fix: Update list of watchers serially to avoid a data race

.github/workflows/tests.yml

@@ -38,4 +38,4 @@ jobs:
         run: make check-gofmt
 
       - name: Run tests
-        run: go test -v ./internal/...
+        run: go test -race -v ./internal/...

Makefile

@@ -56,4 +56,4 @@ check-gofmt:
 	fi
 
 test:
-	go test ./internal/...
+	go test -race ./internal/...

internal/generator/generator.go

@@ -190,11 +190,11 @@ func (g *generator) generateFromEvents() {
 		}
 
 		g.wg.Add(1)
+		watcher := make(chan *docker.APIEvents, 100)
+		watchers = append(watchers, watcher)
 
-		go func(cfg config.Config, watcher chan *docker.APIEvents) {
+		go func(cfg config.Config) {
 			defer g.wg.Done()
-			watchers = append(watchers, watcher)
-
 			debouncedChan := newDebounceChannel(watcher, cfg.Wait)
 			for range debouncedChan {
 				containers, err := g.getContainers()
@@ -210,7 +210,7 @@ func (g *generator) generateFromEvents() {
 				g.runNotifyCmd(cfg)
 				g.sendSignalToContainer(cfg)
 			}
-		}(cfg, make(chan *docker.APIEvents, 100))
+		}(cfg)
 	}
 
 	// maintains docker client connection and passes events to watchers

internal/generator/generator_test.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"os"
 	"strings"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -22,7 +23,7 @@ import (
 func TestGenerateFromEvents(t *testing.T) {
 	log.SetOutput(ioutil.Discard)
 	containerID := "8dfafdbc3a40"
-	counter := 0
+	var counter atomic.Int32
 
 	eventsResponse := `
 {"status":"start","id":"8dfafdbc3a40","from":"base:latest","time":1374067924}
@@ -38,7 +39,7 @@ func TestGenerateFromEvents(t *testing.T) {
 		for rsc.Scan() {
 			w.Write([]byte(rsc.Text()))
 			w.(http.Flusher).Flush()
-			time.Sleep(15 * time.Millisecond)
+			time.Sleep(150 * time.Millisecond)
 		}
 		time.Sleep(500 * time.Millisecond)
 	}))
@@ -67,7 +68,7 @@ func TestGenerateFromEvents(t *testing.T) {
 		json.NewEncoder(w).Encode(result)
 	}))
 	server.CustomHandler(fmt.Sprintf("/containers/%s/json", containerID), http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		counter++
+		counter := counter.Add(1)
 		container := docker.Container{
 			Name:    "docker-gen-test",
 			ID:      containerID,
@@ -165,13 +166,13 @@ func TestGenerateFromEvents(t *testing.T) {
 					Template: tmplFile.Name(),
 					Dest:     destFiles[2].Name(),
 					Watch:    true,
-					Wait:     &config.Wait{Min: 20 * time.Millisecond, Max: 25 * time.Millisecond},
+					Wait:     &config.Wait{Min: 200 * time.Millisecond, Max: 250 * time.Millisecond},
 				},
 				{
 					Template: tmplFile.Name(),
 					Dest:     destFiles[3].Name(),
 					Watch:    true,
-					Wait:     &config.Wait{Min: 25 * time.Millisecond, Max: 100 * time.Millisecond},
+					Wait:     &config.Wait{Min: 250 * time.Millisecond, Max: 1 * time.Second},
 				},
 			},
 		},
@@ -188,12 +189,12 @@ func TestGenerateFromEvents(t *testing.T) {
 
 	// The counter is incremented in each output file in the following sequence:
 	//
-	//       init   0ms    5ms    10ms   15ms   20ms   25ms   30ms   35ms   40ms   45ms   50ms   55ms
+	//       init   150ms  200ms  250ms  300ms  350ms  400ms  450ms  500ms  550ms  600ms  650ms  700ms
 	//       ├──────╫──────┼──────┼──────╫──────┼──────┼──────╫──────┼──────┼──────┼──────┼──────┤
 	// File0 ├─ 1   ║                    ║                    ║
 	// File1 ├─ 1   ╟─ 2                 ╟─ 3                 ╟─ 5
-	// File2 ├─ 1   ╟───── max (25ms) ───║───────────> 4      ╟─────── min (20ms) ──────> 6
-	// File3 └─ 1   ╟──────────────────> ╟──────────────────> ╟─────────── min (25ms) ─────────> 7
+	// File2 ├─ 1   ╟───── max (250ms) ──║───────────> 4      ╟─────── min (200ms) ─────> 6
+	// File3 └─ 1   ╟──────────────────> ╟──────────────────> ╟─────────── min (250ms) ────────> 7
 	//          ┌───╨───┐            ┌───╨──┐             ┌───╨───┐
 	//          │ start │            │ stop │             │ start │
 	//          └───────┘            └──────┘             └───────┘