diff --git a/tools/files.js b/tools/files.js
index d6c7b5a1b2..1cece4d4bb 100644
--- a/tools/files.js
+++ b/tools/files.js
@@ -482,6 +482,14 @@ files.renameDirAlmostAtomically = function (fromDir, toDir) {
     files.rm_recursive(garbageDir);
 };
 
+files.writeFileAtomically = function (filename, contents) {
+  var tmpFile = path.join(
+    path.dirname(filename),
+    '.' + path.basename(filename) + '.' + utils.randomToken());
+  fs.writeFileSync(tmpFile, contents);
+  fs.renameSync(tmpFile, filename);
+};
+
 // Run a program synchronously and, assuming it returns success (0),
 // return whatever it wrote to stdout, as a string. Otherwise (if it
 // did not exit gracefully and return 0) return null. As node has
diff --git a/tools/package-client.js b/tools/package-client.js
index 2907d4ab67..b816059bf1 100644
--- a/tools/package-client.js
+++ b/tools/package-client.js
@@ -85,7 +85,7 @@ var writePackagesToDisk = function (syncToken, collectionData) {
   var filename = config.getPackageStorage();
   // XXX think about permissions?
   files.mkdir_p(path.dirname(filename));
-  fs.writeFileSync(filename, JSON.stringify(finalWrite, null, 2));
+  files.writeFileAtomically(filename, JSON.stringify(finalWrite, null, 2));
 };
 
 loadPackageData = function() {