Move the link loop to a goroutine. (#7244)

enterprise/server/remote_execution/dirtools/dirtools.go

@@ -731,38 +731,41 @@ func (ff *BatchFileFetcher) FetchFiles(filesToFetch FileMap, opts *DownloadTreeO
 	fetchQueue := make(chan digestToFetch, 100)
 
 	linkStart := time.Now()
-	// Note: filesToFetch is keyed by digest, so all files in `filePointers` have
-	// the digest represented by dk.
-	//
-	// Attempt to link digests from the file cache. Digests that are not
-	// present in the filecache will be added to the fetchQueue channel.
-	for dk, filePointers := range filesToFetch {
-		d := dk.ToDigest()
-		filePointers := filePointers
-
-		rn := digest.NewResourceName(dk.ToDigest(), ff.instanceName, rspb.CacheType_CAS, ff.digestFunction)
-		// Write empty files directly (skip checking cache and downloading).
-		if rn.IsEmpty() {
-			for _, fp := range filePointers {
-				if err := writeFile(fp, []byte("")); err != nil {
-					return err
+	linkEG.Go(func() error {
+		// Note: filesToFetch is keyed by digest, so all files in `filePointers` have
+		// the digest represented by dk.
+		//
+		// Attempt to link digests from the file cache. Digests that are not
+		// present in the filecache will be added to the fetchQueue channel.
+		for dk, filePointers := range filesToFetch {
+			d := dk.ToDigest()
+			filePointers := filePointers
+
+			rn := digest.NewResourceName(dk.ToDigest(), ff.instanceName, rspb.CacheType_CAS, ff.digestFunction)
+			// Write empty files directly (skip checking cache and downloading).
+			if rn.IsEmpty() {
+				for _, fp := range filePointers {
+					if err := writeFile(fp, []byte("")); err != nil {
+						return err
+					}
 				}
+				continue
 			}
-			continue
-		}
 
-		linkEG.Go(func() error {
-			// If we linked the digest from the file cache, there's nothing
-			// more to do.
-			if err := ff.linkFromFileCache(filePointers, opts); err == nil {
-				return nil
-			}
+			linkEG.Go(func() error {
+				// If we linked the digest from the file cache, there's nothing
+				// more to do.
+				if err := ff.linkFromFileCache(filePointers, opts); err == nil {
+					return nil
+				}
 
-			// Otherwise, queue the digest to be fetched.
-			fetchQueue <- digestToFetch{d: d, fps: filePointers}
-			return nil
-		})
-	}
+				// Otherwise, queue the digest to be fetched.
+				fetchQueue <- digestToFetch{d: d, fps: filePointers}
+				return nil
+			})
+		}
+		return nil
+	})
 
 	// Read work off the fetchQueue channel and generate batch read requests
 	// to download data.