unpack: only reuse file fs entries if nlink = 1

This matches behavior in bsdtar to prevent overwriting a hardlink with
contents from a File archive entry.

lib/unpack.js

@@ -12,6 +12,7 @@ const wc = require('./winchars.js')
 
 const ONENTRY = Symbol('onEntry')
 const CHECKFS = Symbol('checkFs')
+const ISREUSABLE = Symbol('isReusable')
 const MAKEFS = Symbol('makeFs')
 const FILE = Symbol('file')
 const DIRECTORY = Symbol('directory')
@@ -351,6 +352,17 @@ class Unpack extends Parser {
     entry.resume()
   }
 
+  // Check if we can reuse an existing filesystem entry safely and
+  // overwrite it, rather than unlinking and recreating
+  // Windows doesn't report a useful nlink, so we just never reuse entries
+  [ISREUSABLE] (entry, st) {
+    return entry.type === 'File' &&
+      !this.unlink &&
+      st.isFile() &&
+      st.nlink <= 1 &&
+      process.platform !== 'win32'
+  }
+
   // check if a thing is there, and if so, try to clobber it
   [CHECKFS] (entry) {
     this[PEND]()
@@ -360,7 +372,7 @@ class Unpack extends Parser {
       fs.lstat(entry.absolute, (er, st) => {
         if (st && (this.keep || this.newer && st.mtime > entry.mtime))
           this[SKIP](entry)
-        else if (er || (entry.type === 'File' && !this.unlink && st.isFile()))
+        else if (er || this[ISREUSABLE](entry, st))
           this[MAKEFS](null, entry)
         else if (st.isDirectory()) {
           if (entry.type === 'Directory') {
@@ -422,7 +434,7 @@ class UnpackSync extends Unpack {
       const st = fs.lstatSync(entry.absolute)
       if (this.keep || this.newer && st.mtime > entry.mtime)
         return this[SKIP](entry)
-      else if (entry.type === 'File' && !this.unlink && st.isFile())
+      else if (this[ISREUSABLE](entry, st))
         return this[MAKEFS](null, entry)
       else {
         try {

test/unpack.js

@@ -2286,3 +2286,66 @@ t.test('onentry option is preserved', t => {
 
   t.end()
 })
+
+t.test('do not reuse hardlinks, only nlink=1 files', t => {
+  const basedir = path.resolve(unpackdir, 'hardlink-reuse')
+  mkdirp.sync(basedir)
+  t.teardown(() => rimraf.sync(basedir))
+
+  const now = new Date('2018-04-30T18:30:39.025Z')
+
+  const data = makeTar([
+    {
+      path: 'overwriteme',
+      type: 'File',
+      size: 4,
+      mode: 0o644,
+      mtime: now
+    },
+    'foo\n',
+    {
+      path: 'link',
+      linkpath: 'overwriteme',
+      type: 'Link',
+      mode: 0o644,
+      mtime: now
+    },
+    {
+      path: 'link',
+      type: 'File',
+      size: 4,
+      mode: 0o644,
+      mtime: now
+    },
+    'bar\n',
+    '',
+    ''
+  ])
+
+  const checks = {
+    'link': 'bar\n',
+    'overwriteme': 'foo\n'
+  }
+
+  const check = t => {
+    for (let f in checks) {
+      t.equal(fs.readFileSync(basedir + '/' + f, 'utf8'), checks[f], f)
+      t.equal(fs.statSync(basedir + '/' + f).nlink, 1, f)
+    }
+    t.end()
+  }
+
+  t.test('async', t => {
+    const u = new Unpack({ cwd: basedir })
+    u.on('close', () => check(t))
+    u.end(data)
+  })
+
+  t.test('sync', t => {
+    const u = new UnpackSync({ cwd: basedir })
+    u.end(data)
+    check(t)
+  })
+
+  t.end()
+})