requestdecorator: repurpose the package and rename to useragent

Signed-off-by: Tibor Vass <tibor@docker.com>

useragent/useragent.go

@@ -0,0 +1,60 @@
+// Package useragent provides helper functions to pack
+// version information into a single User-Agent header.
+package useragent
+
+import (
+	"errors"
+	"strings"
+)
+
+var (
+	ErrNilRequest = errors.New("request cannot be nil")
+)
+
+// VersionInfo is used to model UserAgent versions.
+type VersionInfo struct {
+	Name    string
+	Version string
+}
+
+func (vi *VersionInfo) isValid() bool {
+	const stopChars = " \t\r\n/"
+	name := vi.Name
+	vers := vi.Version
+	if len(name) == 0 || strings.ContainsAny(name, stopChars) {
+		return false
+	}
+	if len(vers) == 0 || strings.ContainsAny(vers, stopChars) {
+		return false
+	}
+	return true
+}
+
+// Convert versions to a string and append the string to the string base.
+//
+// Each VersionInfo will be converted to a string in the format of
+// "product/version", where the "product" is get from the name field, while
+// version is get from the version field. Several pieces of verson information
+// will be concatinated and separated by space.
+//
+// Example:
+// AppendVersions("base", VersionInfo{"foo", "1.0"}, VersionInfo{"bar", "2.0"})
+// results in "base foo/1.0 bar/2.0".
+func AppendVersions(base string, versions ...VersionInfo) string {
+	if len(versions) == 0 {
+		return base
+	}
+
+	verstrs := make([]string, 0, 1+len(versions))
+	if len(base) > 0 {
+		verstrs = append(verstrs, base)
+	}
+
+	for _, v := range versions {
+		if !v.isValid() {
+			continue
+		}
+		verstrs = append(verstrs, v.Name+"/"+v.Version)
+	}
+	return strings.Join(verstrs, " ")
+}