Pass upstream client's user agent through to registry on image pulls

Changes how the Engine interacts with Registry servers on image pull.
Previously, Engine sent a User-Agent string to the Registry server
that included only the Engine's version information.  This commit
appends to that string the fields from the User-Agent sent by the
client (e.g., Compose) of the Engine.  This allows Registry server
operators to understand what tools are actually generating pulls on
their registries.

Signed-off-by: Mike Goelzer <mgoelzer@docker.com>

dockerversion/useragent.go

@@ -1,15 +1,19 @@
 package dockerversion
 
 import (
+	"fmt"
 	"runtime"
 
+	"github.com/docker/docker/api/server/httputils"
 	"github.com/docker/docker/pkg/parsers/kernel"
 	"github.com/docker/docker/pkg/useragent"
+	"golang.org/x/net/context"
 )
 
 // DockerUserAgent is the User-Agent the Docker client uses to identify itself.
-// It is populated from version information of different components.
-func DockerUserAgent() string {
+// In accordance with RFC 7231 (5.5.3) is of the form:
+//    [docker client's UA] UpstreamClient([upstream client's UA])
+func DockerUserAgent(upstreamUA string) string {
 	httpVersion := make([]useragent.VersionInfo, 0, 6)
 	httpVersion = append(httpVersion, useragent.VersionInfo{Name: "docker", Version: Version})
 	httpVersion = append(httpVersion, useragent.VersionInfo{Name: "go", Version: runtime.Version()})
@@ -20,5 +24,50 @@ func DockerUserAgent() string {
 	httpVersion = append(httpVersion, useragent.VersionInfo{Name: "os", Version: runtime.GOOS})
 	httpVersion = append(httpVersion, useragent.VersionInfo{Name: "arch", Version: runtime.GOARCH})
 
-	return useragent.AppendVersions("", httpVersion...)
+	dockerUA := useragent.AppendVersions("", httpVersion...)
+	if len(upstreamUA) > 0 {
+		ret := insertUpstreamUserAgent(upstreamUA, dockerUA)
+		return ret
+	}
+	return dockerUA
+}
+
+// GetUserAgentFromContext returns the previously saved user-agent context stored in ctx, if one exists
+func GetUserAgentFromContext(ctx context.Context) string {
+	var upstreamUA string
+	if ctx != nil {
+		var ki interface{} = ctx.Value(httputils.UAStringKey)
+		if ki != nil {
+			upstreamUA = ctx.Value(httputils.UAStringKey).(string)
+		}
+	}
+	return upstreamUA
+}
+
+// escapeStr returns s with every rune in charsToEscape escaped by a backslash
+func escapeStr(s string, charsToEscape string) string {
+	var ret string
+	for _, currRune := range s {
+		appended := false
+		for _, escapeableRune := range charsToEscape {
+			if currRune == escapeableRune {
+				ret += "\\" + string(currRune)
+				appended = true
+				break
+			}
+		}
+		if !appended {
+			ret += string(currRune)
+		}
+	}
+	return ret
+}
+
+// insertUpstreamUserAgent adds the upstream client useragent to create a user-agent
+// string of the form:
+//   $dockerUA UpstreamClient($upstreamUA)
+func insertUpstreamUserAgent(upstreamUA string, dockerUA string) string {
+	charsToEscape := "();\\" //["\\", ";", "(", ")"]string
+	upstreamUAEscaped := escapeStr(upstreamUA, charsToEscape)
+	return fmt.Sprintf("%s UpstreamClient(%s)", dockerUA, upstreamUAEscaped)
 }