Add HEAD support for /_ping endpoint

Monitoring systems and load balancers are usually configured to use HEAD
requests for health monitoring. The /_ping endpoint currently does not
support this type of request, which means that those systems have fallback
to GET requests.

This patch adds support for HEAD requests on the /_ping endpoint.

Although optional, this patch also returns `Content-Type` and `Content-Length`
headers in case of a HEAD request; Refering to RFC 7231, section 4.3.2:

    The HEAD method is identical to GET except that the server MUST NOT
    send a message body in the response (i.e., the response terminates at
    the end of the header section).  The server SHOULD send the same
    header fields in response to a HEAD request as it would have sent if
    the request had been a GET, except that the payload header fields
    (Section 3.3) MAY be omitted.  This method can be used for obtaining
    metadata about the selected representation without transferring the
    representation data and is often used for testing hypertext links for
    validity, accessibility, and recent modification.

    A payload within a HEAD request message has no defined semantics;
    sending a payload body on a HEAD request might cause some existing
    implementations to reject the request.

    The response to a HEAD request is cacheable; a cache MAY use it to
    satisfy subsequent HEAD requests unless otherwise indicated by the
    Cache-Control header field (Section 5.2 of [RFC7234]).  A HEAD
    response might also have an effect on previously cached responses to
    GET; see Section 4.3.5 of [RFC7234].

With this patch applied, either `GET` or `HEAD` requests work; the only
difference is that the body is empty in case of a `HEAD` request;

    curl -i --unix-socket /var/run/docker.sock http://localhost/_ping
    HTTP/1.1 200 OK
    Api-Version: 1.40
    Cache-Control: no-cache, no-store, must-revalidate
    Docker-Experimental: false
    Ostype: linux
    Pragma: no-cache
    Server: Docker/dev (linux)
    Date: Mon, 14 Jan 2019 12:35:16 GMT
    Content-Length: 2
    Content-Type: text/plain; charset=utf-8

    OK

    curl --head -i --unix-socket /var/run/docker.sock http://localhost/_ping
    HTTP/1.1 200 OK
    Api-Version: 1.40
    Cache-Control: no-cache, no-store, must-revalidate
    Content-Length: 0
    Content-Type: text/plain; charset=utf-8
    Docker-Experimental: false
    Ostype: linux
    Pragma: no-cache
    Server: Docker/dev (linux)
    Date: Mon, 14 Jan 2019 12:34:15 GMT

The client is also updated to use `HEAD` by default, but fallback to `GET`
if the daemon does not support this method.

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

api/server/router/system/system.go

@@ -30,6 +30,7 @@ func NewRouter(b Backend, c ClusterBackend, fscache *fscache.FSCache, builder *b
 	r.routes = []router.Route{
 		router.NewOptionsRoute("/{anyroute:.*}", optionsHandler),
 		router.NewGetRoute("/_ping", r.pingHandler),
+		router.NewHeadRoute("/_ping", r.pingHandler),
 		router.NewGetRoute("/events", r.getEvents),
 		router.NewGetRoute("/info", r.getInfo),
 		router.NewGetRoute("/version", r.getVersion),

api/server/router/system/system_routes.go

@@ -34,6 +34,11 @@ func (s *systemRouter) pingHandler(ctx context.Context, w http.ResponseWriter, r
 	if bv := builderVersion; bv != "" {
 		w.Header().Set("Builder-Version", string(bv))
 	}
+	if r.Method == http.MethodHead {
+		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		w.Header().Set("Content-Length", "0")
+		return nil
+	}
 	_, err := w.Write([]byte{'O', 'K'})
 	return err
 }

api/swagger.yaml

@@ -7133,6 +7133,38 @@ paths:
               type: "string"
               default: "no-cache"
       tags: ["System"]
+    head:
+      summary: "Ping"
+      description: "This is a dummy endpoint you can use to test if the server is accessible."
+      operationId: "SystemPingHead"
+      produces: ["text/plain"]
+      responses:
+        200:
+          description: "no error"
+          schema:
+            type: "string"
+            example: "(empty)"
+          headers:
+            API-Version:
+              type: "string"
+              description: "Max API Version the server supports"
+            BuildKit-Version:
+              type: "string"
+              description: "Default version of docker image builder"
+            Docker-Experimental:
+              type: "boolean"
+              description: "If the server is running with experimental mode enabled"
+            Cache-Control:
+              type: "string"
+              default: "no-cache, no-store, must-revalidate"
+            Pragma:
+              type: "string"
+              default: "no-cache"
+        500:
+          description: "server error"
+          schema:
+            $ref: "#/definitions/ErrorResponse"
+      tags: ["System"]
   /commit:
     post:
       summary: "Create a new image from a container"

client/ping.go

@@ -2,34 +2,56 @@ package client // import "github.com/docker/docker/client"
 
 import (
 	"context"
+	"net/http"
 	"path"
 
