fix: Infer owner of all unpacked files

Fix: #2

Infer the uid/gid setttings for unpacked files and mkdir'ed directories
from the ownership of the parent directory into which files are being
unpacked.

This prevents user-owned files from ending up in a root-owned location
(as happens when someone runs `sudo npm i -g ...`) and root-owned files
in a user-owned location (as happens when someone runs `sudo npm i ...`).

The argument can be made that when running a program as root, the user
*shouldn't* be surprised to find root-owned files were created in a
"non-hidden" folder like `node_modules`, even if putting root-owned
files in the `~/.npm` cache is inappropriate.

However:

- In practice, this is indeed unexpected to a huge number of users.
  Since they often don't realize at first that npm installs local deps
  locally in the project, they may think that they need to use `sudo`
  simply because other package managers require it.
- `node_modules` is, for most npm users, just as "hidden" as `~/.npm`.

extract.js

@@ -10,41 +10,54 @@ const optCheck = require('./lib/util/opt-check.js')
 const path = require('path')
 const rimraf = BB.promisify(require('rimraf'))
 const withTarballStream = require('./lib/with-tarball-stream.js')
+const inferOwner = require('infer-owner')
+const chown = BB.promisify(fs.chown)
 
 const truncateAsync = BB.promisify(fs.truncate)
 const readFileAsync = BB.promisify(fs.readFile)
 const appendFileAsync = BB.promisify(fs.appendFile)
 
+// you used to call me on my...
+const selfOwner = process.getuid ? {
+  uid: process.getuid(),
+  gid: process.getgid()
+} : {
+  uid: undefined,
+  gid: undefined
+}
+
 module.exports = extract
 function extract (spec, dest, opts) {
   opts = optCheck(opts)
   spec = npa(spec, opts.where)
   const startTime = Date.now()
-
-  return withTarballStream(spec, opts, stream => {
-    return tryExtract(spec, stream, dest, opts)
-  })
-    .then(() => {
-      if (!opts.resolved) {
-        const pjson = path.join(dest, 'package.json')
-        return readFileAsync(pjson, 'utf8')
-          .then(str => truncateAsync(pjson)
-            .then(() => appendFileAsync(pjson, str.replace(
-              /}\s*$/,
-              `\n,"_resolved": ${
-                JSON.stringify(opts.resolved || '')
-              }\n,"_integrity": ${
-                JSON.stringify(opts.integrity || '')
-              }\n,"_from": ${
-                JSON.stringify(spec.toString())
-              }\n}`
-            ))))
-      }
+  return inferOwner(dest).then(({ uid, gid }) => {
+    opts = opts.concat({ uid, gid })
+    return withTarballStream(spec, opts, stream => {
+      return tryExtract(spec, stream, dest, opts)
     })
-    .then(() => opts.log.silly(
-      'extract',
-      `${spec} extracted to ${dest} (${Date.now() - startTime}ms)`
-    ))
+      .then(() => {
+        if (!opts.resolved) {
+          const pjson = path.join(dest, 'package.json')
+          return readFileAsync(pjson, 'utf8')
+            .then(str => truncateAsync(pjson)
+              .then(() => appendFileAsync(pjson, str.replace(
+                /}\s*$/,
+                `\n,"_resolved": ${
+                  JSON.stringify(opts.resolved || '')
+                }\n,"_integrity": ${
+                  JSON.stringify(opts.integrity || '')
+                }\n,"_from": ${
+                  JSON.stringify(spec.toString())
+                }\n}`
+              ))))
+        }
+      })
+      .then(() => opts.log.silly(
+        'extract',
+        `${spec} extracted to ${dest} (${Date.now() - startTime}ms)`
+      ))
+  })
 }
 
 function tryExtract (spec, tarStream, dest, opts) {
@@ -53,6 +66,14 @@ function tryExtract (spec, tarStream, dest, opts) {
 
     rimraf(dest)
       .then(() => mkdirp(dest))
+      .then(() => {
+        // respect the current ownership of unpack targets
+        if (typeof selfOwner.uid === 'number' &&
+            typeof selfOwner.gid === 'number' &&
+            selfOwner.uid !== opts.uid && selfOwner.gid !== opts.gid) {
+          return chown(dest, opts.uid, opts.gid)
+        }
+      })
       .then(() => {
         const xtractor = extractStream(spec, dest, opts)
         xtractor.on('error', reject)