      this.invalidate()
    }
  }
}