feat: enhance SourceTransformTracker with deduplication logic and improve ModuleTSX for circular import handling

Author: YieldRay

package.json

@@ -15,7 +15,7 @@
     "typescript": "^5.9.3"
   },
   "devDependencies": {
-    "tsdown": "^0.21.5"
+    "tsdown": "^0.20.3"
   },
   "author": "YieldRay",
   "license": "MIT",

src/module-tsx.ts

@@ -204,6 +204,11 @@ export class ModuleTSX extends EventTarget implements IModuleTSX {
         // so we just return the original full URL
         return targetUrl.href;
       } else {
+        // If this URL is already being transformed (circular import), return the raw URL.
+        // The browser handles circular ESM natively; we just need to avoid deadlocking.
+        if (this.sourceTracker.isInFlight("esm", targetUrl.href)) {
+          return targetUrl.href;
+        }
         const blobUrl = await this.transformSourceModule("esm", targetUrl.href, await this.fetchText(targetUrl.href));
         //! ^ transformSourceModule is recursive ^
         return blobUrl;

src/source-tracker.ts

@@ -12,10 +12,15 @@ export class SourceTransformTracker<TSourceType extends string> {
     this.blobMap.set(blobUrl, sourceUrl);
   }
 
+  public isInFlight(sourceType: TSourceType, sourceUrl: string): boolean {
+    return this.inFlightSourceMap.has(this.getSourceKey(sourceType, sourceUrl));
+  }
+
   public getSourceUrlByBlob(blobUrl: string): string | undefined {
     return this.blobMap.get(blobUrl);
   }
 
+  /** Deduplicates concurrent transforms for the same URL. */
   public runWithDedup(
     sourceType: TSourceType,
     sourceUrl: string,
@@ -30,9 +35,11 @@ export class SourceTransformTracker<TSourceType extends string> {
     const task = run();
     this.inFlightSourceMap.set(sourceKey, task);
 
-    return task.finally(() => {
+    task.finally(() => {
       this.inFlightSourceMap.delete(sourceKey);
     });
+
+    return task;
   }
 
   private getSourceKey(sourceType: TSourceType, sourceUrl: string): string {