fix: readdir and readdirSync failing on valid empty directory if another fs fails

src/__tests__/union.test.ts

@@ -161,6 +161,19 @@ describe('union', () => {
           expect(ufs.readdirSync('/bar')).toEqual(['baz', 'qux']);
         });
 
+        // regression test for https://github.com/streamich/unionfs/issues/782
+        it('does not throw error when directory is empty and other fs fails', () => {
+          const vol = Volume.fromJSON({});
+          const vol2 = Volume.fromJSON({
+            '/bar': null,
+          });
+
+          const ufs = new Union();
+          ufs.use(vol as any);
+          ufs.use(vol2 as any);
+          expect(ufs.readdirSync('/bar')).toEqual([]);
+        });
+
         it('honors the withFileTypes: true option', () => {
           const vol = Volume.fromJSON({
             '/foo/bar': 'bar',
@@ -308,6 +321,23 @@ describe('union', () => {
           });
         });
 
+        // regression test for https://github.com/streamich/unionfs/issues/782
+        it('does not throw error when directory is empty and other fs fails', done => {
+          const vol = Volume.fromJSON({});
+          const vol2 = Volume.fromJSON({
+            '/bar': null,
+          });
+
+          const ufs = new Union();
+          ufs.use(vol as any);
+          ufs.use(vol2 as any);
+          ufs.readdir('/bar', (err, files) => {
+            expect(err).toBeNull();
+            expect(files).toEqual([]);
+            done();
+          });
+        });
+
         it('honors the withFileTypes: true option', done => {
           const vol = Volume.fromJSON({
             '/foo/bar': 'bar',

src/union.ts

@@ -176,39 +176,37 @@ export class Union {
       lastarg++;
     }
 
+    let numErrors = 0;
     let lastError: IUnionFsError | null = null;
     let result = new Map<string, readdirEntry>();
     const iterate = (i = 0, error?: IUnionFsError | null) => {
       if (error) {
         error.prev = lastError;
         lastError = error;
+        numErrors++;
       }
 
-      // Already tried all file systems, return the last error.
+      // Already tried all file systems
       if (i >= this.fss.length) {
-        // last one
         if (cb) {
+          // If any previous file system succeeded, don't throw an error.
+          if (numErrors < this.fss.length) {
+            return cb(null, this.sortedArrayFromReaddirResult(result));
+          }
           cb(error || Error('No file systems attached.'));
         }
         return;
       }
 
       // Replace `callback` with our intermediate function.
       args[lastarg] = (err, resArg: readdirEntry[]) => {
-        if (result.size === 0 && err) {
-          return iterate(i + 1, err);
-        }
         if (resArg) {
           for (const res of resArg) {
             result.set(this.pathFromReaddirEntry(res), res);
           }
         }
 
-        if (i === this.fss.length - 1) {
-          return cb(null, this.sortedArrayFromReaddirResult(result));
-        } else {
-          return iterate(i + 1, error);
-        }
+        return iterate(i + 1, err);
       };
 
       const j = this.fss.length - i - 1;
@@ -222,6 +220,7 @@ export class Union {
   };
 
   public readdirSync = (...args): Array<readdirEntry> => {
+    let numErrors = 0;
     let lastError: IUnionFsError | null = null;
     let result = new Map<string, readdirEntry>();
     for (let i = this.fss.length - 1; i >= 0; i--) {
@@ -234,8 +233,9 @@ export class Union {
       } catch (err) {
         err.prev = lastError;
         lastError = err;
-        if (result.size === 0 && !i) {
-          // last one
+        numErrors++;
+        if (numErrors === this.fss.length) {
+          // all filesystems errored
           throw err;
         } else {
           // Ignore error...