[feature] Add system message wrappers for pending replies and placeholder attachments (#3713)

Fixes #3712

internal/typeutils/internaltofrontend_test.go

@@ -1278,7 +1278,7 @@ func (suite *InternalToFrontendTestSuite) TestStatusToFrontendUnknownAttachments
   "muted": false,
   "bookmarked": false,
   "pinned": false,
-  "content": "\u003cp\u003ehi \u003cspan class=\"h-card\"\u003e\u003ca href=\"http://localhost:8080/@admin\" class=\"u-url mention\" rel=\"nofollow noreferrer noopener\" target=\"_blank\"\u003e@\u003cspan\u003eadmin\u003c/span\u003e\u003c/a\u003e\u003c/span\u003e here's some media for ya\u003c/p\u003e\u003chr\u003e\u003cp\u003e\u003ci lang=\"en\"\u003eℹ️ Note from localhost:8080: 2 attachments in this status were not downloaded. Treat the following external links with care:\u003c/i\u003e\u003c/p\u003e\u003cul\u003e\u003cli\u003e\u003ca href=\"http://example.org/fileserver/01HE7Y659ZWZ02JM4AWYJZ176Q/attachment/original/01HE7ZGJYTSYMXF927GF9353KR.svg\" rel=\"nofollow noreferrer noopener\" target=\"_blank\"\u003e01HE7ZGJYTSYMXF927GF9353KR.svg\u003c/a\u003e [SVG line art of a sloth, public domain]\u003c/li\u003e\u003cli\u003e\u003ca href=\"http://example.org/fileserver/01HE7Y659ZWZ02JM4AWYJZ176Q/attachment/original/01HE892Y8ZS68TQCNPX7J888P3.mp3\" rel=\"nofollow noreferrer noopener\" target=\"_blank\"\u003e01HE892Y8ZS68TQCNPX7J888P3.mp3\u003c/a\u003e [Jolly salsa song, public domain.]\u003c/li\u003e\u003c/ul\u003e",
+  "content": "\u003cp\u003ehi \u003cspan class=\"h-card\"\u003e\u003ca href=\"http://localhost:8080/@admin\" class=\"u-url mention\" rel=\"nofollow noreferrer noopener\" target=\"_blank\"\u003e@\u003cspan\u003eadmin\u003c/span\u003e\u003c/a\u003e\u003c/span\u003e here's some media for ya\u003c/p\u003e\u003cdiv class=\"gts-system-message gts-placeholder-attachments\"\u003e\u003chr\u003e\u003cp\u003e\u003ci lang=\"en\"\u003eℹ️ Note from localhost:8080: 2 attachments in this status were not downloaded. Treat the following external links with care:\u003c/i\u003e\u003c/p\u003e\u003cul\u003e\u003cli\u003e\u003ca href=\"http://example.org/fileserver/01HE7Y659ZWZ02JM4AWYJZ176Q/attachment/original/01HE7ZGJYTSYMXF927GF9353KR.svg\" rel=\"nofollow noreferrer noopener\" target=\"_blank\"\u003e01HE7ZGJYTSYMXF927GF9353KR.svg\u003c/a\u003e [SVG line art of a sloth, public domain]\u003c/li\u003e\u003cli\u003e\u003ca href=\"http://example.org/fileserver/01HE7Y659ZWZ02JM4AWYJZ176Q/attachment/original/01HE892Y8ZS68TQCNPX7J888P3.mp3\" rel=\"nofollow noreferrer noopener\" target=\"_blank\"\u003e01HE892Y8ZS68TQCNPX7J888P3.mp3\u003c/a\u003e [Jolly salsa song, public domain.]\u003c/li\u003e\u003c/ul\u003e\u003c/div\u003e",
   "reblog": null,
   "account": {
     "id": "01FHMQX3GAABWSM0S2VZEC2SWC",
@@ -1828,7 +1828,7 @@ func (suite *InternalToFrontendTestSuite) TestStatusToAPIStatusPendingApproval()
   "muted": false,
   "bookmarked": false,
   "pinned": false,
-  "content": "<p>Hi <span class=\"h-card\"><a href=\"http://localhost:8080/@1happyturtle\" class=\"u-url mention\" rel=\"nofollow noreferrer noopener\" target=\"_blank\">@<span>1happyturtle</span></a></span>, can I reply?</p><hr><p><i lang=\"en\">ℹ️ Note from localhost:8080: This reply is pending your approval. You can quickly accept it by liking, boosting or replying to it. You can also accept or reject it at the following link: <a href=\"http://localhost:8080/settings/user/interaction_requests/01J5QVXCCEATJYSXM9H6MZT4JR\" rel=\"noreferrer noopener nofollow\" target=\"_blank\">http://localhost:8080/settings/user/interaction_requests/01J5QVXCCEATJYSXM9H6MZT4JR</a>.</i></p>",
+  "content": "<p>Hi <span class=\"h-card\"><a href=\"http://localhost:8080/@1happyturtle\" class=\"u-url mention\" rel=\"nofollow noreferrer noopener\" target=\"_blank\">@<span>1happyturtle</span></a></span>, can I reply?</p><div class=\"gts-system-message gts-pending-reply\"><hr><p><i lang=\"en\">ℹ️ Note from localhost:8080: This reply is pending your approval. You can quickly accept it by liking, boosting or replying to it. You can also accept or reject it at the following link: <a href=\"http://localhost:8080/settings/user/interaction_requests/01J5QVXCCEATJYSXM9H6MZT4JR\" rel=\"noreferrer noopener nofollow\" target=\"_blank\">http://localhost:8080/settings/user/interaction_requests/01J5QVXCCEATJYSXM9H6MZT4JR</a>.</i></p></div>",
   "reblog": null,
   "application": {
     "name": "superseriousbusiness",

