Use docker error message to negotiate api version as a fallback (#99)

* Use docker error message to negotiate api version as a fallback

* Add comments describing regex

* Use regex.MustCompile

* Hoist regex outside function

* Flatten logic

* Update changelog

CHANGELOG

@@ -44,5 +44,6 @@
     * Updated docker client library to latest
     * Docker client now negotiates a version
 
-0.10.1 /2018-01-08
+0.10.1 /2018-01-24
     * Patch CVE-2017-17512 sensible-utils-0.0.9
+    * Added fallback logic for Docker client version negotiation with Docker server API versions < 1.24

pkg/plugins/docker/producers/docker.go

@@ -3,6 +3,8 @@ package producers
 import (
 	"context"
 	"errors"
+	"log"
+	"regexp"
 
 	"github.com/docker/docker/api/types"
 	docker "github.com/docker/docker/client"
@@ -12,8 +14,44 @@ type Docker struct {
 	client *docker.Client
 }
 
+// this matches server version within error strings like this:
+// `Error response from daemon: client is newer than server (client API version: 1.24, server API version: 1.19)`
+var dockerErrorVersionRegexp *regexp.Regexp = regexp.MustCompile(`server API version:\s*(\d\.\d+)\s*\)`)
+
 func New(client *docker.Client) *Docker {
 	client.NegotiateAPIVersion(context.Background())
+
+	if client.ClientVersion() != "1.24" {
+		return &Docker{client}
+	}
+
+	// there is a possibility that negotiation failed as this is the default value for that case
+	// so we send a ping and check ourselves
+	ping, _ := client.Ping(context.Background())
+	if ping.APIVersion != "" {
+		return &Docker{client}
+	}
+
+	// negotiation failed, so we get to fake it
+	log.Printf("Docker API version negotiation failed. Attempting fallback...")
+	_, err := client.ServerVersion(context.Background())
+
+	if err == nil {
+		// ironically, this is actually a bit of a failure
+		return &Docker{client}
+	}
+
+	matches := dockerErrorVersionRegexp.FindStringSubmatch(err.Error())
+
+	if len(matches) < 2 {
+		log.Printf("Docker API version negotiation fallback failed")
+	} else {
+		log.Printf("Fallback API version detection: %+v", matches[1])
+		var fakePing types.Ping
+		fakePing.APIVersion = matches[1]
+		client.NegotiateAPIVersionPing(fakePing)
+	}
+
 	return &Docker{client}
 }