coder · DanielleMaywood · Jul 14, 2025 · Jul 3, 2025 · Jul 7, 2025 · Jul 7, 2025
diff --git a/agent/agentcontainers/api.go b/agent/agentcontainers/api.go
@@ -2,8 +2,10 @@ package agentcontainers
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
+	"maps"
 	"net/http"
 	"os"
 	"path"
@@ -30,6 +32,7 @@ import (
 	"github.com/coder/coder/v2/codersdk/agentsdk"
 	"github.com/coder/coder/v2/provisioner"
 	"github.com/coder/quartz"
+	"github.com/coder/websocket"
 )
 
 const (
@@ -74,6 +77,7 @@ type API struct {
 
 	mu                       sync.RWMutex  // Protects the following fields.
 	initDone                 chan struct{} // Closed by Init.
+	updateChans              []chan struct{}
 	closed                   bool
 	containers               codersdk.WorkspaceAgentListContainersResponse  // Output from the last list operation.
 	containersErr            error                                          // Error from the last list operation.
@@ -535,6 +539,7 @@ func (api *API) Routes() http.Handler {
 	r.Use(ensureInitDoneMW)
 
 	r.Get("/", api.handleList)
+	r.Get("/watch", api.watchContainers)
 	// TODO(mafredri): Simplify this route as the previous /devcontainers
 	// /-route was dropped. We can drop the /devcontainers prefix here too.
 	r.Route("/devcontainers/{devcontainer}", func(r chi.Router) {
@@ -544,6 +549,65 @@ func (api *API) Routes() http.Handler {
 	return r
 }
 
+func (api *API) watchContainers(rw http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+
+	conn, err := websocket.Accept(rw, r, nil)
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Failed to upgrade connection to websocket.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+
+	// Here we close the websocket for reading, so that the websocket library will handle pings and
+	// close frames.
+	_ = conn.CloseRead(context.Background())
+
+	ctx, wsNetConn := codersdk.WebsocketNetConn(ctx, conn, websocket.MessageText)
+	defer wsNetConn.Close()
+
+	go httpapi.Heartbeat(ctx, conn)
+
+	updateCh := make(chan struct{}, 1)
+
+	api.mu.Lock()
+	api.updateChans = append(api.updateChans, updateCh)
+	api.mu.Unlock()
+
+	defer func() {
+		api.mu.Lock()
+		api.updateChans = slices.DeleteFunc(api.updateChans, func(ch chan struct{}) bool {
+			return ch == updateCh
+		})
+		close(updateCh)
+		api.mu.Unlock()
+	}()
+
+	for {
+		select {
+		case <-api.ctx.Done():
+			return
+
+		case <-ctx.Done():
+			return
+
+		case <-updateCh:
+			ct, err := api.getContainers()
+			if err != nil {
+				api.logger.Error(ctx, "unable to get containers", slog.Error(err))
+				continue
+			}
+
+			if err := json.NewEncoder(wsNetConn).Encode(ct); err != nil {
+				api.logger.Error(ctx, "encode container list", slog.Error(err))
+				return
+			}
+		}
+	}
+}
+
 // handleList handles the HTTP request to list containers.
 func (api *API) handleList(rw http.ResponseWriter, r *http.Request) {
 	ct, err := api.getContainers()
@@ -583,8 +647,32 @@ func (api *API) updateContainers(ctx context.Context) error {
 	api.mu.Lock()
 	defer api.mu.Unlock()
 
+	var previouslyKnownDevcontainers map[string]codersdk.WorkspaceAgentDevcontainer
+	if len(api.updateChans) > 0 {
+		previouslyKnownDevcontainers = maps.Clone(api.knownDevcontainers)
+	}
+
 	api.processUpdatedContainersLocked(ctx, updated)
 
+	if len(api.updateChans) > 0 {
+		statesAreEqual := maps.EqualFunc(
+			previouslyKnownDevcontainers,
+			api.knownDevcontainers,
+			func(dc1, dc2 codersdk.WorkspaceAgentDevcontainer) bool {
+				return dc1.Equals(dc2)
+			})
+
+		if !statesAreEqual {
+			// Broadcast our updates
+			for _, ch := range api.updateChans {
+				select {
+				case ch <- struct{}{}:
+				default:
+				}
+			}
+		}
+	}
+
 	api.logger.Debug(ctx, "containers updated successfully", slog.F("container_count", len(api.containers.Containers)), slog.F("warning_count", len(api.containers.Warnings)), slog.F("devcontainer_count", len(api.knownDevcontainers)))
 
 	return nil

diff --git a/agent/agentcontainers/api_test.go b/agent/agentcontainers/api_test.go
@@ -36,6 +36,7 @@ import (
 	"github.com/coder/coder/v2/pty"
 	"github.com/coder/coder/v2/testutil"
 	"github.com/coder/quartz"
+	"github.com/coder/websocket"
 )
 
