[feature] Federate status language in and out (#2366)

* [feature] Federate status language in + out

* go fmt

* tests, little fix

* improve comments

* unnest a bit

* avoid unnecessary nil check

* use more descriptive variable for contentMap

* prefer instance languages when selecting from contentMap

* update docs to reflect lang selection

* rename rdfLangString -> rdfLangs

* update comments to mention Pollable

* iter through slice instead of map

6 files changed, 411 insertions, 143 deletions

diff --git a/internal/ap/ap_test.go b/internal/ap/ap_test.go
index 6a5073c63..583a37c53 100644
--- a/internal/ap/ap_test.go
+++ b/internal/ap/ap_test.go
@@ -93,6 +93,12 @@ func noteWithMentions1() vocab.ActivityStreamsNote {
 
 	content := streams.NewActivityStreamsContentProperty()
 	content.AppendXMLSchemaString("hey @f0x and @dumpsterqueer")
+
+	rdfLangString := make(map[string]string)
+	rdfLangString["en"] = "hey @f0x and @dumpsterqueer"
+	rdfLangString["fr"] = "bonjour @f0x et @dumpsterqueer"
+	content.AppendRDFLangString(rdfLangString)
+
 	note.SetActivityStreamsContent(content)
 
 	return note
diff --git a/internal/ap/extract.go b/internal/ap/extract.go
index 424f77409..3d92fa2ba 100644
--- a/internal/ap/extract.go
+++ b/internal/ap/extract.go
@@ -631,27 +631,34 @@ func ExtractPublicKey(i WithPublicKey) (
 	return nil, nil, nil, gtserror.New("couldn't find public key")
 }
 
-// ExtractContent returns a string representation of the
-// given interface's Content property, or an empty string
-// if no Content is found.
-func ExtractContent(i WithContent) string {
-	contentProperty := i.GetActivityStreamsContent()
-	if contentProperty == nil {
-		return ""
+// ExtractContent returns an intermediary representation of
+// the given interface's Content and/or ContentMap property.
+func ExtractContent(i WithContent) gtsmodel.Content {
+	content := gtsmodel.Content{}
+
+	contentProp := i.GetActivityStreamsContent()
+	if contentProp == nil {
+		// No content at all.
+		return content
 	}
 
-	for iter := contentProperty.Begin(); iter != contentProperty.End(); iter = iter.Next() {
+	for iter := contentProp.Begin(); iter != contentProp.End(); iter = iter.Next() {
 		switch {
-		// Content may be parsed as IRI, depending on
-		// how it's formatted, so account for this.
-		case iter.IsXMLSchemaString():
-			return iter.GetXMLSchemaString()
-		case iter.IsIRI():
-			return iter.GetIRI().String()
+		case iter.IsRDFLangString() &&
+			len(content.ContentMap) == 0:
+			content.ContentMap = iter.GetRDFLangString()
+
+		case iter.IsXMLSchemaString() &&
+			content.Content == "":
+			content.Content = iter.GetXMLSchemaString()
+
+		case iter.IsIRI() &&
+			content.Content == "":
+			content.Content = iter.GetIRI().String()
 		}
 	}
 
-	return ""
+	return content
 }
 
 // ExtractAttachments attempts to extract barebones MediaAttachment objects from given AS interface type.
diff --git a/internal/ap/extractcontent_test.go b/internal/ap/extractcontent_test.go
index 590d1b931..c899a10e1 100644
--- a/internal/ap/extractcontent_test.go
+++ b/internal/ap/extractcontent_test.go
@@ -30,10 +30,11 @@ type ExtractContentTestSuite struct {
 
 func (suite *ExtractContentTestSuite) TestExtractContent1() {
 	note := suite.noteWithMentions1
-
 	content := ap.ExtractContent(note)
 
