Started work on MIME parser

I'm hoping to mostly rely on the official mime and mime/multipart
packages, but Inbucket is still going to have to decided what to
do with the various parts it reads.

app/inbucket/mime.go

@@ -0,0 +1,81 @@
+package inbucket
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"mime"
+	"mime/multipart"
+	"net/mail"
+)
+
+const MIME_BUF_BYTES = 1024
+
+type MIMEMessage struct {
+	text string
+	html string
+}
+
+func ParseMIMEMessage(mailMsg *mail.Message) (*MIMEMessage, error) {
+	mimeMsg := new(MIMEMessage)
+
+	// Parse top-level multipart
+	ctype := mailMsg.Header.Get("Content-Type")
+	mediatype, params, err := mime.ParseMediaType(ctype)
+	if err != nil {
+		return nil, err
+	}
+	switch mediatype {
+	case "multipart/alternative":
+		// Good
+	default:
+		return nil, fmt.Errorf("Unknown mediatype: %v", mediatype)
+	}
+	boundary := params["boundary"]
+	if boundary == "" {
+		return nil, fmt.Errorf("Unable to locate boundary param in Content-Type header")
+	}
+
+	// Buffer used by Part.Read to hand us chunks of data
+	readBytes := make([]byte, MIME_BUF_BYTES)
+
+	// Loop over MIME parts
+	mr := multipart.NewReader(mailMsg.Body, boundary)
+	for {
+		part, err := mr.NextPart()
+		if err != nil {
+			if err == io.EOF {
+				// This is a clean end-of-message signal
+				break
+			}
+			return nil, err
+		}
+		mediatype, params, err = mime.ParseMediaType(part.Header.Get("Content-Type"))
+		if err != nil {
+			return nil, err
+		}
+		if mediatype == "text/plain" && mimeMsg.text == "" {
+			// First text section, we'll have that, but first we have to deal with
+			// the odd way Part lets us read data.
+			var buf bytes.Buffer
+			for {
+				n, err := part.Read(readBytes)
+				if err != nil {
+					if err == io.EOF {
+						// Clean end of part signal
+						break
+					}
+					return nil, err
+				}
+				// Extra data in readBytes is not cleared, so we must respect
+				// the value returned in 'n'
+				buf.Write(readBytes[:n])
+			}
+
+			mimeMsg.text = buf.String()
+			fmt.Printf("text: '%v'\n", mimeMsg.text)
+		}
+	}
+
+	return mimeMsg, nil
+}