 // fakeContainerCLI implements the agentcontainers.ContainerCLI interface for
@@ -441,6 +442,161 @@ func TestAPI(t *testing.T) {
 		logbuf.Reset()
 	})
 
+	t.Run("Watch", func(t *testing.T) {
+		t.Parallel()
+
+		makeResponse := func(cts ...codersdk.WorkspaceAgentContainer) codersdk.WorkspaceAgentListContainersResponse {
+			return codersdk.WorkspaceAgentListContainersResponse{Containers: cts}
+		}
+
+		fakeContainer1 := fakeContainer(t, func(c *codersdk.WorkspaceAgentContainer) {
+			c.ID = "container1"
+			c.FriendlyName = "devcontainer1"
+			c.Image = "busybox:latest"
+			c.Labels = map[string]string{
+				agentcontainers.DevcontainerLocalFolderLabel: "/home/coder/project1",
+				agentcontainers.DevcontainerConfigFileLabel:  "/home/coder/project1/.devcontainer/devcontainer.json",
+			}
+		})
+
+		fakeContainer2 := fakeContainer(t, func(c *codersdk.WorkspaceAgentContainer) {
+			c.ID = "container2"
+			c.FriendlyName = "devcontainer2"
+			c.Image = "ubuntu:latest"
+			c.Labels = map[string]string{
+				agentcontainers.DevcontainerLocalFolderLabel: "/home/coder/project2",
+				agentcontainers.DevcontainerConfigFileLabel:  "/home/coder/project2/.devcontainer/devcontainer.json",
+			}
+		})
+
+		stages := []struct {
+			containers []codersdk.WorkspaceAgentContainer
+			expected   codersdk.WorkspaceAgentListContainersResponse
+		}{
+			{
+				containers: []codersdk.WorkspaceAgentContainer{fakeContainer1},
+				expected: codersdk.WorkspaceAgentListContainersResponse{
+					Containers: []codersdk.WorkspaceAgentContainer{fakeContainer1},
+					Devcontainers: []codersdk.WorkspaceAgentDevcontainer{
+						{
+							Name:            "project1",
+							WorkspaceFolder: fakeContainer1.Labels[agentcontainers.DevcontainerLocalFolderLabel],
+							ConfigPath:      fakeContainer1.Labels[agentcontainers.DevcontainerConfigFileLabel],
+							Status:          "running",
+							Container:       &fakeContainer1,
+						},
+					},
+				},
+			},
+			{
+				containers: []codersdk.WorkspaceAgentContainer{fakeContainer1, fakeContainer2},
+				expected: codersdk.WorkspaceAgentListContainersResponse{
+					Containers: []codersdk.WorkspaceAgentContainer{fakeContainer1, fakeContainer2},
+					Devcontainers: []codersdk.WorkspaceAgentDevcontainer{
+						{
+							Name:            "project1",
+							WorkspaceFolder: fakeContainer1.Labels[agentcontainers.DevcontainerLocalFolderLabel],
+							ConfigPath:      fakeContainer1.Labels[agentcontainers.DevcontainerConfigFileLabel],
+							Status:          "running",
+							Container:       &fakeContainer1,
+						},
+						{
+							Name:            "project2",
+							WorkspaceFolder: fakeContainer2.Labels[agentcontainers.DevcontainerLocalFolderLabel],
+							ConfigPath:      fakeContainer2.Labels[agentcontainers.DevcontainerConfigFileLabel],
+							Status:          "running",
+							Container:       &fakeContainer2,
+						},
+					},
+				},
+			},
+			{
+				containers: []codersdk.WorkspaceAgentContainer{fakeContainer2},
+				expected: codersdk.WorkspaceAgentListContainersResponse{
+					Containers: []codersdk.WorkspaceAgentContainer{fakeContainer2},
+					Devcontainers: []codersdk.WorkspaceAgentDevcontainer{
+						{
+							Name:            "",
+							WorkspaceFolder: fakeContainer1.Labels[agentcontainers.DevcontainerLocalFolderLabel],
+							ConfigPath:      fakeContainer1.Labels[agentcontainers.DevcontainerConfigFileLabel],
+							Status:          "stopped",
+							Container:       nil,
