Export error descriptors and provide tool generate markdown table

To support accurate specification generation, this changeset includes a quick
and dirty tool to generate a markdown table of error codes generated by the
registry API. Equivalent supports for routes will likely follow.

Exported descriptors could be used to generate other documentation, as well.

cmd/registry-api-doctable-gen/main.go

@@ -0,0 +1,95 @@
+// registry-api-doctable-gen uses various descriptors within the registry code
+// base to generate markdown tables for use in documentation. This is only
+// meant to facilitate updates to documentation and not as an automated tool.
+//
+// For now, this only includes support for error codes:
+//
+// 	$ registry-api-doctable-gen errors
+//
+package main
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"reflect"
+	"strings"
+	"text/tabwriter"
+
+	"github.com/docker/docker-registry/api/errors"
+)
+
+func main() {
+
+	if len(os.Args) < 2 {
+		log.Fatalln("please specify a table to generate: (errors)")
+	}
+
+	switch os.Args[1] {
+	case "errors":
+		dumpErrors(os.Stdout)
+	default:
+		log.Fatalln("unknown descriptor table:", os.Args[1])
+	}
+
+}
+
+func dumpErrors(wr io.Writer) {
+	writer := tabwriter.NewWriter(os.Stdout, 8, 8, 0, '\t', 0)
+	defer writer.Flush()
+
+	fmt.Fprint(writer, "|")
+	dtype := reflect.TypeOf(errors.ErrorDescriptor{})
+	var fieldsPrinted int
+	for i := 0; i < dtype.NumField(); i++ {
+		field := dtype.Field(i)
+		if field.Name == "Value" {
+			continue
+		}
+
+		fmt.Fprint(writer, field.Name, "|")
+		fieldsPrinted++
+	}
+
+	divider := strings.Repeat("-", 8)
+	var parts []string
+	for i := 0; i < fieldsPrinted; i++ {
+		parts = append(parts, divider)
+	}
+	divider = strings.Join(parts, "|")
+
+	fmt.Fprintln(writer, "\n"+divider)
+
+	for _, descriptor := range errors.Descriptors {
+		fmt.Fprint(writer, "|")
+
+		v := reflect.ValueOf(descriptor)
+		for i := 0; i < dtype.NumField(); i++ {
+			value := v.Field(i).Interface()
+			field := v.Type().Field(i)
+			if field.Name == "Value" {
+				continue
+			} else if field.Name == "Description" {
+				value = strings.Replace(value.(string), "\n", " ", -1)
+			} else if field.Name == "Code" {
+				value = fmt.Sprintf("`%s`", value)
+			} else if field.Name == "HTTPStatusCodes" {
+				if len(value.([]int)) > 0 {
+					var codes []string
+					for _, code := range value.([]int) {
+						codes = append(codes, fmt.Sprint(code))
+					}
+					value = strings.Join(codes, ", ")
+				} else {
+					value = "Any"
+				}
+
+			}
+
+			fmt.Fprint(writer, value, "|")
+		}
+
+		fmt.Fprint(writer, "\n")
+	}
+}