feat!: Abusive content reporting (#6977)

This implements milestones 1. and 4. from **Task F. Moderation features: Reporting** (part of [amendment of the workplan](https://codeberg.org/forgejo/sustainability/src/branch/main/2022-12-01-nlnet/2025-02-07-extended-workplan.md#task-f-moderation-features-reporting) for NLnet 2022-12-035):

> 1. A reporting feature is implemented in the database. It ensures that content remains available for review, even if a user deletes it after a report was sent.

> 4. Users can report the most relevant content types (at least: issue comments, repositories, users)

### See also:
- forgejo/discussions#291
- forgejo/discussions#304
- forgejo/design#30

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6977
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: floss4good <floss4good@disroot.org>
Co-committed-by: floss4good <floss4good@disroot.org>

models/user/moderation.go

@@ -0,0 +1,112 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package user
+
+import (
+	"context"
+	"reflect"
+	"slices"
+	"sync"
+
+	"forgejo.org/models/moderation"
+	"forgejo.org/modules/json"
+	"forgejo.org/modules/timeutil"
+
+	"xorm.io/xorm/names"
+)
+
+// UserData represents a trimmed down user that is used for preserving
+// only the fields needed for abusive content reports (mainly string fields).
+type UserData struct { //revive:disable-line:exported
+	Name        string
+	FullName    string
+	Email       string
+	LoginName   string
+	Location    string
+	Website     string
+	Pronouns    string
+	Description string
+	CreatedUnix timeutil.TimeStamp
+	UpdatedUnix timeutil.TimeStamp
+	// This field was intentionally renamed so that is not the same with the one from User struct.
+	// If we keep it the same as in User, during login it might trigger the creation of a shadow copy.
+	// TODO: Should we decide that this field is not that relevant for abuse reporting purposes, better remove it.
+	LastLogin   timeutil.TimeStamp `json:"LastLoginUnix"`
+	Avatar      string
+	AvatarEmail string
+}
+
+// newUserData creates a trimmed down user to be used just to create a JSON structure
+// (keeping only the fields relevant for moderation purposes)
+func newUserData(user *User) UserData {
+	return UserData{
+		Name:        user.Name,
+		FullName:    user.FullName,
+		Email:       user.Email,
+		LoginName:   user.LoginName,
+		Location:    user.Location,
+		Website:     user.Website,
+		Pronouns:    user.Pronouns,
+		Description: user.Description,
+		CreatedUnix: user.CreatedUnix,
+		UpdatedUnix: user.UpdatedUnix,
+		LastLogin:   user.LastLoginUnix,
+		Avatar:      user.Avatar,
+		AvatarEmail: user.AvatarEmail,
+	}
+}
+
+// userDataColumnNames builds (only once) and returns a slice with the column names
+// (e.g. FieldName -> field_name) corresponding to UserData struct fields.
+var userDataColumnNames = sync.OnceValue(func() []string {
+	mapper := new(names.GonicMapper)
+	udType := reflect.TypeOf(UserData{})
+	columnNames := make([]string, 0, udType.NumField())
+	for i := 0; i < udType.NumField(); i++ {
+		columnNames = append(columnNames, mapper.Obj2Table(udType.Field(i).Name))
+	}
+	return columnNames
+})
+
+// IfNeededCreateShadowCopyForUser checks if for the given user there are any reports of abusive content submitted
+// and if found a shadow copy of relevant user fields will be stored into DB and linked to the above report(s).
+// This function should be called before a user is deleted or updated.
+//
+// For deletions alteredCols argument must be omitted.
+//
+// In case of updates it will first checks whether any of the columns being updated (alteredCols argument)
+// is relevant for moderation purposes (i.e. included in the UserData struct).
+func IfNeededCreateShadowCopyForUser(ctx context.Context, user *User, alteredCols ...string) error {
+	// TODO: this can be triggered quite often (e.g. by routers/web/repo/middlewares.go SetDiffViewStyle())
+
+	shouldCheckIfNeeded := len(alteredCols) == 0 // no columns being updated, therefore a deletion
+	if !shouldCheckIfNeeded {
+		// for updates we need to go further only if certain column are being changed
+		for _, colName := range userDataColumnNames() {
+			if shouldCheckIfNeeded = slices.Contains(alteredCols, colName); shouldCheckIfNeeded {
+				break
+			}
+		}
+	}
+
+	if !shouldCheckIfNeeded {
+		return nil
+	}
+
+	shadowCopyNeeded, err := moderation.IsShadowCopyNeeded(ctx, moderation.ReportedContentTypeUser, user.ID)
+	if err != nil {
+		return err
+	}
+
+	if shadowCopyNeeded {
+		userData := newUserData(user)
+		content, err := json.Marshal(userData)
+		if err != nil {
+			return err
+		}
+		return moderation.CreateShadowCopyForUser(ctx, user.ID, string(content))
+	}
+
+	return nil
+}