imapserver: return proper response for FETCH of "BODY[1.MIME]" where 1 is a message

MIME returns the part headers. If 1 is a message, i.e. a message/rfc822 or
message/global, for example when top-level is a multipart/mixed, we were
returning the MIME headers from the message, not from the part.

We also shouldn't be returning a MIME-Version header or the separating newline
for MIME. Those are for MIME headers of a message, but the "MIME" fetch body
part is always about the part.

Found after looking into FETCH BODY handling for issue #327.

imapserver/fetch.go

@@ -826,35 +826,40 @@ func (cmd *fetchCmd) xpartnumsDeref(nums []uint32, p *message.Part) *message.Par
 }
 
 func (cmd *fetchCmd) xsection(section *sectionSpec, p *message.Part) io.Reader {
+	// msgtext is not nil, i.e. HEADER* or TEXT (not MIME), for the top-level part (a message).
 	if section.part == nil {
 		return cmd.xsectionMsgtext(section.msgtext, p)
 	}
 
 	p = cmd.xpartnumsDeref(section.part.part, p)
 
+	// If there is no sectionMsgText, then this isn't for HEADER*, TEXT or MIME, i.e. a
+	// part body, e.g. "BODY[1]".
 	if section.part.text == nil {
 		return p.RawReader()
 	}
 
-	// ../rfc/9051:4535
+	// MIME is defined for all parts. Otherwise it's HEADER* or TEXT, which is only
-	if p.Message != nil {
+	// defined for parts that are messages. ../rfc/9051:4500 ../rfc/9051:4517
+	if !section.part.text.mime {
+		if p.Message == nil {
+			cmd.xerrorf("part is not a message, cannot request header* or text")
+		}
+
 		err := p.SetMessageReaderAt()
 		cmd.xcheckf(err, "preparing submessage")
 		p = p.Message
-	}
 
-	if !section.part.text.mime {
 		return cmd.xsectionMsgtext(section.part.text.msgtext, p)
 	}
 
-	// MIME header, see ../rfc/9051:4534 ../rfc/2045:1645
+	// MIME header, see ../rfc/9051:4514 ../rfc/2045:1652
 	h, err := io.ReadAll(p.HeaderReader())
 	cmd.xcheckf(err, "reading header")
 
 	matchesFields := func(line []byte) bool {
 		k := textproto.CanonicalMIMEHeaderKey(string(bytes.TrimRight(bytes.SplitN(line, []byte(":"), 2)[0], " \t")))
-		// Only add MIME-Version and additional CRLF for messages, not other parts. ../rfc/2045:1645 ../rfc/2045:1652
+		return strings.HasPrefix(k, "Content-")
-		return (p.Envelope != nil && k == "Mime-Version") || strings.HasPrefix(k, "Content-")
 	}
 
 	var match bool
@@ -868,7 +873,7 @@ func (cmd *fetchCmd) xsection(section *sectionSpec, p *message.Part) io.Reader {
 		h = h[len(line):]
 
 		match = matchesFields(line) || match && (bytes.HasPrefix(line, []byte(" ")) || bytes.HasPrefix(line, []byte("\t")))
-		if match || len(line) == 2 {
+		if match {
 			hb.Write(line)
 		}
 	}
@@ -876,11 +881,10 @@ func (cmd *fetchCmd) xsection(section *sectionSpec, p *message.Part) io.Reader {
 }
 
 func (cmd *fetchCmd) xsectionMsgtext(smt *sectionMsgtext, p *message.Part) io.Reader {
-	if smt.s == "HEADER" {
-		return p.HeaderReader()
-	}
-
 	switch smt.s {
+	case "HEADER":
+		return p.HeaderReader()
+
 	case "HEADER.FIELDS":
 		return cmd.xmodifiedHeader(p, smt.headers, false)
 
@@ -888,8 +892,8 @@ func (cmd *fetchCmd) xsectionMsgtext(smt *sectionMsgtext, p *message.Part) io.Re
 		return cmd.xmodifiedHeader(p, smt.headers, true)
 
 	case "TEXT":
-		// It appears imap clients expect to get the body of the message, not a "text body"
+		// TEXT the body (excluding headers) of a message, either the top-level message, or
-		// which sounds like it means a text/* part of a message. ../rfc/9051:4517
+		// a nested as message/rfc822 or message/global. ../rfc/9051:4517
 		return p.RawReader()
 	}
 	panic(serverError{fmt.Errorf("missing case")})

imapserver/fetch_test.go

@@ -78,9 +78,7 @@ func TestFetch(t *testing.T) {
 	headerSplit := strings.SplitN(exampleMsgHeader, "\r\n", 2)
 	dateheader1 := imapclient.FetchBody{RespAttr: "BODY[HEADER.FIELDS (Date)]", Section: "HEADER.FIELDS (Date)", Body: headerSplit[0] + "\r\n\r\n"}
 	nodateheader1 := imapclient.FetchBody{RespAttr: "BODY[HEADER.FIELDS.NOT (Date)]", Section: "HEADER.FIELDS.NOT (Date)", Body: headerSplit[1]}
-	date1header1 := imapclient.FetchBody{RespAttr: "BODY[1.HEADER.FIELDS (Date)]", Section: "1.HEADER.FIELDS (Date)", Body: headerSplit[0] + "\r\n\r\n"}
+	mime1 := imapclient.FetchBody{RespAttr: "BODY[1.MIME]", Section: "1.MIME", Body: "Content-Type: TEXT/PLAIN; CHARSET=US-ASCII\r\n"}
-	nodate1header1 := imapclient.FetchBody{RespAttr: "BODY[1.HEADER.FIELDS.NOT (Date)]", Section: "1.HEADER.FIELDS.NOT (Date)", Body: headerSplit[1]}
-	mime1 := imapclient.FetchBody{RespAttr: "BODY[1.MIME]", Section: "1.MIME", Body: "MIME-Version: 1.0\r\nContent-Type: TEXT/PLAIN; CHARSET=US-ASCII\r\n\r\n"}
 
