Remote plugins plumbing.

Signed-off-by: David Calavera <david.calavera@gmail.com>

plugins/client.go

@@ -0,0 +1,100 @@
+package plugins
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"net"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/Sirupsen/logrus"
+)
+
+const (
+	versionMimetype = "appplication/vnd.docker.plugins.v1+json"
+	defaultTimeOut  = 120
+)
+
+func NewClient(addr string) *Client {
+	tr := &http.Transport{}
+	protoAndAddr := strings.Split(addr, "://")
+	configureTCPTransport(tr, protoAndAddr[0], protoAndAddr[1])
+	return &Client{&http.Client{Transport: tr}, protoAndAddr[1]}
+}
+
+type Client struct {
+	http *http.Client
+	addr string
+}
+
+func (c *Client) Call(serviceMethod string, args interface{}, ret interface{}) error {
+	var buf bytes.Buffer
+	if err := json.NewEncoder(&buf).Encode(args); err != nil {
+		return err
+	}
+
+	req, err := http.NewRequest("POST", "/"+serviceMethod, &buf)
+	if err != nil {
+		return err
+	}
+	req.Header.Add("Accept", versionMimetype)
+	req.URL.Scheme = "http"
+	req.URL.Host = c.addr
+
+	var retries int
+	start := time.Now()
+
+	for {
+		resp, err := c.http.Do(req)
+		if err != nil {
+			timeOff := backoff(retries)
+			if timeOff+time.Since(start) > defaultTimeOut {
+				return err
+			}
+			retries++
+			logrus.Warn("Unable to connect to plugin: %s, retrying in %ds\n", c.addr, timeOff)
+			time.Sleep(timeOff)
+			continue
+		}
+
+		if resp.StatusCode != http.StatusOK {
+			remoteErr, err := ioutil.ReadAll(resp.Body)
+			if err != nil {
+				return nil
+			}
+			return fmt.Errorf("Plugin Error: %s", remoteErr)
+		}
+
+		return json.NewDecoder(resp.Body).Decode(&ret)
+	}
+}
+
+func backoff(retries int) time.Duration {
+	b, max := float64(1), float64(defaultTimeOut)
+	for b < max && retries > 0 {
+		b *= 2
+		retries--
+	}
+	if b > max {
+		b = max
+	}
+	return time.Duration(b)
+}
+
+func configureTCPTransport(tr *http.Transport, proto, addr string) {
+	// Why 32? See https://github.com/docker/docker/pull/8035.
+	timeout := 32 * time.Second
+	if proto == "unix" {
+		// No need for compression in local communications.
+		tr.DisableCompression = true
+		tr.Dial = func(_, _ string) (net.Conn, error) {
+			return net.DialTimeout(proto, addr, timeout)
+		}
+	} else {
+		tr.Proxy = http.ProxyFromEnvironment
+		tr.Dial = (&net.Dialer{Timeout: timeout}).Dial
+	}
+}