Add client.WithAPIVersionNegotiation() option

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.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/client_test.go

@@ -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) {