+						},
+						{
+							Name:            "project2",
+							WorkspaceFolder: fakeContainer2.Labels[agentcontainers.DevcontainerLocalFolderLabel],
+							ConfigPath:      fakeContainer2.Labels[agentcontainers.DevcontainerConfigFileLabel],
+							Status:          "running",
+							Container:       &fakeContainer2,
+						},
+					},
+				},
+			},
+		}
+
+		var (
+			ctx               = testutil.Context(t, testutil.WaitShort)
+			mClock            = quartz.NewMock(t)
+			updaterTickerTrap = mClock.Trap().TickerFunc("updaterLoop")
+			mCtrl             = gomock.NewController(t)
+			mLister           = acmock.NewMockContainerCLI(mCtrl)
+			logger            = slogtest.Make(t, &slogtest.Options{IgnoreErrors: true}).Leveled(slog.LevelDebug)
+		)
+
+		mLister.EXPECT().List(gomock.Any()).Return(makeResponse(), nil)
+
+		api := agentcontainers.NewAPI(logger,
+			agentcontainers.WithClock(mClock),
+			agentcontainers.WithContainerCLI(mLister),
+			agentcontainers.WithWatcher(watcher.NewNoop()),
+		)
+		api.Start()
+		defer api.Close()
+
+		srv := httptest.NewServer(api.Routes())
+		defer srv.Close()
+
+		updaterTickerTrap.MustWait(ctx).MustRelease(ctx)
+		defer updaterTickerTrap.Close()
+
+		client, res, err := websocket.Dial(ctx, srv.URL+"/watch", nil)
+		require.NoError(t, err)
+		if res != nil && res.Body != nil {
+			defer res.Body.Close()
+		}
+
+		mLister.EXPECT().DetectArchitecture(gomock.Any(), gomock.Any()).Return("<none>", nil).AnyTimes()
+
+		for _, stage := range stages {
+			mLister.EXPECT().List(gomock.Any()).Return(codersdk.WorkspaceAgentListContainersResponse{Containers: stage.containers}, nil)
+
+			// Given: We allow the update loop to progress
+			_, aw := mClock.AdvanceNext()
+			aw.MustWait(ctx)
+
+			// When: We attempt to read a message from the socket.
+			mt, msg, err := client.Read(ctx)
+			require.NoError(t, err)
+			require.Equal(t, websocket.MessageText, mt)
+
+			// Then: We expect the receieved message matches the expected response.
+			var got codersdk.WorkspaceAgentListContainersResponse
+			err = json.Unmarshal(msg, &got)
+			require.NoError(t, err)
+
+			require.Equal(t, stage.expected.Containers, got.Containers)
+			require.Len(t, got.Devcontainers, len(stage.expected.Devcontainers))
+			for j, expectedDev := range stage.expected.Devcontainers {
+				gotDev := got.Devcontainers[j]
+				require.Equal(t, expectedDev.Name, gotDev.Name)
+				require.Equal(t, expectedDev.WorkspaceFolder, gotDev.WorkspaceFolder)
+				require.Equal(t, expectedDev.ConfigPath, gotDev.ConfigPath)
+				require.Equal(t, expectedDev.Status, gotDev.Status)
+				require.Equal(t, expectedDev.Container, gotDev.Container)
+			}
+		}
+	})
+
 	// List tests the API.getContainers method using a mock
 	// implementation. It specifically tests caching behavior.
 	t.Run("List", func(t *testing.T) {

diff --git a/coderd/apidoc/docs.go b/coderd/apidoc/docs.go
diff --git a/coderd/apidoc/swagger.json b/coderd/apidoc/swagger.json
diff --git a/coderd/coderd.go b/coderd/coderd.go
@@ -1351,6 +1351,7 @@ func New(options *Options) *API {
 				r.Get("/listening-ports", api.workspaceAgentListeningPorts)
 				r.Get("/connection", api.workspaceAgentConnection)
 				r.Get("/containers", api.workspaceAgentListContainers)
+				r.Get("/containers/watch", api.watchWorkspaceAgentContainers)
 				r.Post("/containers/devcontainers/{devcontainer}/recreate", api.workspaceAgentRecreateDevcontainer)
 				r.Get("/coordinate", api.workspaceAgentClientCoordinate)