internal/typeutils/util.go

@@ -128,12 +128,14 @@ func misskeyReportInlineURLs(content string) []*url.URL {
 //
 // Example:
 //
+//	<div class="gts-system-message gts-placeholder-attachments">
 //		<hr>
 //		<p><i lang="en">ℹ️ Note from your.instance.com: 2 attachment(s) in this status were not downloaded. Treat the following external link(s) with care:</i></p>
 //		<ul>
 //			<li><a href="http://example.org/fileserver/01HE7Y659ZWZ02JM4AWYJZ176Q/attachment/original/01HE7ZGJYTSYMXF927GF9353KR.svg" rel="nofollow noreferrer noopener" target="_blank">01HE7ZGJYTSYMXF927GF9353KR.svg</a> [SVG line art of a sloth, public domain]</li>
 //			<li><a href="http://example.org/fileserver/01HE7Y659ZWZ02JM4AWYJZ176Q/attachment/original/01HE892Y8ZS68TQCNPX7J888P3.mp3" rel="nofollow noreferrer noopener" target="_blank">01HE892Y8ZS68TQCNPX7J888P3.mp3</a> [Jolly salsa song, public domain.]</li>
 //		</ul>
+//	</div>
 func placeholderAttachments(arr []*apimodel.Attachment) (string, []*apimodel.Attachment) {
 
 	// Extract non-locally stored attachments into a
@@ -187,7 +189,7 @@ func placeholderAttachments(arr []*apimodel.Attachment) (string, []*apimodel.Att
 	}
 	note.WriteString(`</ul>`)
 
-	return text.SanitizeToHTML(note.String()), arr
+	return systemMessage("gts-placeholder-attachments", note.String()), arr
 }
 
 func (c *Converter) pendingReplyNote(
@@ -228,7 +230,27 @@ func (c *Converter) pendingReplyNote(
 	note.WriteString(`</a>.`)
 	note.WriteString(`</i></p>`)
 
-	return text.SanitizeToHTML(note.String()), nil
+	return systemMessage("gts-pending-reply", note.String()), nil
+}
+
+// systemMessage wraps a note with a div with semantic classes that aren't allowed through the sanitizer,
+// but may be emitted to the client as an addition to the status's actual content.
+// Clients may want to display these specially or suppress them in favor of their own UI.
+//
+// messageClass must be valid inside an HTML attribute and should be one or more classes starting with `gts-`.
+func systemMessage(
+	messageClass string,
+	unsanitizedNoteHTML string,
+) string {
+	var wrappedNote strings.Builder
+
+	wrappedNote.WriteString(`<div class="gts-system-message `)
+	wrappedNote.WriteString(messageClass)
+	wrappedNote.WriteString(`">`)
+	wrappedNote.WriteString(text.SanitizeToHTML(unsanitizedNoteHTML))
+	wrappedNote.WriteString(`</div>`)
+
+	return wrappedNote.String()
 }
 
 // ContentToContentLanguage tries to