Make server middleware standalone functions.

Removing direct dependencies from the server configuration.

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

api/server/middleware/version.go

@@ -0,0 +1,38 @@
+package middleware
+
+import (
+	"fmt"
+	"net/http"
+	"runtime"
+
+	"github.com/docker/docker/api/server/httputils"
+	"github.com/docker/docker/errors"
+	"github.com/docker/docker/pkg/version"
+	"golang.org/x/net/context"
+)
+
+// NewVersionMiddleware creates a new Version middleware.
+func NewVersionMiddleware(versionCheck string, defaultVersion, minVersion version.Version) Middleware {
+	serverVersion := version.Version(versionCheck)
+
+	return func(handler httputils.APIFunc) httputils.APIFunc {
+		return func(ctx context.Context, w http.ResponseWriter, r *http.Request, vars map[string]string) error {
+			apiVersion := version.Version(vars["version"])
+			if apiVersion == "" {
+				apiVersion = defaultVersion
+			}
+
+			if apiVersion.GreaterThan(defaultVersion) {
+				return errors.ErrorCodeNewerClientVersion.WithArgs(apiVersion, defaultVersion)
+			}
+			if apiVersion.LessThan(minVersion) {
+				return errors.ErrorCodeOldClientVersion.WithArgs(apiVersion, minVersion)
+			}
+
+			header := fmt.Sprintf("Docker/%s (%s)", serverVersion, runtime.GOOS)
+			w.Header().Set("Server", header)
+			ctx = context.WithValue(ctx, httputils.APIVersionKey, apiVersion)
+			return handler(ctx, w, r, vars)
+		}
+	}
+}