Add stats options to not prime the stats

Metrics collectors generally don't need the daemon to prime the stats
with something to compare since they already have something to compare
with.
Before this change, the API does 2 collection cycles (which takes
roughly 2s) in order to provide comparison for CPU usage over 1s. This
was primarily added so that `docker stats --no-stream` had something to
compare against.

Really the CLI should have just made a 2nd call and done the comparison
itself rather than forcing it on all API consumers.
That ship has long sailed, though.

With this change, clients can set an option to just pull a single stat,
which is *at least* a full second faster:

Old:
```
time curl --unix-socket
/go/src/github.com/docker/docker/bundles/test-integration-shell/docker.sock
http://./containers/test/stats?stream=false\&one-shot=false > /dev/null
2>&1

real0m1.864s
user0m0.005s
sys0m0.007s

time curl --unix-socket
/go/src/github.com/docker/docker/bundles/test-integration-shell/docker.sock
http://./containers/test/stats?stream=false\&one-shot=false > /dev/null
2>&1

real0m1.173s
user0m0.010s
sys0m0.006s
```

New:
```
time curl --unix-socket
/go/src/github.com/docker/docker/bundles/test-integration-shell/docker.sock
http://./containers/test/stats?stream=false\&one-shot=true > /dev/null
2>&1
real0m0.680s
user0m0.008s
sys0m0.004s

time curl --unix-socket
/go/src/github.com/docker/docker/bundles/test-integration-shell/docker.sock
http://./containers/test/stats?stream=false\&one-shot=true > /dev/null
2>&1

real0m0.156s
user0m0.007s
sys0m0.007s
```

This fixes issues with downstreams ability to use the stats API to
collect metrics.

Signed-off-by: Brian Goff <[email protected]>

api/server/router/container/container_routes.go

@@ -105,9 +105,14 @@ func (s *containerRouter) getContainersStats(ctx context.Context, w http.Respons
 	if !stream {
 		w.Header().Set("Content-Type", "application/json")
 	}
+	var oneShot bool
+	if versions.GreaterThanOrEqualTo(httputils.VersionFromContext(ctx), "1.41") {
+		oneShot = httputils.BoolValueOrDefault(r, "one-shot", false)
+	}
 
 	config := &backend.ContainerStatsConfig{
 		Stream:    stream,
+		OneShot:   oneShot,
 		OutStream: w,
 		Version:   httputils.VersionFromContext(ctx),
 	}

api/swagger.yaml

@@ -5685,6 +5685,11 @@ paths:
           description: "Stream the output. If false, the stats will be output once and then it will disconnect."
           type: "boolean"
           default: true
+        - name: "one-shot"
+          in: "query"
+          description: "Only get a single stat instead of waiting for 2 cycles. Must be used with stream=false"
+          type: "boolean"
+          default: false
       tags: ["Container"]
   /containers/{id}/resize:
     post:

api/types/backend/backend.go

@@ -73,6 +73,7 @@ type LogSelector struct {
 // behavior of a backend.ContainerStats() call.
 type ContainerStatsConfig struct {
 	Stream    bool
+	OneShot   bool
 	OutStream io.Writer
 	Version   string
 }

client/container_stats.go

@@ -24,3 +24,19 @@ func (cli *Client) ContainerStats(ctx context.Context, containerID string, strea
 	osType := getDockerOS(resp.header.Get("Server"))
 	return types.ContainerStats{Body: resp.body, OSType: osType}, err
 }
+
+// ContainerStatsOneShot gets a single stat entry from a container.
+// It differs from `ContainerStats` in that the API should not wait to prime the stats
+func (cli *Client) ContainerStatsOneShot(ctx context.Context, containerID string) (types.ContainerStats, error) {
+	query := url.Values{}
+	query.Set("stream", "0")
+	query.Set("one-shot", "1")
+
+	resp, err := cli.get(ctx, "/containers/"+containerID+"/stats", query, nil)
+	if err != nil {
+		return types.ContainerStats{}, err
+	}
+
+	osType := getDockerOS(resp.header.Get("Server"))
+	return types.ContainerStats{Body: resp.body, OSType: osType}, err
+}

