From 3c8cab74be23a35b0bfd5f693a7d661b0ace3e16 Mon Sep 17 00:00:00 2001
From: Drew Powers <drew@pow.rs>
Date: Mon, 23 Aug 2021 11:48:25 -0600
Subject: [PATCH] PR feedback

---
 snowpack/src/sources/local.ts | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/snowpack/src/sources/local.ts b/snowpack/src/sources/local.ts
index 6b00b7ab2f..0f99e7e7ae 100644
--- a/snowpack/src/sources/local.ts
+++ b/snowpack/src/sources/local.ts
@@ -499,9 +499,19 @@ export class PackageSourceLocal implements PackageSource {
       let packageFormatted = spec + colors.dim('@' + packageVersion);
       const existingImportMapLoc = path.join(installDest, 'import-map.json');
       let existingImportMap: ImportMap | undefined = memoizedImportMap[packageName];
-      if (!existingImportMap && existsSync(existingImportMapLoc)) {
-        existingImportMap = JSON.parse(await fs.readFile(existingImportMapLoc, 'utf8'));
-        memoizedImportMap[packageName] = existingImportMap as ImportMap;
+      if (!existingImportMap) {
+        // note: this must happen BEFORE the check on disk to prevent a race condition.
+        // If two lookups occur at once from different sources, then we mark this as “taken” immediately and finish the lookup async
+        memoizedImportMap[packageName] = {imports: {}}; // TODO: this may not exist; should we throw an error?
+        const importMapHandle = await fs.open(existingImportMapLoc, 'r+').catch(() => {
+          delete memoizedResolve[packageName]; // if there was trouble reading this, free up memoization
+        });
+        if (importMapHandle) {
+          const importMapData = await importMapHandle.readFile('utf8');
+          existingImportMap = importMapData ? JSON.parse(importMapData) : null;
+          memoizedImportMap[packageName] = existingImportMap as ImportMap;
+          await importMapHandle.close();
+        }
       }
 
       // Kick off a build, if needed.