From f6affeacb9aa944a01ed4bafa84e8c8a24c272fc Mon Sep 17 00:00:00 2001
From: Justin Bay <justin.bay@outlook.com>
Date: Mon, 9 Oct 2017 21:33:02 -0400
Subject: [PATCH] ignore cache write EPERM errors on windows

if we're reasonably sure they came from a cross-process race condition
---
 .../jest-runtime/src/script_transformer.js     | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/packages/jest-runtime/src/script_transformer.js b/packages/jest-runtime/src/script_transformer.js
index cea308731189..21c176b65204 100644
--- a/packages/jest-runtime/src/script_transformer.js
+++ b/packages/jest-runtime/src/script_transformer.js
@@ -402,6 +402,10 @@ const writeCacheFile = (cachePath: Path, fileData: string) => {
   try {
     writeFileAtomic.sync(cachePath, fileData, {encoding: 'utf8'});
   } catch (e) {
+    if (cacheWriteErrorSafeToIgnore(e, cachePath)) {
+      return;
+    }
+
     e.message =
       'jest: failed to cache transform results in: ' +
       cachePath +
@@ -412,6 +416,20 @@ const writeCacheFile = (cachePath: Path, fileData: string) => {
   }
 };
 
+/**
+ * On Windows, renames are not atomic, leading to EPERM exceptions when two
+ * processes attempt to rename to the same target file at the same time.
+ * If the target file exists we can be reasonably sure another process has
+ * legitimately won a cache write race and ignore the error.
+ */
+const cacheWriteErrorSafeToIgnore = (e: Error, cachePath: Path) => {
+  return (
+    process.platform === 'win32' &&
+    e.code === 'EPERM' &&
+    fs.existsSync(cachePath)
+  );
+};
+
 const readCacheFile = (cachePath: Path): ?string => {
   if (!fs.existsSync(cachePath)) {
     return null;