feat: do not regenerate an existing random avatar

Leave aside the refactoring part of
https://github.com/go-gitea/gitea/pull/33433, keep the improvement.

Also get a test from https://github.com/go-gitea/gitea/pull/31365
while we're at it.

(cherry picked from commit 4ffc54f59a7723eb5aef21955129bdd329ee1d4f)

models/user/avatar.go

@@ -38,14 +38,18 @@ func GenerateRandomAvatar(ctx context.Context, u *User) error {
 
 	u.Avatar = avatars.HashEmail(seed)
 
-	// Don't share the images so that we can delete them easily
-	if err := storage.SaveFrom(storage.Avatars, u.CustomAvatarRelativePath(), func(w io.Writer) error {
-		if err := png.Encode(w, img); err != nil {
-			log.Error("Encode: %v", err)
+	_, err = storage.Avatars.Stat(u.CustomAvatarRelativePath())
+	if err != nil {
+		// If unable to Stat the avatar file (usually it means non-existing), then try to save a new one
+		// Don't share the images so that we can delete them easily
+		if err := storage.SaveFrom(storage.Avatars, u.CustomAvatarRelativePath(), func(w io.Writer) error {
+			if err := png.Encode(w, img); err != nil {
+				log.Error("Encode: %v", err)
+			}
+			return nil
+		}); err != nil {
+			return fmt.Errorf("failed to save avatar %s: %w", u.CustomAvatarRelativePath(), err)
 		}
-		return err
-	}); err != nil {
-		return fmt.Errorf("Failed to create dir %s: %w", u.CustomAvatarRelativePath(), err)
 	}
 
 	if _, err := db.GetEngine(ctx).ID(u.ID).Cols("avatar").Update(u); err != nil {

models/user/avatar_test.go

@@ -0,0 +1,68 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package user
+
+import (
+	"context"
+	"io"
+	"strings"
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/storage"
+	"code.gitea.io/gitea/modules/test"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestUserAvatarLink(t *testing.T) {
+	defer test.MockVariableValue(&setting.AppURL, "https://localhost/")()
+	defer test.MockVariableValue(&setting.AppSubURL, "")()
+
+	u := &User{ID: 1, Avatar: "avatar.png"}
+	link := u.AvatarLink(db.DefaultContext)
+	assert.Equal(t, "https://localhost/avatars/avatar.png", link)
+
+	setting.AppURL = "https://localhost/sub-path/"
+	setting.AppSubURL = "/sub-path"
+	link = u.AvatarLink(db.DefaultContext)
+	assert.Equal(t, "https://localhost/sub-path/avatars/avatar.png", link)
+}
+
+func TestUserAvatarGenerate(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+	var err error
+	tmpDir := t.TempDir()
+	storage.Avatars, err = storage.NewLocalStorage(context.Background(), &setting.Storage{Path: tmpDir})
+	require.NoError(t, err)
+
+	u := unittest.AssertExistsAndLoadBean(t, &User{ID: 2})
+
+	// there was no avatar, generate a new one
+	assert.Empty(t, u.Avatar)
+	err = GenerateRandomAvatar(db.DefaultContext, u)
+	require.NoError(t, err)
+	assert.NotEmpty(t, u.Avatar)
+
+	// make sure the generated one exists
+	oldAvatarPath := u.CustomAvatarRelativePath()
+	_, err = storage.Avatars.Stat(u.CustomAvatarRelativePath())
+	require.NoError(t, err)
+	// and try to change its content
+	_, err = storage.Avatars.Save(u.CustomAvatarRelativePath(), strings.NewReader("abcd"), 4)
+	require.NoError(t, err)
+
+	// try to generate again
+	err = GenerateRandomAvatar(db.DefaultContext, u)
+	require.NoError(t, err)
+	assert.Equal(t, oldAvatarPath, u.CustomAvatarRelativePath())
+	f, err := storage.Avatars.Open(u.CustomAvatarRelativePath())
+	require.NoError(t, err)
+	defer f.Close()
+	content, _ := io.ReadAll(f)
+	assert.Equal(t, "abcd", string(content))
+}