 	"github.com/docker/docker/api/types"
 )
 
-// Ping pings the server and returns the value of the "Docker-Experimental", "Builder-Version", "OS-Type" & "API-Version" headers
+// Ping pings the server and returns the value of the "Docker-Experimental",
+// "Builder-Version", "OS-Type" & "API-Version" headers. It attempts to use
+// a HEAD request on the endpoint, but falls back to GET if HEAD is not supported
+// by the daemon.
 func (cli *Client) Ping(ctx context.Context) (types.Ping, error) {
 	var ping types.Ping
-	req, err := cli.buildRequest("GET", path.Join(cli.basePath, "/_ping"), nil, nil)
+	req, err := cli.buildRequest("HEAD", path.Join(cli.basePath, "/_ping"), nil, nil)
 	if err != nil {
 		return ping, err
 	}
 	serverResp, err := cli.doRequest(ctx, req)
+	if err == nil {
+		defer ensureReaderClosed(serverResp)
+		switch serverResp.statusCode {
+		case http.StatusOK, http.StatusInternalServerError:
+			// Server handled the request, so parse the response
+			return parsePingResponse(cli, serverResp)
+		}
+	}
+
+	req, err = cli.buildRequest("GET", path.Join(cli.basePath, "/_ping"), nil, nil)
+	if err != nil {
+		return ping, err
+	}
+	serverResp, err = cli.doRequest(ctx, req)
 	if err != nil {
 		return ping, err
 	}
 	defer ensureReaderClosed(serverResp)
+	return parsePingResponse(cli, serverResp)
+}
 
-	if serverResp.header != nil {
-		ping.APIVersion = serverResp.header.Get("API-Version")
-
-		if serverResp.header.Get("Docker-Experimental") == "true" {
-			ping.Experimental = true
-		}
-		ping.OSType = serverResp.header.Get("OSType")
-		if bv := serverResp.header.Get("Builder-Version"); bv != "" {
-			ping.BuilderVersion = types.BuilderVersion(bv)
-		}
+func parsePingResponse(cli *Client, resp serverResponse) (types.Ping, error) {
+	var ping types.Ping
+	if resp.header == nil {
+		return ping, cli.checkResponseErr(resp)
+	}
+	ping.APIVersion = resp.header.Get("API-Version")
+	ping.OSType = resp.header.Get("OSType")
+	if resp.header.Get("Docker-Experimental") == "true" {
+		ping.Experimental = true
+	}
+	if bv := resp.header.Get("Builder-Version"); bv != "" {
+		ping.BuilderVersion = types.BuilderVersion(bv)
 	}
-	return ping, cli.checkResponseErr(serverResp)
+	return ping, cli.checkResponseErr(resp)
 }

client/ping_test.go

@@ -81,3 +81,49 @@ func TestPingSuccess(t *testing.T) {
 	assert.Check(t, is.Equal(true, ping.Experimental))
 	assert.Check(t, is.Equal("awesome", ping.APIVersion))
 }
+
+// TestPingHeadFallback tests that the client falls back to GET if HEAD fails.
+func TestPingHeadFallback(t *testing.T) {
+	tests := []struct {
+		status   int
+		expected string
+	}{
+		{
+			status:   http.StatusOK,
+			expected: "HEAD",
+		},
+		{
+			status:   http.StatusInternalServerError,
+			expected: "HEAD",
+		},
+		{
+			status:   http.StatusNotFound,
+			expected: "HEAD, GET",
+		},
+		{
+			status:   http.StatusMethodNotAllowed,
+			expected: "HEAD, GET",
+		},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(http.StatusText(tc.status), func(t *testing.T) {
+			var reqs []string
+			client := &Client{
+				client: newMockClient(func(req *http.Request) (*http.Response, error) {
+					reqs = append(reqs, req.Method)
+					resp := &http.Response{StatusCode: http.StatusOK}
+					if req.Method == http.MethodHead {
+						resp.StatusCode = tc.status
+					}
+					resp.Header = http.Header{}
+					resp.Header.Add("API-Version", strings.Join(reqs, ", "))
+					return resp, nil
+				}),
+			}
+			ping, _ := client.Ping(context.Background())
+			assert.Check(t, is.Equal(ping.APIVersion, tc.expected))
+		})
+	}
+}

client/request.go

@@ -195,17 +195,21 @@ func (cli *Client) checkResponseErr(serverResp serverResponse) error {
 		return nil
 	}
 
