fix #878: concurrent map write with inject

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+* Fix a concurrent map write with the `--inject:` feature ([#878](https://github.com/evanw/esbuild/issues/878))
+
+    This release fixes an issue where esbuild could potentially crash sometimes with a concurrent map write when using injected files and entry points that were neither relative nor absolute paths. This was an edge case where esbuild's low-level file subsystem was being used without being behind a mutex lock. This regression was likely introduced in version 0.8.21. The cause of the crash has been fixed.
+
 ## 0.8.51
 
 * The stderr log format now contains line numbers after file names ([#865](https://github.com/evanw/esbuild/issues/865))

internal/fs/fs_real.go

@@ -12,7 +12,8 @@ import (
 
 type realFS struct {
 	// Stores the file entries for directories we've listed before
-	entries map[string]entriesOrErr
+	entriesMutex sync.Mutex
+	entries      map[string]entriesOrErr
 
 	// If true, do not use the "entries" cache
 	doNotCacheEntries bool
@@ -117,7 +118,13 @@ func RealFS(options RealFSOptions) (FS, error) {
 func (fs *realFS) ReadDirectory(dir string) (DirEntries, error) {
 	if !fs.doNotCacheEntries {
 		// First, check the cache
-		if cached, ok := fs.entries[dir]; ok {
+		cached, ok := func() (cached entriesOrErr, ok bool) {
+			fs.entriesMutex.Lock()
+			defer fs.entriesMutex.Unlock()
+			cached, ok = fs.entries[dir]
+			return
+		}()
+		if ok {
 			// Cache hit: stop now
 			return cached.entries, cached.err
 		}
@@ -166,6 +173,8 @@ func (fs *realFS) ReadDirectory(dir string) (DirEntries, error) {
 		entries.data = nil
 	}
 	if !fs.doNotCacheEntries {
+		fs.entriesMutex.Lock()
+		defer fs.entriesMutex.Unlock()
 		fs.entries[dir] = entriesOrErr{entries: entries, err: err}
 	}
 	return entries, err

internal/resolver/resolver.go

@@ -126,7 +126,6 @@ type resolver struct {
 	log     logger.Log
 	caches  *cache.CacheSet
 	options config.Options
-	mutex   sync.Mutex
 
 	// A special filtered import order for CSS "@import" imports.
 	//
@@ -152,6 +151,16 @@ type resolver struct {
 	// picture but it's better than some alternatives and probably pretty good.
 	atImportExtensionOrder []string
 
+	// This mutex serves two purposes. First of all, it guards access to "dirCache"
+	// which is potentially mutated during path resolution. But this mutex is also
+	// necessary for performance. The "React admin" benchmark mysteriously runs
+	// twice as fast when this mutex is locked around the whole resolve operation
+	// instead of around individual accesses to "dirCache". For some reason,
+	// reducing parallelism in the resolver helps the rest of the bundler go
+	// faster. I'm not sure why this is but please don't change this unless you
+	// do a lot of testing with various benchmarks and there aren't any regressions.
+	mutex sync.Mutex
+
 	// This cache maps a directory path to information about that directory and
 	// all parent directories
 	dirCache map[string]*dirInfo