-	suite.Equal("hey @f0x and @dumpsterqueer", content)
+	suite.Equal("hey @f0x and @dumpsterqueer", content.Content)
+	suite.Equal("bonjour @f0x et @dumpsterqueer", content.ContentMap["fr"])
+	suite.Equal("hey @f0x and @dumpsterqueer", content.ContentMap["en"])
 }
 
 func TestExtractContentTestSuite(t *testing.T) {
diff --git a/internal/ap/normalize.go b/internal/ap/normalize.go
index 192a2d740..a27527b84 100644
--- a/internal/ap/normalize.go
+++ b/internal/ap/normalize.go
@@ -20,11 +20,12 @@ package ap
 import (
 	"github.com/superseriousbusiness/activity/pub"
 	"github.com/superseriousbusiness/activity/streams"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
 	"github.com/superseriousbusiness/gotosocial/internal/text"
 )
 
 /*
-	NORMALIZE INCOMING
+	INCOMING NORMALIZATION
 	The below functions should be called to normalize the content
 	of messages *COMING INTO* GoToSocial via the federation API,
 	either as the result of delivery from a remote instance to this
@@ -84,39 +85,84 @@ func NormalizeIncomingActivity(activity pub.Activity, rawJSON map[string]interfa
 	}
 }
 
-// NormalizeIncomingContent replaces the Content of the given item
-// with the sanitized version of the raw 'content' value from the
-// raw json object map.
+// normalizeContent normalizes the given content
+// string by sanitizing its HTML and minimizing it.
 //
-// noop if there was no content in the json object map or the
-// content was not a plain string.
-func NormalizeIncomingContent(item WithContent, rawJSON map[string]interface{}) {
-	rawContent, ok := rawJSON["content"]
-	if !ok {
-		// No content in rawJSON.
-		// TODO: In future we might also
-		// look for "contentMap" property.
-		return
+// Noop for non-string content.
+func normalizeContent(rawContent interface{}) string {
+	if rawContent == nil {
+		// Nothing to fix.
+		return ""
 	}
 
 	content, ok := rawContent.(string)
 	if !ok {
-		// Not interested in content arrays.
-		return
+		// Not interested in
+		// content slices etc.
+		return ""
+	}
+
+	if content == "" {
+		// Nothing to fix.
+		return ""
 	}
 
-	// Content should be HTML encoded by default:
+	// Content entries should be HTML encoded by default:
 	// https://www.w3.org/TR/activitystreams-vocabulary/#dfn-content
 	//
 	// TODO: sanitize differently based on mediaType.
 	// https://www.w3.org/TR/activitystreams-vocabulary/#dfn-mediatype
 	content = text.SanitizeToHTML(content)
 	content = text.MinifyHTML(content)
+	return content
+}
 
-	// Set normalized content property from the raw string;
-	// this replaces any existing content property on the item.
+// NormalizeIncomingContent replaces the Content property of the given
+// item with the normalized versions of the raw 'content' and 'contentMap'
+// values from the raw json object map.
+//
+// noop if there was no 'content' or 'contentMap' in the json object map.
+func NormalizeIncomingContent(item WithContent, rawJSON map[string]interface{}) {
+	var (
+		rawContent    = rawJSON["content"]
+		rawContentMap = rawJSON["contentMap"]
+	)
+
+	if rawContent == nil &&
+		rawContentMap == nil {
+		// Nothing to normalize,
+		// leave no content on item.
+		return
+	}
+
+	// Create wrapper for normalized content.
 	contentProp := streams.NewActivityStreamsContentProperty()
-	contentProp.AppendXMLSchemaString(content)
+
+	// Fix 'content' if applicable.
+	content := normalizeContent(rawContent)
+	if content != "" {
+		contentProp.AppendXMLSchemaString(content)
+	}
+
+	// Fix 'contentMap' if applicable.
+	contentMap, ok := rawContentMap.(map[string]interface{})
+	if ok {
+		rdfLangs := make(map[string]string, len(contentMap))
+
+		for lang, rawContent := range contentMap {
+			content := normalizeContent(rawContent)
+			if content != "" {
+				rdfLangs[lang] = content
+			}
+		}
+
+		if len(rdfLangs) != 0 {
+			contentProp.AppendRDFLangString(rdfLangs)
+		}
+	}
+
+	// Replace any existing content property
+	// on the item with normalized version.
 	item.SetActivityStreamsContent(contentProp)
 }
 
