Improved the search to be concurrent

- Add support for querying multiple sources concurrently
- Return the combined results from all sources
- If any errors occur, return the first error and discard the results

[source/multi.go]
- Add `sync.WaitGroup` for waiting on all source queries to finish
- Add `errs` and `resChan` channels for collecting errors and results from sources
- Add goroutines for querying sources, collecting errors, and collecting results
- If any errors occurred, return the first error and discard the results
- Return the combined results from all sources

Signed-off-by: naveensrinivasan <172697+naveensrinivasan@users.noreply.github.com>

source/multi.go

@@ -14,7 +14,10 @@
 
 package source
 
-import "context"
+import (
+	"context"
+	"sync"
+)
 
 type MultiSource struct {
 	sources []Sourcer
@@ -24,16 +27,59 @@ func NewMultiSource(sources ...Sourcer) *MultiSource {
 	return &MultiSource{sources}
 }
 
+// Search concurrently queries all sources and returns the combined results.
 func (s *MultiSource) Search(ctx context.Context, collectionName string, subjectDigests, attestations []string) ([]CollectionEnvelope, error) {
-	results := make([]CollectionEnvelope, 0)
-	for _, source := range s.sources {
-		res, err := source.Search(ctx, collectionName, subjectDigests, attestations)
-		if err != nil {
-			return results, err
+	results := []CollectionEnvelope{}
+	errors := []error{}
+
+	errs := make(chan error)                   // Channel for collecting errors from each source
+	resChan := make(chan []CollectionEnvelope) // Channel for collecting results from each source
+
+	errdone := make(chan bool)    // Signal channel indicating when error collection is done
+	readerDone := make(chan bool) // Signal channel indicating when result collection is done
+
+	// Goroutine for collecting results from the result channel
+	go func() {
+		for item := range resChan {
+			results = append(results, item...)
 		}
+		readerDone <- true
+	}()
 
-		results = append(results, res...)
+	// Goroutine for collecting errors from the error channel
+	go func() {
+		for err := range errs {
+			errors = append(errors, err)
+		}
+		errdone <- true
+	}()
+
+	var wg sync.WaitGroup // WaitGroup for waiting on all source queries to finish
+	for _, source := range s.sources {
+		source := source
+		wg.Add(1)
+		// Goroutine for querying a source and collecting the results or error
+		go func(src Sourcer) {
+			defer wg.Done()
+			res, err := src.Search(ctx, collectionName, subjectDigests, attestations)
+			if err != nil {
+				errs <- err
+			} else {
+				resChan <- res
+			}
+		}(source)
 	}
+	wg.Wait()      // Wait for all source queries to finish
+	close(resChan) // Close the result channel
+	close(errs)    // Close the error channel
 
+	<-errdone    // Wait for error collection to finish
+	<-readerDone // Wait for result collection to finish
+
+	// If any errors occurred, return the first error and discard the results
+	if len(errors) > 0 {
+		return nil, errors[0]
+	}
+	// Return the combined results from all sources
 	return results, nil
 }