Check and warn for the absence of fs.realpath.native (#953)

* Revert "Remove check for fs.realpath.native support, since it's everywhere (#887)"

This reverts commit f4a880d.

* Added warning and fallback support for missing fs.realpath.native

Signed-off-by: Lam Wei Li <peteriman@mail.com>

* Removed NodeJS check and fs.realpath.native fallback

Signed-off-by: Lam Wei Li <peteriman@mail.com>

* Leaving original listener intact during test and better phasing for warning

Signed-off-by: Lam Wei Li <peteriman@mail.com>

* Using done() for proper async test

Signed-off-by: Lam Wei Li <peteriman@mail.com>

* Updated comments for clarity

Co-authored-by: Ryan Zimmerman <17342435+RyanZim@users.noreply.github.com>

Co-authored-by: Ryan Zimmerman <17342435+RyanZim@users.noreply.github.com>

lib/fs/__tests__/realpath.test.js

@@ -1,11 +1,67 @@
 'use strict'
 
-const fse = require('../..')
+const fs = require('fs')
+const path = require('path')
 const assert = require('assert')
 
 /* eslint-env mocha */
 
+describe('realpath.native does not exist', () => {
+  let warning
+  const warningListener = error => {
+    if (error.name === 'Warning') {
+      if (error.code.startsWith('fs-extra-WARN0003')) {
+        warning = error
+      }
+    }
+  }
+
+  const realpathNativeBackup = fs.realpath.native
+  const clearFseCache = () => {
+    const fsePath = path.dirname(require.resolve('../../..'))
+    for (const entry in require.cache) {
+      if (entry.startsWith(fsePath)) {
+        delete require.cache[entry]
+      }
+    }
+  }
+
+  before(() => {
+    process.on('warning', warningListener)
+
+    // clear existing require.cache
+    clearFseCache()
+
+    // simulate fs monkey-patch
+    delete fs.realpath.native
+  })
+
+  after(() => {
+    process.off('warning', warningListener)
+
+    // clear stubbed require.cache
+    clearFseCache()
+
+    // reinstate fs.realpath.native
+    fs.realpath.native = realpathNativeBackup
+  })
+
+  it('fse should not export realpath.native', done => {
+    const fse = require('../..')
+
+    // next event loop to allow event emitter/listener to happen
+    setImmediate(() => {
+      assert(warning, 'fs-extra-WARN0003 should be emitted')
+      done()
+    })
+
+    assert(!fse.realpath.native)
+  })
+})
+
 describe('realpath.native', () => {
+  const fse = require('../..')
+
   it('works with callbacks', () => {
     fse.realpath.native(__dirname, (err, path) => {
       assert.ifError(err)

lib/fs/index.js

@@ -54,7 +54,6 @@ Object.assign(exports, fs)
 api.forEach(method => {
   exports[method] = u(fs[method])
 })
-exports.realpath.native = u(fs.realpath.native)
 
 // We differ from mz/fs in that we still ship the old, broken, fs.exists()
 // since we are a drop-in replacement for the native module
@@ -117,3 +116,13 @@ if (typeof fs.writev === 'function') {
     })
   }
 }
+
+// fs.realpath.native sometimes not available if fs is monkey-patched
+if (typeof fs.realpath.native === 'function') {
+  exports.realpath.native = u(fs.realpath.native)
+} else {
+  process.emitWarning(
+    'fs.realpath.native is not a function. Is fs being monkey-patched?',
+    'Warning', 'fs-extra-WARN0003'
+  )
+}