@@ -299,3 +345,204 @@ func NormalizeIncomingPollOptions(item WithOneOf, rawJSON map[string]interface{}
 		NormalizeIncomingName(choiceable, rawChoice)
 	}
 }
+
+/*
+	OUTGOING NORMALIZATION
+	The below functions should be called to normalize the content
+	of messages *GOING OUT OF* GoToSocial via the federation API,
+	either as the result of delivery to a remote instance from this
+	instance, or as a result of a remote instance doing an http call
+	to us to dereference something.
+*/
+
+// NormalizeOutgoingAttachmentProp replaces single-entry Attachment objects with
+// single-entry arrays, for better compatibility with other AP implementations.
+//
+// Ie:
+//
+//	"attachment": {
+//	  ...
+//	}
+//
+// becomes:
+//
+//	"attachment": [
+//	  {
+//	    ...
+//	  }
+//	]
+//
+// Noop for items with no attachments, or with attachments that are already a slice.
+func NormalizeOutgoingAttachmentProp(item WithAttachment, rawJSON map[string]interface{}) {
+	attachment, ok := rawJSON["attachment"]
+	if !ok {
+		// No 'attachment',
+		// nothing to change.
+		return
+	}
+
+	if _, ok := attachment.([]interface{}); ok {
+		// Already slice,
+		// nothing to change.
+		return
+	}
+
+	// Coerce single-object to slice.
+	rawJSON["attachment"] = []interface{}{attachment}
+}
+
+// NormalizeOutgoingContentProp normalizes go-fed's funky formatting of content and
+// contentMap properties to a format better understood by other AP implementations.
+//
+// Ie., incoming "content" property like this:
+//
+//	"content": [
+//	  "hello world!",
+//	  {
+//	    "en": "hello world!"
+//	  }
+//	]
+//
+// Is unpacked to:
+//
+//	"content": "hello world!",
+//	"contentMap": {
+//	  "en": "hello world!"
+//	}
+//
+// Noop if neither content nor contentMap are set.
+func NormalizeOutgoingContentProp(item WithContent, rawJSON map[string]interface{}) {
+	contentProp := item.GetActivityStreamsContent()
+	if contentProp == nil {
+		// Nothing to do,
+		// bail early.
+		return
+	}
+
+	contentPropLen := contentProp.Len()
+	if contentPropLen == 0 {
+		// Nothing to do,
+		// bail early.
+		return
+	}
+
+	var (
+		content    string
+		contentMap map[string]string
+	)
+
+	for iter := contentProp.Begin(); iter != contentProp.End(); iter = iter.Next() {
+		switch {
+		case iter.IsRDFLangString() &&
+			contentMap == nil:
+			contentMap = iter.GetRDFLangString()
+
+		case content == "" &&
+			iter.IsXMLSchemaString():
+			content = iter.GetXMLSchemaString()
+		}
+	}
+
+	if content != "" {
+		rawJSON["content"] = content
+	} else {
+		delete(rawJSON, "content")
+	}
+
+	if contentMap != nil {
+		rawJSON["contentMap"] = contentMap
+	} else {
+		delete(rawJSON, "contentMap")
+	}
+}
+
+// NormalizeOutgoingObjectProp normalizes each Object entry in the rawJSON of the given
+// item by calling custom serialization / normalization functions on them in turn.
+//
+// This function also unnests single-entry arrays, so that:
+//
+//	"object": [
+//	  {
+//	    ...
+//	  }
+//	]
+//
+// Becomes:
+//
+//	"object": {
+//	  ...
+//	}
+//
+// Noop for each Object entry that isn't an Accountable or Statusable.
+func NormalizeOutgoingObjectProp(item WithObject, rawJSON map[string]interface{}) error {
+	objectProp := item.GetActivityStreamsObject()
+	if objectProp == nil {
+		// Nothing to do,
+		// bail early.
+		return nil
+	}
+
+	objectPropLen := objectProp.Len()
+	if objectPropLen == 0 {
+		// Nothing to do,
+		// bail early.
+		return nil
+	}
+
+	// The thing we already serialized has objects
+	// on it, so we should see if we need to custom
+	// serialize any of those objects, and replace
+	// them on the data map as necessary.
+	objects := make([]interface{}, 0, objectPropLen)
+	for iter := objectProp.Begin(); iter != objectProp.End(); iter = iter.Next() {
+		if iter.IsIRI() {
+			// Plain IRIs don't need custom serialization.
+			objects = append(objects, iter.GetIRI().String())
+			continue
+		}
+
+		var (
+			objectType = iter.GetType()
+			objectSer  map[string]interface{}
+		)
+
+		if objectType == nil {
+			// This is awkward.
+			return gtserror.Newf("could not resolve object iter %T to vocab.Type", iter)
+		}
+
+		var err error
+
+		// In the below accountable and statusable serialization,
+		// `@context` will be included in the wrapping type already,
+		// so we shouldn't also include it in the object itself.
+		switch tn := objectType.GetTypeName(); {
+		case IsAccountable(tn):
+			objectSer, err = serializeAccountable(objectType, false)
+
+		case IsStatusable(tn):
+			// IsStatusable includes Pollable as well.
+			objectSer, err = serializeStatusable(objectType, false)
+
+		default:
+			// No custom serializer for this type; serialize as normal.
+			objectSer, err = objectType.Serialize()
+		}
+
+		if err != nil {
+			return err
+		}
+
+		objects = append(objects, objectSer)
+	}
+
+	if objectPropLen == 1 {
+		// Unnest single object.
+		rawJSON["object"] = objects[0]
+	} else {
+		// Array of objects.
+		rawJSON["object"] = objects
+	}
+
+	return nil
+}
diff --git a/internal/ap/normalize_test.go b/internal/ap/normalize_test.go
index cd1affe60..33b1f6ea6 100644
--- a/internal/ap/normalize_test.go
+++ b/internal/ap/normalize_test.go
@@ -46,6 +46,9 @@ func (suite *NormalizeTestSuite) getStatusable() (vocab.ActivityStreamsNote, map
 		  "https://example.org/users/someone/followers"
 		],
 		"content": "UPDATE: As of this morning there are now more than 7 million Mastodon users, most from the <a class=\"hashtag\" data-tag=\"twittermigration\" href=\"https://example.org/tag/twittermigration\" rel=\"tag ugc\">#TwitterMigration</a>.<br><br>In fact, 100,000 new accounts have been created since last night.<br><br>Since last night&#39;s spike 8,000-12,000 new accounts are being created every hour.<br><br>Yesterday, I estimated that Mastodon would have 8 million users by the end of the week. That might happen a lot sooner if this trend continues.",
+		"contentMap": {
+			"en": "UPDATE: As of this morning there are now more than 7 million Mastodon users, most from the <a class=\"hashtag\" data-tag=\"twittermigration\" href=\"https://example.org/tag/twittermigration\" rel=\"tag ugc\">#TwitterMigration</a>.<br><br>In fact, 100,000 new accounts have been created since last night.<br><br>Since last night&#39;s spike 8,000-12,000 new accounts are being created every hour.<br><br>Yesterday, I estimated that Mastodon would have 8 million users by the end of the week. That might happen a lot sooner if this trend continues."
+		},
 		"context": "https://example.org/contexts/01GX0MSHPER1E0FT022Q209EJZ",
 		"conversation": "https://example.org/contexts/01GX0MSHPER1E0FT022Q209EJZ",
 		"id": "https://example.org/objects/01GX0MT2PA58JNSMK11MCS65YD",
@@ -182,7 +185,15 @@ func (suite *NormalizeTestSuite) getAccountable() (vocab.ActivityStreamsPerson,
 
