add option to restrict number of parallel vcs workers

cmd/gomodproxy/main.go

@@ -106,6 +106,7 @@ func main() {
 	dir := flag.String("dir", filepath.Join(os.Getenv("HOME"), ".gomodproxy/cache"), "modules cache directory")
 	gitdir := flag.String("gitdir", filepath.Join(os.Getenv("HOME"), ".gomodproxy/git"), "git cache directory")
 	memLimit := flag.Int64("mem", 256, "in-memory cache size in MB")
+	workers := flag.Int("workers", 1, "number of parallel VCS workers")
 	flag.Var(&gitPaths, "git", "list of git settings")
 
 	flag.Parse()
@@ -138,6 +139,7 @@ func main() {
 	}
 
 	options = append(options,
+		api.VCSWorkers(*workers),
 		api.GitDir(*gitdir),
 		api.Memory(logger, *memLimit*1024*1024),
 		api.CacheDir(*dir),

pkg/api/api.go

@@ -26,6 +26,7 @@ type api struct {
 	gitdir   string
 	vcsPaths []vcsPath
 	stores   []store.Store
+	semc     chan struct{}
 }
 
 type vcsPath struct {
@@ -53,7 +54,7 @@ var (
 
 // New returns a configured http.Handler which implements GOPROXY API.
 func New(options ...Option) http.Handler {
-	api := &api{log: func(...interface{}) {}}
+	api := &api{log: func(...interface{}) {}, semc: make(chan struct{}, 1)}
 	for _, opt := range options {
 		opt(api)
 	}
@@ -99,6 +100,15 @@ func CacheDir(dir string) Option {
 	}
 }
 
+// VCSWorkers configures API to use at most n parallel workers when fetching
+// from the VCS. The reason to restrict number of workers is to limit their
+// memory usage.
+func VCSWorkers(n int) Option {
+	return func(api *api) {
+		api.semc = make(chan struct{}, n)
+	}
+}
+
 func decodeBangs(s string) string {
 	buf := []rune{}
 	bang := false
@@ -170,6 +180,10 @@ func (api *api) module(ctx context.Context, module string, version vcs.Version)
 	}
 	cacheMisses.Add(module, 1)
 
+	// wait for semaphore
+	api.semc <- struct{}{}
+	defer func() { <-api.semc }()
+
 	timestamp, err := api.vcs(ctx, module).Timestamp(ctx, version)
 	if err != nil {
 		return nil, time.Time{}, err