 	flagsSeen := imapclient.FetchFlags{`\Seen`}
 
@@ -212,11 +210,10 @@ func TestFetch(t *testing.T) {
 	tc.xuntagged(imapclient.UntaggedFetch{Seq: 1, Attrs: []imapclient.FetchAttr{uid1, dateheader1}})
 	tc.transactf("ok", "fetch 1 body.peek[header.fields.not (date)]")
 	tc.xuntagged(imapclient.UntaggedFetch{Seq: 1, Attrs: []imapclient.FetchAttr{uid1, nodateheader1}})
-	// For non-multipart messages, 1 means the whole message. ../rfc/9051:4481
+	// For non-multipart messages, 1 means the whole message, but since it's not of
-	tc.transactf("ok", "fetch 1 body.peek[1.header.fields (date)]")
+	// type message/{rfc822,global} (a message), you can't get the message headers.
-	tc.xuntagged(imapclient.UntaggedFetch{Seq: 1, Attrs: []imapclient.FetchAttr{uid1, date1header1}})
+	// ../rfc/9051:4481
-	tc.transactf("ok", "fetch 1 body.peek[1.header.fields.not (date)]")
+	tc.transactf("no", "fetch 1 body.peek[1.header]")
-	tc.xuntagged(imapclient.UntaggedFetch{Seq: 1, Attrs: []imapclient.FetchAttr{uid1, nodate1header1}})
 
 	// MIME, part 1 for non-multipart messages is the message itself. ../rfc/9051:4481
 	tc.transactf("ok", "fetch 1 body.peek[1.mime]")
@@ -407,9 +404,7 @@ Content-Disposition: inline; filename=image.jpg
 	tc.transactf("ok", "fetch 2 body.peek[3]")
 	tc.xuntagged(imapclient.UntaggedFetch{Seq: 2, Attrs: []imapclient.FetchAttr{uid2, imapclient.FetchBody{RespAttr: "BODY[3]", Section: "3", Body: part3}}})
 
-	part2mime := tocrlf(`Content-type: text/plain; charset=US-ASCII
+	part2mime := "Content-type: text/plain; charset=US-ASCII\r\n"
-
-`)
 	tc.transactf("ok", "fetch 2 body.peek[2.mime]")
 	tc.xuntagged(imapclient.UntaggedFetch{Seq: 2, Attrs: []imapclient.FetchAttr{uid2, imapclient.FetchBody{RespAttr: "BODY[2.MIME]", Section: "2.MIME", Body: part2mime}}})
 
@@ -434,19 +429,26 @@ Content-Transfer-Encoding: Quoted-printable
 	tc.transactf("ok", "fetch 2 body.peek[5.header]")
 	tc.xuntagged(imapclient.UntaggedFetch{Seq: 2, Attrs: []imapclient.FetchAttr{uid2, imapclient.FetchBody{RespAttr: "BODY[5.HEADER]", Section: "5.HEADER", Body: part5header}}})
 
-	part5mime := tocrlf(`Content-Type: Text/plain; charset=ISO-8859-1
+	part5mime := tocrlf(`Content-Type: message/rfc822
-Content-Transfer-Encoding: Quoted-printable
+Content-MD5: MDEyMzQ1Njc4OWFiY2RlZjAxMjM0NTY3ODlhYmNkZWY=
-
+Content-Language: en,de
+Content-Location: http://localhost
 `)
 	tc.transactf("ok", "fetch 2 body.peek[5.mime]")
 	tc.xuntagged(imapclient.UntaggedFetch{Seq: 2, Attrs: []imapclient.FetchAttr{uid2, imapclient.FetchBody{RespAttr: "BODY[5.MIME]", Section: "5.MIME", Body: part5mime}}})
 
 	part5text := "  ... Additional text in ISO-8859-1 goes here ...\r\n"
+
 	tc.transactf("ok", "fetch 2 body.peek[5.text]")
 	tc.xuntagged(imapclient.UntaggedFetch{Seq: 2, Attrs: []imapclient.FetchAttr{uid2, imapclient.FetchBody{RespAttr: "BODY[5.TEXT]", Section: "5.TEXT", Body: part5text}}})
 
+	part5body := "  ... Additional text in ISO-8859-1 goes here ...\r\n"
 	tc.transactf("ok", "fetch 2 body.peek[5.1]")
-	tc.xuntagged(imapclient.UntaggedFetch{Seq: 2, Attrs: []imapclient.FetchAttr{uid2, imapclient.FetchBody{RespAttr: "BODY[5.1]", Section: "5.1", Body: part5text}}})
+	tc.xuntagged(imapclient.UntaggedFetch{Seq: 2, Attrs: []imapclient.FetchAttr{uid2, imapclient.FetchBody{RespAttr: "BODY[5.1]", Section: "5.1", Body: part5body}}})
+
+	// 5.1 is the part that is the sub message, but not as message/rfc822, but as part,
+	// so we cannot request a header.
+	tc.transactf("no", "fetch 2 body.peek[5.1.header]")
 
 	// In case of EXAMINE instead of SELECT, we should not be seeing any changed \Seen flags for non-peek commands.
 	tc.client.StoreFlagsClear("1", true, `\Seen`)