 func (suite *NormalizeTestSuite) TestNormalizeActivityObject() {
 	note, rawNote := suite.getStatusable()
-	suite.Equal(`update: As of this morning there are now more than 7 million Mastodon users, most from the <a class="hashtag" data-tag="twittermigration" href="https://example.org/tag/twittermigration" rel="tag ugc">#TwitterMigration%3C/a%3E.%3Cbr%3E%3Cbr%3EIn%20fact,%20100,000%20new%20accounts%20have%20been%20created%20since%20last%20night.%3Cbr%3E%3Cbr%3ESince%20last%20night&%2339;s%20spike%208,000-12,000%20new%20accounts%20are%20being%20created%20every%20hour.%3Cbr%3E%3Cbr%3EYesterday,%20I%20estimated%20that%20Mastodon%20would%20have%208%20million%20users%20by%20the%20end%20of%20the%20week.%20That%20might%20happen%20a%20lot%20sooner%20if%20this%20trend%20continues.`, ap.ExtractContent(note))
+	content := ap.ExtractContent(note)
+	suite.Equal(
+		`update: As of this morning there are now more than 7 million Mastodon users, most from the <a class="hashtag" data-tag="twittermigration" href="https://example.org/tag/twittermigration" rel="tag ugc">#TwitterMigration%3C/a%3E.%3Cbr%3E%3Cbr%3EIn%20fact,%20100,000%20new%20accounts%20have%20been%20created%20since%20last%20night.%3Cbr%3E%3Cbr%3ESince%20last%20night&%2339;s%20spike%208,000-12,000%20new%20accounts%20are%20being%20created%20every%20hour.%3Cbr%3E%3Cbr%3EYesterday,%20I%20estimated%20that%20Mastodon%20would%20have%208%20million%20users%20by%20the%20end%20of%20the%20week.%20That%20might%20happen%20a%20lot%20sooner%20if%20this%20trend%20continues.`,
+		content.Content,
+	)
+
+	// Malformed contentMap entry
+	// will not be extractable yet.
+	suite.Empty(content.ContentMap["en"])
 
