email: bound the MIME walk and drop two copies of every body (V-581)

The MIME tree walk had no depth limit, and the nesting comes off the wire.
A boundary line is a few bytes, so one message inside MaxMessageBytes can
declare tens of thousands of multipart levels and pick the recursion depth
of a daemon reading his mail. MaxMIMEDepth stops the walk at 12, well past
the three levels real mail uses, and the headers still come through.

ParseMessage converted the raw message to a string to read it, which copied
up to 2 MiB per mail on a box already holding the resident model. It reads
the bytes directly now. decodeCP1251 collected runes and then copied them
into a string, four bytes a character for the whole body, and writes into a
Builder instead.

No behaviour change to what is read: EXAMINE and BODY.PEEK are still the
only mailbox commands, and no credential reaches a log line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/email/message.go

@@ -18,6 +18,7 @@
 package email
 
 import (
+	"bytes"
 	"encoding/base64"
 	"fmt"
 	"io"
@@ -57,7 +58,10 @@ type Message struct {
 // through, because a subject line alone is often the whole task ("Счёт за
 // интернет"). Only a message whose headers cannot be read at all is an error.
 func ParseMessage(uid uint32, raw []byte) (Message, error) {
-	m, err := mail.ReadMessage(strings.NewReader(string(raw)))
+	// bytes.NewReader, not strings.NewReader(string(raw)): the conversion copied
+	// the whole message, and MaxMessageBytes lets that be 2 MiB per mail on a box
+	// already holding the resident model.
+	m, err := mail.ReadMessage(bytes.NewReader(raw))
 	if err != nil {
 		return Message{}, fmt.Errorf("email: parse message: %w", err)
 	}
@@ -82,6 +86,23 @@ func ParseMessage(uid uint32, raw []byte) (Message, error) {
 // wholesale — an attachment is a file, not a sentence, and reading one would
 // mean parsing arbitrary formats from the network.
 func plaintextBody(contentType, encoding string, body io.Reader) (string, error) {
+	return plaintextBodyAt(contentType, encoding, body, 0)
+}
+
+// MaxMIMEDepth — how deep the MIME tree is walked.
+//
+// The nesting comes off the wire, so the recursion depth is the sender's to
+// pick: a boundary line is a few bytes, and one message inside MaxMessageBytes
+// can declare tens of thousands of multipart levels. Real mail is three deep
+// (mixed, then alternative, then related), so a message past this is malformed
+// or hostile and truncating the walk costs a body nobody was going to read.
+const MaxMIMEDepth = 12
+
+// plaintextBodyAt is plaintextBody carrying the current nesting depth.
+func plaintextBodyAt(contentType, encoding string, body io.Reader, depth int) (string, error) {
+	if depth > MaxMIMEDepth {
+		return "", nil
+	}
 	mediaType, params, err := mime.ParseMediaType(contentType)
 	if contentType == "" || err != nil {
 		// No Content-Type at all is legal and means text/plain; a broken one is
@@ -94,7 +115,7 @@ func plaintextBody(contentType, encoding string, body io.Reader) (string, error)
 		if boundary == "" {
 			return "", fmt.Errorf("email: multipart without boundary")
 		}
-		plain, html, err := multipartText(multipart.NewReader(body, boundary))
+		plain, html, err := multipartText(multipart.NewReader(body, boundary), depth+1)
 		if err != nil {
 			return "", err
 		}
@@ -124,7 +145,7 @@ func plaintextBody(contentType, encoding string, body io.Reader) (string, error)
 // contribute either kind. Folding a nested level's answer into one string put
 // HTML-derived text in the plain bucket, and a real text/plain sibling later in
 // the message was then thrown away by the "plain is already set" guard.
-func multipartText(mr *multipart.Reader) (plain, html string, err error) {
+func multipartText(mr *multipart.Reader, depth int) (plain, html string, err error) {
 	for {
 		part, err := mr.NextPart()
 		if err == io.EOF {
@@ -143,8 +164,8 @@ func multipartText(mr *multipart.Reader) (plain, html string, err error) {
 		switch {
 		case strings.HasPrefix(mediaType, "multipart/"):
 			var np, nh string
-			if b := params["boundary"]; b != "" {
-				np, nh, _ = multipartText(multipart.NewReader(part, b))
+			if b := params["boundary"]; b != "" && depth <= MaxMIMEDepth {
+				np, nh, _ = multipartText(multipart.NewReader(part, b), depth+1)
 			}
 			part.Close()
 			if plain == "" {
@@ -154,7 +175,7 @@ func multipartText(mr *multipart.Reader) (plain, html string, err error) {
 				html = nh
 			}
 		default:
-			text, terr := plaintextBody(ct, part.Header.Get("Content-Transfer-Encoding"), part)
+			text, terr := plaintextBodyAt(ct, part.Header.Get("Content-Transfer-Encoding"), part, depth)
 			part.Close()
 			if terr != nil || strings.TrimSpace(text) == "" {
 				continue