client/interface.go

@@ -67,6 +67,7 @@ type ContainerAPIClient interface {
 	ContainerRestart(ctx context.Context, container string, timeout *time.Duration) error
 	ContainerStatPath(ctx context.Context, container, path string) (types.ContainerPathStat, error)
 	ContainerStats(ctx context.Context, container string, stream bool) (types.ContainerStats, error)
+	ContainerStatsOneShot(ctx context.Context, container string) (types.ContainerStats, error)
 	ContainerStart(ctx context.Context, container string, options types.ContainerStartOptions) error
 	ContainerStop(ctx context.Context, container string, timeout *time.Duration) error
 	ContainerTop(ctx context.Context, container string, arguments []string) (containertypes.ContainerTopOKBody, error)

daemon/stats.go

@@ -12,6 +12,7 @@ import (
 	"github.com/docker/docker/api/types/versions"
 	"github.com/docker/docker/api/types/versions/v1p20"
 	"github.com/docker/docker/container"
+	"github.com/docker/docker/errdefs"
 	"github.com/docker/docker/pkg/ioutils"
 )
 
@@ -30,6 +31,10 @@ func (daemon *Daemon) ContainerStats(ctx context.Context, prefixOrName string, c
 		return err
 	}
 
+	if config.Stream && config.OneShot {
+		return errdefs.InvalidParameter(errors.New("cannot have stream=true and one-shot=true"))
+	}
+
 	// If the container is either not running or restarting and requires no stream, return an empty stats.
 	if (!container.IsRunning() || container.IsRestarting()) && !config.Stream {
 		return json.NewEncoder(config.OutStream).Encode(&types.StatsJSON{
@@ -63,7 +68,7 @@ func (daemon *Daemon) ContainerStats(ctx context.Context, prefixOrName string, c
 	updates := daemon.subscribeToContainerStats(container)
 	defer daemon.unsubscribeToContainerStats(container, updates)
 
-	noStreamFirstFrame := true
+	noStreamFirstFrame := !config.OneShot
 	for {
 		select {
 		case v, ok := <-updates:

docs/api/version-history.md

@@ -57,6 +57,8 @@ keywords: "API, Docker, rcli, REST, documentation"
   service.
 * `GET /tasks/{id}` now includes `JobIteration` on the task if spawned from a
   job-mode service.
+* `GET /containers/{id}/stats` now accepts a query param (`one-shot`) which, when used with `stream=false` fetches a
+  single set of stats instead of waiting for two collection cycles to have 2 CPU stats over a 1 second period.
 
 ## v1.40 API changes
 

integration/container/stats_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"io"
+	"reflect"
 	"testing"
 	"time"
 
@@ -33,10 +34,23 @@ func TestStats(t *testing.T) {
 	assert.NilError(t, err)
 	defer resp.Body.Close()
 
-	var v *types.Stats
+	var v types.Stats
 	err = json.NewDecoder(resp.Body).Decode(&v)
 	assert.NilError(t, err)
 	assert.Check(t, is.Equal(int64(v.MemoryStats.Limit), info.MemTotal))
+	assert.Check(t, !reflect.DeepEqual(v.PreCPUStats, types.CPUStats{}))
+	err = json.NewDecoder(resp.Body).Decode(&v)
+	assert.Assert(t, is.ErrorContains(err, ""), io.EOF)
+
+	resp, err = client.ContainerStatsOneShot(ctx, cID)
+	assert.NilError(t, err)
+	defer resp.Body.Close()
+
+	v = types.Stats{}
+	err = json.NewDecoder(resp.Body).Decode(&v)
+	assert.NilError(t, err)
+	assert.Check(t, is.Equal(int64(v.MemoryStats.Limit), info.MemTotal))
+	assert.Check(t, is.DeepEqual(v.PreCPUStats, types.CPUStats{}))
 	err = json.NewDecoder(resp.Body).Decode(&v)
 	assert.Assert(t, is.ErrorContains(err, ""), io.EOF)
 }