 	create := testrig.WrapAPNoteInCreate(
 		testrig.URLMustParse("https://example.org/create_something"),
@@ -192,7 +203,18 @@ func (suite *NormalizeTestSuite) TestNormalizeActivityObject() {
 	)
 
 	ap.NormalizeIncomingActivity(create, map[string]interface{}{"object": rawNote})
-	suite.Equal(`UPDATE: As of this morning there are now more than 7 million Mastodon users, most from the <a class="hashtag" href="https://example.org/tag/twittermigration" rel="tag ugc nofollow noreferrer noopener" target="_blank">#TwitterMigration</a>.<br><br>In fact, 100,000 new accounts have been created since last night.<br><br>Since last night's spike 8,000-12,000 new accounts are being created every hour.<br><br>Yesterday, I estimated that Mastodon would have 8 million users by the end of the week. That might happen a lot sooner if this trend continues.`, ap.ExtractContent(note))
+	content = ap.ExtractContent(note)
+
+	suite.Equal(
+		`UPDATE: As of this morning there are now more than 7 million Mastodon users, most from the <a class="hashtag" href="https://example.org/tag/twittermigration" rel="tag ugc nofollow noreferrer noopener" target="_blank">#TwitterMigration</a>.<br><br>In fact, 100,000 new accounts have been created since last night.<br><br>Since last night's spike 8,000-12,000 new accounts are being created every hour.<br><br>Yesterday, I estimated that Mastodon would have 8 million users by the end of the week. That might happen a lot sooner if this trend continues.`,
+		content.Content,
+	)
+
+	// Content map entry should now be extractable.
+	suite.Equal(
+		`UPDATE: As of this morning there are now more than 7 million Mastodon users, most from the <a class="hashtag" href="https://example.org/tag/twittermigration" rel="tag ugc nofollow noreferrer noopener" target="_blank">#TwitterMigration</a>.<br><br>In fact, 100,000 new accounts have been created since last night.<br><br>Since last night's spike 8,000-12,000 new accounts are being created every hour.<br><br>Yesterday, I estimated that Mastodon would have 8 million users by the end of the week. That might happen a lot sooner if this trend continues.`,
+		content.ContentMap["en"],
+	)
 }
 
