imapserver: for the "bodystructure" fetch response item, add the content-type parameters for multiparts so clients will get the mime boundary without having to parse the message themselves

"bodystructure" is like "body", but bodystructure allows returning more
information. we chose not to do that, initially because it was easier to
implement, and more recently because we can't easily return the additional
content-md5 field for leaf parts (since we don't have it in parsed form). but
now we just return the extended form for multiparts, and non-extended form for
leaf parts. likely no one would be looking for any content-md5-value for leaf
parts anyway. knowing the boundary is much more likely to be useful.

for issue #217 by danieleggert, thanks for reporting!

imapserver/fetch.go

@@ -406,7 +406,7 @@ func (cmd *fetchCmd) xprocessAtt(a fetchAtt) []token {
 
 	case "BODYSTRUCTURE":
 		_, part := cmd.xensureParsed()
-		bs := xbodystructure(part)
+		bs := xbodystructure(part, true)
 		return []token{bare("BODYSTRUCTURE"), bs}
 
 	case "BODY":
@@ -660,7 +660,7 @@ func (cmd *fetchCmd) xbody(a fetchAtt) (string, token) {
 
 	if a.section == nil {
 		// Non-extensible form of BODYSTRUCTURE.
-		return a.field, xbodystructure(part)
+		return a.field, xbodystructure(part, false)
 	}
 
 	cmd.peekOrSeen(a.peek)
@@ -865,20 +865,33 @@ func bodyFldEnc(s string) token {
 
 // xbodystructure returns a "body".
 // calls itself for multipart messages and message/{rfc822,global}.
-func xbodystructure(p *message.Part) token {
+func xbodystructure(p *message.Part, extensible bool) token {
 	if p.MediaType == "MULTIPART" {
 		// Multipart, ../rfc/9051:6355 ../rfc/9051:6411
 		var bodies concat
 		for i := range p.Parts {
-			bodies = append(bodies, xbodystructure(&p.Parts[i]))
+			bodies = append(bodies, xbodystructure(&p.Parts[i], extensible))
 		}
-		return listspace{bodies, string0(p.MediaSubType)}
+		r := listspace{bodies, string0(p.MediaSubType)}
+		if extensible {
+			if len(p.ContentTypeParams) == 0 {
+				r = append(r, nilt)
+			} else {
+				params := make(listspace, 0, 2*len(p.ContentTypeParams))
+				for k, v := range p.ContentTypeParams {
+					params = append(params, string0(k), string0(v))
+				}
+				r = append(r, params)
+			}
+		}
+		return r
 	}
 
 	// ../rfc/9051:6355
+	var r listspace
 	if p.MediaType == "TEXT" {
 		// ../rfc/9051:6404 ../rfc/9051:6418
-		return listspace{
+		r = listspace{
 			dquote("TEXT"), string0(p.MediaSubType), // ../rfc/9051:6739
 			// ../rfc/9051:6376
 			bodyFldParams(p.ContentTypeParams), // ../rfc/9051:6401
@@ -891,7 +904,7 @@ func xbodystructure(p *message.Part) token {
 	} else if p.MediaType == "MESSAGE" && (p.MediaSubType == "RFC822" || p.MediaSubType == "GLOBAL") {
 		// ../rfc/9051:6415
 		// note: we don't have to prepare p.Message for reading, because we aren't going to read from it.
-		return listspace{
+		r = listspace{
 			dquote("MESSAGE"), dquote(p.MediaSubType), // ../rfc/9051:6732
 			// ../rfc/9051:6376
 			bodyFldParams(p.ContentTypeParams), // ../rfc/9051:6401
@@ -900,10 +913,10 @@ func xbodystructure(p *message.Part) token {
 			bodyFldEnc(p.ContentTransferEncoding),
 			number(p.EndOffset - p.BodyOffset),
 			xenvelope(p.Message),
-			xbodystructure(p.Message),
+			xbodystructure(p.Message, extensible),
 			number(p.RawLineCount), // todo: or mp.RawLineCount?
 		}
-	}
+	} else {
 		var media token
 		switch p.MediaType {
 		case "APPLICATION", "AUDIO", "IMAGE", "FONT", "MESSAGE", "MODEL", "VIDEO":
@@ -912,7 +925,7 @@ func xbodystructure(p *message.Part) token {
 			media = string0(p.MediaType)
 		}
 		// ../rfc/9051:6404 ../rfc/9051:6407
-	return listspace{
+		r = listspace{
 			media, string0(p.MediaSubType), // ../rfc/9051:6723
 			// ../rfc/9051:6376
 			bodyFldParams(p.ContentTypeParams), // ../rfc/9051:6401
@@ -921,4 +934,7 @@ func xbodystructure(p *message.Part) token {
 			bodyFldEnc(p.ContentTransferEncoding),
 			number(p.EndOffset - p.BodyOffset),
 		}
+	}
+	// todo: if "extensible", we could add the value of the "content-md5" header. we don't have it in our parsed data structure, so we don't add it. likely no one would use it, also not any of the other optional fields. ../rfc/9051:6366
+	return r
 }

imapserver/fetch_test.go

@@ -241,6 +241,7 @@ func TestFetch(t *testing.T) {
 						imapclient.BodyTypeBasic{MediaType: "IMAGE", MediaSubtype: "JPEG", BodyFields: imapclient.BodyFields{CTE: "BASE64"}},
 					},
 					MediaSubtype: "PARALLEL",
+					Ext:          &imapclient.BodyExtensionMpart{Params: [][2]string{{"boundary", "unique-boundary-2"}}},
 				},
 				imapclient.BodyTypeText{MediaType: "TEXT", MediaSubtype: "ENRICHED", BodyFields: imapclient.BodyFields{Octets: 145}, Lines: 5},
 				imapclient.BodyTypeMsg{
@@ -260,6 +261,7 @@ func TestFetch(t *testing.T) {
 				},
 			},
 			MediaSubtype: "MIXED",
+			Ext:          &imapclient.BodyExtensionMpart{Params: [][2]string{{"boundary", "unique-boundary-1"}}},
 		},
 	}
 	tc.client.Append("inbox", nil, &received, []byte(nestedMessage))