Do not rewrite ssh keys files when deleting a user without one (#6097)

### Problem

Big instances can have huge authorized_keys files when using OpenSSH instead of the internal ssh server. Forgejo always re-generates the contents of that file when a user is deleted, even if he does not even have a public key uploaded. In case of codeberg.org, a 15MB file gets rewritten. If we batch delete 100 Spam users without ssh keys, we rewrite 1.5GB, this takes time and wears the SSD. In addition, there is a high chance of hitting a race contidion bug, when deleting users in parallel.

### Solution / Mitigation

This patch prevents rewriting authorized_keys files, when not necessary. It greatly speeds up deleting malicious users, saves IO bandwidth and SSD wear. It also greatly reduces the chance of hitting a race condition bug. Fixing the race condition is not the scope of this patch though.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6097
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Andreas Shimokawa <shimokawa@fsfe.org>
Co-committed-by: Andreas Shimokawa <shimokawa@fsfe.org>

services/user/user_test.go

@@ -5,11 +5,14 @@ package user
 
 import (
 	"fmt"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"time"
 
 	"code.gitea.io/gitea/models"
+	asymkey_model "code.gitea.io/gitea/models/asymkey"
 	"code.gitea.io/gitea/models/auth"
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/models/organization"
@@ -17,6 +20,7 @@ import (
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
 	"code.gitea.io/gitea/modules/timeutil"
 
 	"github.com/stretchr/testify/assert"
@@ -63,20 +67,61 @@ func TestDeleteUser(t *testing.T) {
 }
 
 func TestPurgeUser(t *testing.T) {
-	test := func(userID int64) {
+	defer unittest.OverrideFixtures(
+		unittest.FixturesOptions{
+			Dir:  filepath.Join(setting.AppWorkPath, "models/fixtures/"),
+			Base: setting.AppWorkPath,
+			Dirs: []string{"services/user/TestPurgeUser/"},
+		},
+	)()
+	require.NoError(t, unittest.PrepareTestDatabase())
+	defer test.MockVariableValue(&setting.SSH.RootPath, t.TempDir())()
+	defer test.MockVariableValue(&setting.SSH.CreateAuthorizedKeysFile, true)()
+	defer test.MockVariableValue(&setting.SSH.CreateAuthorizedPrincipalsFile, true)()
+	defer test.MockVariableValue(&setting.SSH.StartBuiltinServer, false)()
+	require.NoError(t, asymkey_model.RewriteAllPublicKeys(db.DefaultContext))
+	require.NoError(t, asymkey_model.RewriteAllPrincipalKeys(db.DefaultContext))
+
+	test := func(userID int64, modifySSHKey bool) {
 		require.NoError(t, unittest.PrepareTestDatabase())
 		user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: userID})
 
-		err := DeleteUser(db.DefaultContext, user, true)
+		fAuthorizedKeys, err := os.Open(filepath.Join(setting.SSH.RootPath, "authorized_keys"))
 		require.NoError(t, err)
+		authorizedKeysStatBefore, err := fAuthorizedKeys.Stat()
+		require.NoError(t, err)
+		fAuthorizedPrincipals, err := os.Open(filepath.Join(setting.SSH.RootPath, "authorized_principals"))
+		require.NoError(t, err)
+		authorizedPrincipalsBefore, err := fAuthorizedPrincipals.Stat()
+		require.NoError(t, err)
+
+		require.NoError(t, DeleteUser(db.DefaultContext, user, true))
 
 		unittest.AssertNotExistsBean(t, &user_model.User{ID: userID})
 		unittest.CheckConsistencyFor(t, &user_model.User{}, &repo_model.Repository{})
+
+		fAuthorizedKeys, err = os.Open(filepath.Join(setting.SSH.RootPath, "authorized_keys"))
+		require.NoError(t, err)
+		fAuthorizedPrincipals, err = os.Open(filepath.Join(setting.SSH.RootPath, "authorized_principals"))
+		require.NoError(t, err)
+
+		authorizedKeysStatAfter, err := fAuthorizedKeys.Stat()
+		require.NoError(t, err)
+		authorizedPrincipalsAfter, err := fAuthorizedPrincipals.Stat()
+		require.NoError(t, err)
+
+		if modifySSHKey {
+			assert.Greater(t, authorizedKeysStatAfter.ModTime(), authorizedKeysStatBefore.ModTime())
+			assert.Greater(t, authorizedPrincipalsAfter.ModTime(), authorizedPrincipalsBefore.ModTime())
+		} else {
+			assert.Equal(t, authorizedKeysStatAfter.ModTime(), authorizedKeysStatBefore.ModTime())
+			assert.Equal(t, authorizedPrincipalsAfter.ModTime(), authorizedPrincipalsBefore.ModTime())
+		}
 	}
-	test(2)
-	test(4)
-	test(8)
-	test(11)
+	test(2, true)
+	test(4, false)
+	test(8, false)
+	test(11, false)
 
 	org := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 3})
 	require.Error(t, DeleteUser(db.DefaultContext, org, false))