[bugfix] Prevent multiple mentions being created for one mention (#4127)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

This pull request fixes an issue where multiple mentions were being created for one statuses, when delivered to multiple inboxes on an instance, but only the final mention ended up actually being used. Also adds tests to make sure that a notification is received when a status is edited to include a mention.

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4127
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

internal/federation/dereferencing/status.go

@@ -670,9 +670,9 @@ func (d *Dereferencer) fetchStatusMentions(
 			alreadyExists bool
 		)
 
-		// Search existing status for a mention already stored,
+		// Search existing status + db for a mention already stored,
 		// else ensure new mention's target account is populated.
-		mention, alreadyExists, err = d.populateMentionTarget(ctx,
+		mention, alreadyExists, err = d.newOrExistingMention(ctx,
 			requestUser,
 			existing,
 			mention,
@@ -683,8 +683,8 @@ func (d *Dereferencer) fetchStatusMentions(
 		}
 
 		if alreadyExists {
-			// This mention was already attached
-			// to the status, use it and continue.
+			// This mention was already
+			// stored, use it and continue.
 			status.Mentions[i] = mention
 			status.MentionIDs[i] = mention.ID
 			continue
@@ -1294,14 +1294,15 @@ func (d *Dereferencer) handleStatusEdit(
 	return cols, nil
 }
 
-// populateMentionTarget tries to populate the given
+// newOrExistingMention tries to populate the given
 // mention with the correct TargetAccount and (if not
 // yet set) TargetAccountURI, returning the populated
 // mention.
 //
-// Will check on the existing status if the mention
-// is already there and populated; if so, existing
-// mention will be returned along with `true`.
+// Will check on the existing status and in the db
+// if the mention is already there and populated;
+// if so, existing mention will be returned along
+// with `true` to indicate that it already existed.
 //
 // Otherwise, this function will try to parse first
 // the Href of the mention, and then the namestring,
@@ -1312,7 +1313,7 @@ func (d *Dereferencer) handleStatusEdit(
 // rather than a URI, but because some remotes do
 // silly things like only provide `@username` instead
 // of `@username@domain`, we try by URI first.
-func (d *Dereferencer) populateMentionTarget(
+func (d *Dereferencer) newOrExistingMention(
 	ctx context.Context,
 	requestUser string,
 	existing *gtsmodel.Status,
@@ -1325,11 +1326,13 @@ func (d *Dereferencer) populateMentionTarget(
 	// Mentions can be created using `name` or `href`.
 	//
 	// Prefer `href` (TargetAccountURI), fall back to Name.
-	if mention.TargetAccountURI != "" {
-
-		// Look for existing mention with target account's URI, if so use this.
+	switch {
+	case mention.TargetAccountURI != "":
+		// Look on the status for existing mention with target account's URI.
 		existingMention, ok := existing.GetMentionByTargetURI(mention.TargetAccountURI)
 		if ok && existingMention.ID != "" {
+			// Already populated
+			// mention, use this.
 			return existingMention, true, nil
 		}
 
@@ -1354,8 +1357,25 @@ func (d *Dereferencer) populateMentionTarget(
 			err := gtserror.Newf("failed to dereference account %s: %w", targetAccountURI, err)
 			return nil, false, err
 		}
-	} else {
 
+		// Look in the db for this existing mention.
+		existingMention, err = d.state.DB.GetMentionByTargetAcctStatus(
+			ctx,
+			mention.TargetAccount.ID,
+			existing.ID,
+		)
+		if err != nil && !errors.Is(err, db.ErrNoEntries) {
+			err := gtserror.Newf("db error looking for existing mention: %w", err)
+			return nil, false, err
+		}
+
+		if existingMention != nil {
+			// Already had stored
+			// mention, use this.
+			return existingMention, true, nil
+		}
+
+	case mention.NameString != "":
 		// Href wasn't set, extract the username and domain parts from namestring.
 		username, domain, err := util.ExtractNamestringParts(mention.NameString)
 		if err != nil {
@@ -1363,9 +1383,11 @@ func (d *Dereferencer) populateMentionTarget(
 			return nil, false, err
 		}
 
-		// Look for existing mention with username domain target, if so use this.
+		// Look on the status for existing mention with username domain target.
 		existingMention, ok := existing.GetMentionByUsernameDomain(username, domain)
 		if ok && existingMention.ID != "" {
+			// Already populated
+			// mention, use this.
 			return existingMention, true, nil
 		}
 
@@ -1395,11 +1417,34 @@ func (d *Dereferencer) populateMentionTarget(
 			return nil, false, err
 		}
 
-		// Look for existing mention with target account's URI, if so use this.
+		// Look on the status for existing mention with target account's URI.
 		existingMention, ok = existing.GetMentionByTargetURI(mention.TargetAccountURI)
 		if ok && existingMention.ID != "" {
+			// Already populated
+			// mention, use this.
 			return existingMention, true, nil
 		}
+
+		// Look in the db for this existing mention.
+		existingMention, err = d.state.DB.GetMentionByTargetAcctStatus(
+			ctx,
+			mention.TargetAccount.ID,
+			existing.ID,
+		)
+		if err != nil && !errors.Is(err, db.ErrNoEntries) {
+			err := gtserror.Newf("db error looking for existing mention: %w", err)
+			return nil, false, err
+		}
+
+		if existingMention != nil {
+			// Already had stored
+			// mention, use this.
+			return existingMention, true, nil
+		}
+
+	default:
+		const errText = "neither target uri nor namestring were set on mention, cannot process it"
+		return nil, false, gtserror.New(errText)
 	}
 
 	// At this point, mention.TargetAccountURI