@@ -81,6 +81,15 @@ type Client struct {

 	customHTTPHeaders map[string]string

 	// manualOverride is set to true when the version was set by users.

 	manualOverride bool

+

+	// negotiateVersion indicates if the client should automatically negotiate

+	// the API version to use when making requests. API version negotiation is

+	// performed on the first request, after which negotiated is set to "true"

+	// so that subsequent requests do not re-negotiate.

+	negotiateVersion bool

+

+	// negotiated indicates that API version negotiation took place

+	negotiated bool

 }

 // CheckRedirect specifies the policy for dealing with redirect responses:

@@ -169,8 +178,11 @@ func (cli *Client) Close() error {

 // getAPIPath returns the versioned request path to call the api.

 // It appends the query parameters to the path if they are not empty.

-func (cli *Client) getAPIPath(p string, query url.Values) string {

+func (cli *Client) getAPIPath(ctx context.Context, p string, query url.Values) string {

 	var apiPath string

+	if cli.negotiateVersion && !cli.negotiated {

+		cli.NegotiateAPIVersion(ctx)

+	}

 	if cli.version != "" {

 		v := strings.TrimPrefix(cli.version, "v")

 		apiPath = path.Join(cli.basePath, "/v"+v, p)

@@ -186,19 +198,31 @@ func (cli *Client) ClientVersion() string {

 }

 // NegotiateAPIVersion queries the API and updates the version to match the

-// API version. Any errors are silently ignored.

+// API version. Any errors are silently ignored. If a manual override is in place,

+// either through the `DOCKER_API_VERSION` environment variable, or if the client

+// was initialized with a fixed version (`opts.WithVersion(xx)`), no negotiation

+// will be performed.

 func (cli *Client) NegotiateAPIVersion(ctx context.Context) {

-	ping, _ := cli.Ping(ctx)

-	cli.NegotiateAPIVersionPing(ping)

+	if !cli.manualOverride {

+		ping, _ := cli.Ping(ctx)

+		cli.negotiateAPIVersionPing(ping)

+	}

 }

 // NegotiateAPIVersionPing updates the client version to match the Ping.APIVersion

-// if the ping version is less than the default version.

+// if the ping version is less than the default version.  If a manual override is

+// in place, either through the `DOCKER_API_VERSION` environment variable, or if

+// the client was initialized with a fixed version (`opts.WithVersion(xx)`), no

+// negotiation is performed.

 func (cli *Client) NegotiateAPIVersionPing(p types.Ping) {

-	if cli.manualOverride {

-		return

+	if !cli.manualOverride {

+		cli.negotiateAPIVersionPing(p)

 	}

+}

+// negotiateAPIVersionPing queries the API and updates the version to match the

+// API version. Any errors are silently ignored.

+func (cli *Client) negotiateAPIVersionPing(p types.Ping) {

 	// try the latest version before versioning headers existed

 	if p.APIVersion == "" {

 		p.APIVersion = "1.24"

@@ -213,6 +237,12 @@ func (cli *Client) NegotiateAPIVersionPing(p types.Ping) {

 	if versions.LessThan(p.APIVersion, cli.version) {

 		cli.version = p.APIVersion

 	}

+

+	// Store the results, so that automatic API version negotiation (if enabled)

+	// won't be performed on the next request.

+	if cli.negotiateVersion {

+		cli.negotiated = true

+	}

 }

 // DaemonHost returns the host address used by the client

@@ -2,10 +2,13 @@ package client // import "github.com/docker/docker/client"

 import (

 	"bytes"

+	"context"

+	"io/ioutil"

 	"net/http"

 	"net/url"

 	"os"

 	"runtime"

+	"strings"

 	"testing"

 	"github.com/docker/docker/api"

@@ -123,9 +126,10 @@ func TestGetAPIPath(t *testing.T) {

 		{"v1.22", "/networks/kiwl$%^", nil, "/v1.22/networks/kiwl$%25%5E"},

 	}

+	ctx := context.TODO()

 	for _, testcase := range testcases {

 		c := Client{version: testcase.version, basePath: "/"}

-		actual := c.getAPIPath(testcase.path, testcase.query)

+		actual := c.getAPIPath(ctx, testcase.path, testcase.query)

 		assert.Check(t, is.Equal(actual, testcase.expected))

 	}

 }

@@ -265,6 +269,35 @@ func TestNegotiateAPVersionOverride(t *testing.T) {

 	assert.Check(t, is.Equal(expected, client.version))

 }

+func TestNegotiateAPIVersionAutomatic(t *testing.T) {

+	var pingVersion string

+	httpClient := newMockClient(func(req *http.Request) (*http.Response, error) {

+		resp := &http.Response{StatusCode: http.StatusOK, Header: http.Header{}}

+		resp.Header.Set("API-Version", pingVersion)

+		resp.Body = ioutil.NopCloser(strings.NewReader("OK"))

+		return resp, nil

+	})

+

+	client, err := NewClientWithOpts(

+		WithHTTPClient(httpClient),

+		WithAPIVersionNegotiation(),

+	)

+	assert.NilError(t, err)

+

+	ctx := context.Background()

+	assert.Equal(t, client.ClientVersion(), api.DefaultVersion)

+

+	// First request should trigger negotiation

+	pingVersion = "1.35"

+	_, _ = client.Info(ctx)

+	assert.Equal(t, client.ClientVersion(), "1.35")

+

+	// Once successfully negotiated, subsequent requests should not re-negotiate

+	pingVersion = "1.25"

+	_, _ = client.Info(ctx)

+	assert.Equal(t, client.ClientVersion(), "1.35")

+}

+

 // TestNegotiateAPIVersionWithEmptyVersion asserts that initializing a client

 // with an empty version string does still allow API-version negotiation

 func TestNegotiateAPIVersionWithEmptyVersion(t *testing.T) {

@@ -23,7 +23,7 @@ func (cli *Client) postHijacked(ctx context.Context, path string, query url.Valu

 		return types.HijackedResponse{}, err

 	}

-	apiPath := cli.getAPIPath(path, query)

+	apiPath := cli.getAPIPath(ctx, path, query)

 	req, err := http.NewRequest("POST", apiPath, bodyEncoded)

 	if err != nil {

 		return types.HijackedResponse{}, err

@@ -150,3 +150,14 @@ func WithVersion(version string) Opt {

 		return nil

 	}

 }

+

+// WithAPIVersionNegotiation enables automatic API version negotiation for the client.

+// With this option enabled, the client automatically negotiates the API version

+// to use when making requests. API version negotiation is performed on the first

+// request; subsequent requests will not re-negotiate.

+func WithAPIVersionNegotiation() Opt {

+	return func(c *Client) error {

+		c.negotiateVersion = true

+		return nil

+	}

+}

@@ -115,7 +115,7 @@ func (cli *Client) buildRequest(method, path string, body io.Reader, headers hea

 }

 func (cli *Client) sendRequest(ctx context.Context, method, path string, query url.Values, body io.Reader, headers headers) (serverResponse, error) {

-	req, err := cli.buildRequest(method, cli.getAPIPath(path, query), body, headers)

+	req, err := cli.buildRequest(method, cli.getAPIPath(ctx, path, query), body, headers)

 	if err != nil {

 		return serverResponse{}, err

 	}