 func (suite *NormalizeTestSuite) TestNormalizeStatusableAttachmentsOneAttachment() {
@@ -202,12 +224,14 @@ func (suite *NormalizeTestSuite) TestNormalizeStatusableAttachmentsOneAttachment
 	// the attachment(s) should be all jacked up.
 	suite.Equal(`{
   "@context": "https://www.w3.org/ns/activitystreams",
-  "attachment": {
-    "mediaType": "image/jpeg",
-    "name": "description: here's \u003c\u003ca\u003e\u003e picture of a #cat,%20it%27s%20cute!%20here%27s%20some%20special%20characters:%20%22%22%20%5C%20weeee%27%27%27%27",
-    "type": "Document",
-    "url": "https://files.example.org/media_attachments/files/110/258/459/579/509/026/original/b65392ebe0fb04ef.jpeg"
-  },
+  "attachment": [
+    {
+      "mediaType": "image/jpeg",
+      "name": "description: here's \u003c\u003ca\u003e\u003e picture of a #cat,%20it%27s%20cute!%20here%27s%20some%20special%20characters:%20%22%22%20%5C%20weeee%27%27%27%27",
+      "type": "Document",
+      "url": "https://files.example.org/media_attachments/files/110/258/459/579/509/026/original/b65392ebe0fb04ef.jpeg"
+    }
+  ],
   "attributedTo": "https://example.org/users/hourlycatbot",
   "id": "https://example.org/users/hourlycatbot/statuses/01GYW48H311PZ78C5G856MGJJJ",
   "to": "https://www.w3.org/ns/activitystreams#Public",
@@ -222,12 +246,14 @@ func (suite *NormalizeTestSuite) TestNormalizeStatusableAttachmentsOneAttachment
 	// attachment should no longer be all jacked up.
 	suite.Equal(`{
   "@context": "https://www.w3.org/ns/activitystreams",
-  "attachment": {
-    "mediaType": "image/jpeg",
-    "name": "DESCRIPTION: here's \u003c\u003e picture of a #cat, it's cute! here's some special characters: \"\" \\ weeee''''",
-    "type": "Document",
-    "url": "https://files.example.org/media_attachments/files/110/258/459/579/509/026/original/b65392ebe0fb04ef.jpeg"
-  },
+  "attachment": [
+    {
+      "mediaType": "image/jpeg",
+      "name": "DESCRIPTION: here's \u003c\u003e picture of a #cat, it's cute! here's some special characters: \"\" \\ weeee''''",
+      "type": "Document",
+      "url": "https://files.example.org/media_attachments/files/110/258/459/579/509/026/original/b65392ebe0fb04ef.jpeg"
+    }
+  ],
   "attributedTo": "https://example.org/users/hourlycatbot",
   "id": "https://example.org/users/hourlycatbot/statuses/01GYW48H311PZ78C5G856MGJJJ",
   "to": "https://www.w3.org/ns/activitystreams#Public",
@@ -243,12 +269,14 @@ func (suite *NormalizeTestSuite) TestNormalizeStatusableAttachmentsOneAttachment
 	// the attachment(s) should be all jacked up.
 	suite.Equal(`{
   "@context": "https://www.w3.org/ns/activitystreams",
-  "attachment": {
-    "mediaType": "image/jpeg",
-    "name": "description: here's \u003c\u003ca\u003e\u003e picture of a #cat,%20it%27s%20cute!%20here%27s%20some%20special%20characters:%20%22%22%20%5C%20weeee%27%27%27%27",
-    "type": "Document",
-    "url": "https://files.example.org/media_attachments/files/110/258/459/579/509/026/original/b65392ebe0fb04ef.jpeg"
-  },
+  "attachment": [
+    {
+      "mediaType": "image/jpeg",
+      "name": "description: here's \u003c\u003ca\u003e\u003e picture of a #cat,%20it%27s%20cute!%20here%27s%20some%20special%20characters:%20%22%22%20%5C%20weeee%27%27%27%27",
+      "type": "Document",
+      "url": "https://files.example.org/media_attachments/files/110/258/459/579/509/026/original/b65392ebe0fb04ef.jpeg"
+    }
+  ],
   "attributedTo": "https://example.org/users/hourlycatbot",
   "id": "https://example.org/users/hourlycatbot/statuses/01GYW48H311PZ78C5G856MGJJJ",
   "to": "https://www.w3.org/ns/activitystreams#Public",
@@ -263,12 +291,14 @@ func (suite *NormalizeTestSuite) TestNormalizeStatusableAttachmentsOneAttachment
 	// attachment should no longer be all jacked up.
 	suite.Equal(`{
   "@context": "https://www.w3.org/ns/activitystreams",
-  "attachment": {
-    "mediaType": "image/jpeg",
-    "name": "DESCRIPTION: here's \u003c\u003e picture of a #cat, it's cute! here's some special characters: \"\" \\ weeee''''",
-    "type": "Document",
-    "url": "https://files.example.org/media_attachments/files/110/258/459/579/509/026/original/b65392ebe0fb04ef.jpeg"
-  },
+  "attachment": [
+    {
+      "mediaType": "image/jpeg",
+      "name": "DESCRIPTION: here's \u003c\u003e picture of a #cat, it's cute! here's some special characters: \"\" \\ weeee''''",
+      "type": "Document",
+      "url": "https://files.example.org/media_attachments/files/110/258/459/579/509/026/original/b65392ebe0fb04ef.jpeg"
+    }
+  ],
   "attributedTo": "https://example.org/users/hourlycatbot",
   "id": "https://example.org/users/hourlycatbot/statuses/01GYW48H311PZ78C5G856MGJJJ",
   "to": "https://www.w3.org/ns/activitystreams#Public",
diff --git a/internal/ap/serialize.go b/internal/ap/serialize.go
index 368d7f9a2..944e67407 100644
--- a/internal/ap/serialize.go
+++ b/internal/ap/serialize.go
@@ -18,10 +18,9 @@
 package ap
 
