Stop unsymlinking destinations that are symlinks

Fixes: #5

index.js

@@ -15,6 +15,11 @@ module.exports = function writeFile (filename, data, options, callback) {
     options = null
   }
   if (!options) options = {}
+  fs.realpath(filename, function (_, realname) {
+    _writeFile(realname || filename, data, options, callback)
+  })
+}
+function _writeFile (filename, data, options, callback) {
   var tmpfile = getTmpname(filename)
 
   if (options.mode && options.chown) {
@@ -51,6 +56,11 @@ module.exports = function writeFile (filename, data, options, callback) {
 
 module.exports.sync = function writeFileSync (filename, data, options) {
   if (!options) options = {}
+  try {
+    filename = fs.realpathSync(filename)
+  } catch (ex) {
+    // it's ok, it'll happen on a not yet existing file
+  }
   var tmpfile = getTmpname(filename)
 
   try {

test/basic.js

@@ -3,6 +3,9 @@ var test = require('tap').test
 var requireInject = require('require-inject')
 var writeFileAtomic = requireInject('../index', {
   'graceful-fs': {
+    realpath: function (filename, cb) {
+      return cb(null, filename)
+    },
     writeFile: function (tmpfile, data, options, cb) {
       if (/nowrite/.test(tmpfile)) return cb(new Error('ENOWRITE'))
       cb()
@@ -27,6 +30,9 @@ var writeFileAtomic = requireInject('../index', {
       if (/nostat/.test(tmpfile)) return cb(new Error('ENOSTAT'))
       cb()
     },
+    realpathSync: function (filename, cb) {
+      return filename
+    },
     writeFileSync: function (tmpfile, data, options) {
       if (/nowrite/.test(tmpfile)) throw new Error('ENOWRITE')
     },

test/integration.js

@@ -84,6 +84,20 @@ test('writes simple file (async)', function (t) {
   })
 })
 
+test('writes to symlinks without clobbering (async)', function (t) {
+  t.plan(5)
+  var file = tmpFile()
+  var link = tmpFile()
+  fs.writeFileSync(file, '42')
+  fs.symlinkSync(file, link)
+  didWriteFileAtomic(t, currentUser(), link, '43', function (err) {
+    t.ifError(err, 'no error')
+    t.is(readFile(file), '43', 'target content ok')
+    t.is(readFile(link), '43', 'link content ok')
+    t.ok(fs.lstatSync(link).isSymbolicLink(), 'link is link')
+  })
+})
+
 test('runs chown on given file (async)', function (t) {
   var file = tmpFile()
   didWriteFileAtomic(t, {uid: 42, gid: 43}, file, '42', { chown: { uid: 42, gid: 43 } }, function (err) {
@@ -156,6 +170,18 @@ test('writes simple file (sync)', function (t) {
   t.is(readFile(file), '42')
 })
 
+test('writes to symlinks without clobbering (sync)', function (t) {
+  t.plan(4)
+  var file = tmpFile()
+  var link = tmpFile()
+  fs.writeFileSync(file, '42')
+  fs.symlinkSync(file, link)
+  didWriteFileAtomicSync(t, currentUser(), link, '43')
+  t.is(readFile(file), '43', 'target content ok')
+  t.is(readFile(link), '43', 'link content ok')
+  t.ok(fs.lstatSync(link).isSymbolicLink(), 'link is link')
+})
+
 test('runs chown on given file (sync)', function (t) {
   t.plan(1)
   var file = tmpFile()