mime/multipart and HTTP multipart/form-data support

Somewhat of a work-in-progress (in that MIME is a large spec), but this is
functional and enough for discussion and/or code review.

In addition to the unit tests, I've tested with curl and Chrome with
a variety of test files, making sure the digests of files are unaltered
when read via a multipart Part.

R=rsc, adg, dsymonds1, agl1
CC=golang-dev
https://golang.org/cl/1681049

src/pkg/http/request.go

@@ -16,6 +16,8 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"mime"
+	"mime/multipart"
 	"os"
 	"strconv"
 	"strings"
@@ -40,6 +42,8 @@ var (
 	ErrNotSupported         = &ProtocolError{"feature not supported"}
 	ErrUnexpectedTrailer    = &ProtocolError{"trailer header without chunked transfer encoding"}
 	ErrMissingContentLength = &ProtocolError{"missing ContentLength in HEAD response"}
+	ErrNotMultipart         = &ProtocolError{"request Content-Type isn't multipart/form-data"}
+	ErrMissingBoundary      = &ProtocolError{"no multipart boundary param Content-Type"}
 )
 
 type badStringError struct {
@@ -139,6 +143,24 @@ func (r *Request) ProtoAtLeast(major, minor int) bool {
 		r.ProtoMajor == major && r.ProtoMinor >= minor
 }
 
+// MultipartReader returns a MIME multipart reader if this is a
+// multipart/form-data POST request, else returns nil and an error.
+func (r *Request) MultipartReader() (multipart.Reader, os.Error) {
+	v, ok := r.Header["Content-Type"]
+	if !ok {
+		return nil, ErrNotMultipart
+	}
+	d, params := mime.ParseMediaType(v)
+	if d != "multipart/form-data" {
+		return nil, ErrNotMultipart
+	}
+	boundary, ok := params["boundary"]
+	if !ok {
+		return nil, ErrMissingBoundary
+	}
+	return multipart.NewReader(r.Body, boundary), nil
+}
+
 // Return value if nonempty, def otherwise.
 func valueOrDefault(value, def string) string {
 	if value != "" {