 import (
-	"fmt"
-
 	"github.com/superseriousbusiness/activity/streams"
 	"github.com/superseriousbusiness/activity/streams/vocab"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
 )
 
 // Serialize is a custom serializer for ActivityStreams types.
@@ -35,17 +34,20 @@ import (
 //
 // Currently, the following things will be custom serialized:
 //
-//   - OrderedCollection: 'orderedItems' property will always be made into an array.
-//   - Any Accountable type: 'attachment' property will always be made into an array.
-//   - Update: any Accountable 'object's set on an update will be custom serialized as above.
+//   - OrderedCollection:       'orderedItems' property will always be made into an array.
+//   - Any Accountable type:    'attachment' property will always be made into an array.
+//   - Any Statusable type:     'attachment' property will always be made into an array; 'content' and 'contentMap' will be normalized.
+//   - Any Activityable type:   any 'object's set on an activity will be custom serialized as above.
 func Serialize(t vocab.Type) (m map[string]interface{}, e error) {
-	switch t.GetTypeName() {
-	case ObjectOrderedCollection:
+	switch tn := t.GetTypeName(); {
+	case tn == ObjectOrderedCollection:
 		return serializeOrderedCollection(t)
-	case ActorApplication, ActorGroup, ActorOrganization, ActorPerson, ActorService:
+	case IsAccountable(tn):
 		return serializeAccountable(t, true)
-	case ActivityUpdate:
-		return serializeWithObject(t)
+	case IsStatusable(tn):
+		return serializeStatusable(t, true)
+	case IsActivityable(tn):
+		return serializeActivityable(t, true)
 	default:
 		// No custom serializer necessary.
 		return streams.Serialize(t)
@@ -61,8 +63,8 @@ func Serialize(t vocab.Type) (m map[string]interface{}, e error) {
 // See:
 //   - https://github.com/go-fed/activity/issues/139
 //   - https://github.com/mastodon/mastodon/issues/24225
-func serializeOrderedCollection(orderedCollection vocab.Type) (map[string]interface{}, error) {
-	data, err := streams.Serialize(orderedCollection)
+func serializeOrderedCollection(t vocab.Type) (map[string]interface{}, error) {
+	data, err := streams.Serialize(t)
 	if err != nil {
 		return nil, err
 	}
@@ -99,7 +101,12 @@ func serializeOrderedCollection(orderedCollection vocab.Type) (map[string]interf
 // If the accountable is being serialized as part of another object (eg., as the
 // object of an activity), then includeContext should be set to false, as the
 // @context entry should be included on the top-level/wrapping activity/object.
-func serializeAccountable(accountable vocab.Type, includeContext bool) (map[string]interface{}, error) {
+func serializeAccountable(t vocab.Type, includeContext bool) (map[string]interface{}, error) {
+	accountable, ok := t.(Accountable)
+	if !ok {
+		return nil, gtserror.Newf("vocab.Type %T not accountable", t)
+	}
+
 	var (
 		data map[string]interface{}
 		err  error
@@ -115,91 +122,61 @@ func serializeAccountable(accountable vocab.Type, includeContext bool) (map[stri
 		return nil, err
 	}
 
