gopls/internal/lsp/source: guard against concurrent writes in xrefs

Add a missing mutex to guard map writes while expanding the references
search.

Change-Id: Ie02f5ba18ec3d78a802e2c373c9c6d18f45b883b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/473675
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/internal/lsp/source/references.go

@@ -393,23 +393,27 @@ func expandMethodSearch(ctx context.Context, snapshot Snapshot, method *types.Fu
 	if err != nil {
 		return err
 	}
+	var mu sync.Mutex // guards scope and targets
 	var group errgroup.Group
 	for i, index := range indexes {
 		i := i
 		index := index
 		group.Go(func() error {
 			// Consult index for matching methods.
 			results := index.Search(key, method.Name())
+			if len(results) == 0 {
+				return nil
+			}
 
 			// Expand global search scope to include rdeps of this pkg.
-			if len(results) > 0 {
-				rdeps, err := snapshot.ReverseDependencies(ctx, allIDs[i], true)
-				if err != nil {
-					return err
-				}
-				for _, rdep := range rdeps {
-					scope[rdep.ID] = rdep
-				}
+			rdeps, err := snapshot.ReverseDependencies(ctx, allIDs[i], true)
+			if err != nil {
+				return err
+			}
+			mu.Lock()
+			defer mu.Unlock()
+			for _, rdep := range rdeps {
+				scope[rdep.ID] = rdep
 			}
 
 			// Add each corresponding method the to set of global search targets.