fix #556: guard "ProbeResolvePackageAsRelative"

CHANGELOG.md

@@ -1,5 +1,19 @@
 # Changelog
 
+## Unreleased
+
+* Fix a concurrency bug caused by an error message change ([#556](https://github.com/evanw/esbuild/issues/556))
+
+    An improvement to the error message for path resolution was introduced in version 0.8.12. It detects when a relative path is being interpreted as a package path because you forgot to start the path with `./`:
+
+    ```
+     > src/posts/index.js: error: Could not resolve "PostCreate" (use "./PostCreate" to import "src/posts/PostCreate.js")
+        2 │ import PostCreate from 'PostCreate';
+          ╵                        ~~~~~~~~~~~~
+    ```
+
+    This is implemented by re-running path resolution for package path resolution failures as a relative path instead. Unfortunately, this second path resolution operation wasn't guarded by a mutex and could result in concurrency bugs. This issue only occurs when path resolution fails. It is fixed in this release.
+
 ## 0.8.13
 
 * Assigning to a `const` symbol is now an error when bundling

internal/resolver/resolver.go

@@ -253,6 +253,10 @@ func (r *resolver) ResolveAbs(absPath string) *ResolveResult {
 
 func (r *resolver) ProbeResolvePackageAsRelative(sourceDir string, importPath string, kind ast.ImportKind) *ResolveResult {
 	absPath := r.fs.Join(sourceDir, importPath)
+
+	r.mutex.Lock()
+	defer r.mutex.Unlock()
+
 	if pair, ok := r.loadAsFileOrDirectory(absPath, kind); ok {
 		return r.finalizeResolve(ResolveResult{PathPair: pair})
 	}