-	attachment, ok := data["attachment"]
-	if !ok {
-		// No 'attachment', nothing to change.
-		return data, nil
-	}
-
-	if _, ok := attachment.([]interface{}); ok {
-		// Already slice.
-		return data, nil
-	}
-
-	// Coerce single-object to slice.
-	data["attachment"] = []interface{}{attachment}
+	NormalizeOutgoingAttachmentProp(accountable, data)
 
 	return data, nil
 }
 
-func serializeWithObject(t vocab.Type) (map[string]interface{}, error) {
-	withObject, ok := t.(WithObject)
+func serializeStatusable(t vocab.Type, includeContext bool) (map[string]interface{}, error) {
+	statusable, ok := t.(Statusable)
 	if !ok {
-		return nil, fmt.Errorf("serializeWithObject: could not resolve %T to WithObject", t)
+		return nil, gtserror.Newf("vocab.Type %T not statusable", t)
+	}
+
+	var (
+		data map[string]interface{}
+		err  error
+	)
+
+	if includeContext {
+		data, err = streams.Serialize(statusable)
+	} else {
+		data, err = statusable.Serialize()
 	}
 
-	data, err := streams.Serialize(t)
 	if err != nil {
 		return nil, err
 	}
 
-	object := withObject.GetActivityStreamsObject()
-	if object == nil {
-		// Nothing to do, bail early.
-		return data, nil
+	NormalizeOutgoingAttachmentProp(statusable, data)
+	NormalizeOutgoingContentProp(statusable, data)
+
+	return data, nil
+}
+
+func serializeActivityable(t vocab.Type, includeContext bool) (map[string]interface{}, error) {
+	activityable, ok := t.(Activityable)
+	if !ok {
+		return nil, gtserror.Newf("vocab.Type %T not activityable", t)
 	}
 
-	objectLen := object.Len()
-	if objectLen == 0 {
-		// Nothing to do, bail early.
-		return data, nil
+	var (
+		data map[string]interface{}
+		err  error
+	)
+
+	if includeContext {
+		data, err = streams.Serialize(activityable)
+	} else {
+		data, err = activityable.Serialize()
 	}
 
-	// The thing we already serialized has objects
-	// on it, so we should see if we need to custom
-	// serialize any of those objects, and replace
-	// them on the data map as necessary.
-	objects := make([]interface{}, 0, objectLen)
-	for iter := object.Begin(); iter != object.End(); iter = iter.Next() {
-		if iter.IsIRI() {
-			// Plain IRIs don't need custom serialization.
-			objects = append(objects, iter.GetIRI().String())
-			continue
-		}
-
-		var (
-			objectType = iter.GetType()
-			objectSer  map[string]interface{}
-		)
-
-		if objectType == nil {
-			// This is awkward.
-			return nil, fmt.Errorf("serializeWithObject: could not resolve object iter %T to vocab.Type", iter)
-		}
-
-		switch objectType.GetTypeName() {
-		case ActorApplication, ActorGroup, ActorOrganization, ActorPerson, ActorService:
-			// @context will be included in wrapping type already,
-			// we don't need to include it in the object itself.
-			objectSer, err = serializeAccountable(objectType, false)
-		default:
-			// No custom serializer for this type; serialize as normal.
-			objectSer, err = objectType.Serialize()
-		}
-
-		if err != nil {
-			return nil, err
-		}
-
-		objects = append(objects, objectSer)
+	if err != nil {
+		return nil, err
 	}
 
-	if objectLen == 1 {
-		// Unnest single object.
-		data["object"] = objects[0]
-	} else {
-		// Array of objects.
-		data["object"] = objects
+	if err := NormalizeOutgoingObjectProp(activityable, data); err != nil {
+		return nil, err
 	}
 
 	return data, nil