-	bodyMax := 1 * 1024 * 1024 // 1 MiB
-	bodyR := &io.LimitedReader{
-		R: serverResp.body,
-		N: int64(bodyMax),
-	}
-	body, err := ioutil.ReadAll(bodyR)
-	if err != nil {
-		return err
-	}
-	if bodyR.N == 0 {
-		return fmt.Errorf("request returned %s with a message (> %d bytes) for API route and version %s, check if the server supports the requested API version", http.StatusText(serverResp.statusCode), bodyMax, serverResp.reqURL)
+	var body []byte
+	var err error
+	if serverResp.body != nil {
+		bodyMax := 1 * 1024 * 1024 // 1 MiB
+		bodyR := &io.LimitedReader{
+			R: serverResp.body,
+			N: int64(bodyMax),
+		}
+		body, err = ioutil.ReadAll(bodyR)
+		if err != nil {
+			return err
+		}
+		if bodyR.N == 0 {
+			return fmt.Errorf("request returned %s with a message (> %d bytes) for API route and version %s, check if the server supports the requested API version", http.StatusText(serverResp.statusCode), bodyMax, serverResp.reqURL)
+		}
 	}
 	if len(body) == 0 {
 		return fmt.Errorf("request returned %s for API route and version %s, check if the server supports the requested API version", http.StatusText(serverResp.statusCode), serverResp.reqURL)

docs/api/version-history.md

@@ -17,9 +17,14 @@ keywords: "API, Docker, rcli, REST, documentation"
 
 [Docker Engine API v1.40](https://docs.docker.com/engine/api/v1.40/) documentation
 
-* `GET /_ping` now sets `Cache-Control` and `Pragma` headers to prevent the result
-  from being cached. This change is not versioned, and affects all API versions
-  if the daemon has this patch.
+* The `/_ping` endpoint can now be accessed both using `GET` or `HEAD` requests.
+  when accessed using a `HEAD` request, all headers are returned, but the body
+  is empty (`Content-Length: 0`). This change is not versioned, and affects all
+  API versions if the daemon has this patch. Clients are recommended to try
+  using `HEAD`, but fallback to `GET` if the `HEAD` requests fails.
+* `GET /_ping` and `HEAD /_ping` now set `Cache-Control` and `Pragma` headers to
+  prevent the result from being cached. This change is not versioned, and affects
+  all API versions if the daemon has this patch.
 * `GET /services` now returns `Sysctls` as part of the `ContainerSpec`.
 * `GET /services/{id}` now returns `Sysctls` as part of the `ContainerSpec`.
 * `POST /services/create` now accepts `Sysctls` as part of the `ContainerSpec`.

integration/system/ping_test.go

@@ -5,8 +5,10 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/docker/docker/api/types/versions"
 	"github.com/docker/docker/internal/test/request"
 	"gotest.tools/assert"
+	"gotest.tools/skip"
 )
 
 func TestPingCacheHeaders(t *testing.T) {
@@ -20,6 +22,33 @@ func TestPingCacheHeaders(t *testing.T) {
 	assert.Equal(t, hdr(res, "Pragma"), "no-cache")
 }
 
+func TestPingGet(t *testing.T) {
+	defer setupTest(t)()
+
+	res, body, err := request.Get("/_ping")
+	assert.NilError(t, err)
+
+	b, err := request.ReadBody(body)
+	assert.NilError(t, err)
+	assert.Equal(t, string(b), "OK")
+	assert.Equal(t, res.StatusCode, http.StatusOK)
+	assert.Check(t, hdr(res, "API-Version") != "")
+}
+
+func TestPingHead(t *testing.T) {
+	skip.If(t, versions.LessThan(testEnv.DaemonAPIVersion(), "1.40"), "skip test from new feature")
+	defer setupTest(t)()
+
+	res, body, err := request.Head("/_ping")
+	assert.NilError(t, err)
+
+	b, err := request.ReadBody(body)
+	assert.NilError(t, err)
+	assert.Equal(t, 0, len(b))
+	assert.Equal(t, res.StatusCode, http.StatusOK)
+	assert.Check(t, hdr(res, "API-Version") != "")
+}
+
 func hdr(res *http.Response, name string) string {
 	val, ok := res.Header[http.CanonicalHeaderKey(name)]
 	if !ok || len(val) == 0 {

internal/test/request/request.go

@@ -77,6 +77,11 @@ func Get(endpoint string, modifiers ...func(*Options)) (*http.Response, io.ReadC
 	return Do(endpoint, modifiers...)
 }
 
+// Head creates and execute a HEAD request on the specified host and endpoint, with the specified request modifiers
+func Head(endpoint string, modifiers ...func(*Options)) (*http.Response, io.ReadCloser, error) {
+	return Do(endpoint, append(modifiers, Method(http.MethodHead))...)
+}
+
 // Do creates and execute a request on the specified endpoint, with the specified request modifiers
 func Do(endpoint string, modifiers ...func(*Options)) (*http.Response, io.ReadCloser, error) {
 	opts := &Options{