Detect changes in running containers

* Naive attempt to detect changes based on container metadata
* And my best effort to test it

app/discovery/provider/docker.go

@@ -177,12 +177,48 @@ func (d *Docker) events(ctx context.Context, eventsCh chan<- discovery.ProviderI
 	ticker := time.NewTicker(dockerPollingInterval)
 	defer ticker.Stop()
 
+	// Keep track of running containers
+	saved := make(map[string]containerInfo)
+
+	update := func() {
+		containers, err := d.listContainers()
+		if err != nil {
+			log.Printf("[ERROR] failed to fetch running containers: %s", err)
+			return
+		}
+
+		refresh := false
+		seen := make(map[string]bool)
+
+		for _, c := range containers {
+			old, exists := saved[c.ID]
+
+			if !exists || c.IP != old.IP || c.State != old.State || !c.TS.Equal(old.TS) {
+				refresh = true
+			}
+
+			seen[c.ID] = true
+		}
+
+		if len(saved) != len(seen) || refresh {
+			log.Printf("[INFO] changes in running containers detected: refreshing routes")
+			for k := range saved {
+				delete(saved, k)
+			}
+			for _, c := range containers {
+				saved[c.ID] = c
+			}
+			eventsCh <- discovery.PIDocker
+		}
+	}
+
+	update() // Refresh immediately
 	for {
 		select {
 		case <-ctx.Done():
 			return ctx.Err()
 		case <-ticker.C:
-			eventsCh <- discovery.PIDocker
+			update()
 		}
 	}
 }

app/discovery/provider/docker_test.go

@@ -1,8 +1,10 @@
 package provider
 
 import (
+	"context"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -11,6 +13,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"github.com/umputun/reproxy/app/discovery"
 )
 
 func TestDocker_List(t *testing.T) {
@@ -66,6 +69,62 @@ func TestDocker_List(t *testing.T) {
 	assert.Equal(t, "*", res[2].Server)
 }
 
+func TestDocker_refresh(t *testing.T) {
+	containers := make(chan []containerInfo)
+
+	d := Docker{DockerClient: &DockerClientMock{
+		ListContainersFunc: func() ([]containerInfo, error) {
+			return <-containers, nil
+		},
+	}}
+
+	events := make(chan discovery.ProviderID)
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+
+	stub := func(id string) containerInfo {
+		return containerInfo{ID: id, Name: id, State: "running", IP: "127.0.0." + id, Ports: []int{12345}}
+	}
+
+	recv := func() {
+		select {
+		case <-events:
+			return
+		case <-time.After(time.Second):
+			t.Fatal("No refresh notification was received after 1s")
+		}
+	}
+
+	go func() {
+		dockerPollingInterval = time.Microsecond
+		if err := d.events(ctx, events); err != context.Canceled {
+			log.Fatal(err)
+		}
+	}()
+
+	// Start some
+	containers <- []containerInfo{stub("1"), stub("2")}
+	recv()
+
+	// Nothing changed
+	containers <- []containerInfo{stub("1"), stub("2")}
+	time.Sleep(time.Millisecond)
+	assert.Empty(t, events, "unexpected refresh notification")
+
+	// Stopped
+	containers <- []containerInfo{stub("1")}
+	recv()
+
+	// One changed
+	containers <- []containerInfo{
+		{ID: "1", Name: "1", State: "running", IP: "127.42.42.42", Ports: []int{12345}},
+	}
+	recv()
+
+	time.Sleep(time.Millisecond)
+	assert.Empty(t, events, "unexpect refresh notification from events channel")
+}
+
 func TestDockerClient(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		require.Equal(t, "/v1.41/containers/json", r.URL.Path)