move: preserve dest node_modules when moving

Credit: @iarna
Reviewed-By: @zkat
PR-URL: npm#10655

lib/install/action/move.js

@@ -9,6 +9,12 @@ var rmStuff = require('../../unbuild.js').rmStuff
 var lifecycle = require('../../utils/lifecycle.js')
 var updatePackageJson = require('../update-package-json')
 
+/*
+  Move a module from one point in the node_modules tree to another.
+  Do not disturb either the source or target location's node_modules
+  folders.
+*/
+
 module.exports = function (top, buildpath, pkg, log, next) {
   log.silly('move', pkg.fromPath, pkg.path)
   chain([
@@ -33,10 +39,31 @@ function removeEmptyParents (pkgdir, next) {
 
 function moveModuleOnly (from, to, log, done) {
   var fromModules = path.join(from, 'node_modules')
-  var tempModules = from + '.node_modules'
+  var tempFromModules = from + '.node_modules'
+  var toModules = path.join(to, 'node_modules')
+  var tempToModules = to + '.node_modules'
+
+  log.silly('move', 'move existing destination node_modules away', toModules)
+
+  fs.rename(toModules, tempToModules, removeDestination(done))
 
-  log.silly('move', 'remove existing destination', to)
-  rimraf(to, iferr(done, makeDestination(done)))
+  function removeDestination (next) {
+    return function (er) {
+      log.silly('move', 'remove existing destination', to)
+      if (er) {
+        rimraf(to, iferr(next, makeDestination(next)))
+      } else {
+        rimraf(to, iferr(next, makeDestination(iferr(next, moveToModulesBack(next)))))
+      }
+    }
+  }
+
+  function moveToModulesBack (next) {
+    return function () {
+      log.silly('move', 'move existing destination node_modules back', toModules)
+      fs.rename(tempToModules, toModules, iferr(done, next))
+    }
+  }
 
   function makeDestination (next) {
     return function () {
@@ -48,7 +75,7 @@ function moveModuleOnly (from, to, log, done) {
   function moveNodeModules (next) {
     return function () {
       log.silly('move', 'move source node_modules away', fromModules)
-      fs.rename(fromModules, tempModules, iferr(doMove(next), doMove(moveNodeModulesBack(next))))
+      fs.rename(fromModules, tempFromModules, iferr(doMove(next), doMove(moveNodeModulesBack(next))))
     }
   }
 
@@ -63,7 +90,7 @@ function moveModuleOnly (from, to, log, done) {
     return function () {
       mkdirp(from, iferr(done, function () {
         log.silly('move', 'put source node_modules back', fromModules)
-        fs.rename(tempModules, fromModules, iferr(done, next))
+        fs.rename(tempFromModules, fromModules, iferr(done, next))
       }))
     }
   }