cache: atomic de-race-ified package.json writing

a. Use a tmp file with the pid, and mv into place
b. Only write it once, use inflight to hop on it if in progress

1 files changed, 12 insertions, 2 deletions

diff --git a/lib/cache.js b/lib/cache.js
index 9905ea4a7..37bba5a06 100644
--- a/lib/cache.js
+++ b/lib/cache.js
@@ -71,6 +71,7 @@ var npm = require("./npm.js")
   , addLocal = require("./cache/add-local.js")
   , addRemoteTarball = require("./cache/add-remote-tarball.js")
   , addRemoteGit = require("./cache/add-remote-git.js")
+  , inflight = require("inflight")
 
 cache.usage = "npm cache add <tarball file>"
             + "\nnpm cache add <folder>"
@@ -317,8 +318,17 @@ function afterAdd (arg, cb) { return function (er, data) {
   var name = data.name
   var ver = data.version
   var pj = path.join(npm.cache, name, ver, "package", "package.json")
-  fs.writeFile(pj, JSON.stringify(data), "utf8", function (er) {
-    cb(er, data)
+  var tmp = pj + "." + process.pid
+
+  var done = inflight(pj, cb)
+
+  if (!done) return
+
+  fs.writeFile(tmp, JSON.stringify(data), "utf8", function (er) {
+    if (er) return done(er)
+    fs.rename(tmp, pj, function (er) {
+      done(er, data)
+    })
   })
 }}