Refactor: Move login out of models (#16199)

`models` does far too much. In particular it handles all `UserSignin`.

It shouldn't be responsible for calling LDAP, SMTP or PAM for signing in.

Therefore we should move this code out of `models`.

This code has to depend on `models` - therefore it belongs in `services`.

There is a package in `services` called `auth` and clearly this functionality belongs in there.

Plan:

- [x] Change `auth.Auth` to `auth.Method` - as they represent methods of authentication.
- [x] Move `models.UserSignIn` into `auth`
- [x] Move `models.ExternalUserLogin`
- [x] Move most of the `LoginVia*` methods to `auth` or subpackages
- [x] Move Resynchronize functionality to `auth`
  - Involved some restructuring of `models/ssh_key.go` to reduce the size of this massive file and simplify its files.
- [x] Move the rest of the LDAP functionality in to the ldap subpackage
- [x] Re-factor the login sources to express an interfaces `auth.Source`?
  - I've done this through some smaller interfaces Authenticator and Synchronizable - which would allow us to extend things in future
- [x] Now LDAP is out of models - need to think about modules/auth/ldap and I think all of that functionality might just be moveable
- [x] Similarly a lot Oauth2 functionality need not be in models too and should be moved to services/auth/source/oauth2
  - [x] modules/auth/oauth2/oauth2.go uses xorm... This is naughty - probably need to move this into models.
  - [x] models/oauth2.go - mostly should be in modules/auth/oauth2 or services/auth/source/oauth2 
- [x] More simplifications of login_source.go may need to be done
- Allow wiring in of notify registration -  *this can now easily be done - but I think we should do it in another PR*  - see #16178 
- More refactors...?
  - OpenID should probably become an auth Method but I think that can be left for another PR
  - Methods should also probably be cleaned up  - again another PR I think.
  - SSPI still needs more refactors.* Rename auth.Auth auth.Method
* Restructure ssh_key.go

- move functions from models/user.go that relate to ssh_key to ssh_key
- split ssh_key.go to try create clearer function domains for allow for
future refactors here.

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/ssh_key_fingerprint.go

@@ -0,0 +1,97 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package models
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/process"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
+	"golang.org/x/crypto/ssh"
+)
+
+// ___________.__                                         .__        __
+// \_   _____/|__| ____    ____   ________________________|__| _____/  |_
+//  |    __)  |  |/    \  / ___\_/ __ \_  __ \____ \_  __ \  |/    \   __\
+//  |     \   |  |   |  \/ /_/  >  ___/|  | \/  |_> >  | \/  |   |  \  |
+//  \___  /   |__|___|  /\___  / \___  >__|  |   __/|__|  |__|___|  /__|
+//      \/            \//_____/      \/      |__|                 \/
+//
+// This file contains functions for fingerprinting SSH keys
+//
+// The database is used in checkKeyFingerprint however most of these functions probably belong in a module
+
+// checkKeyFingerprint only checks if key fingerprint has been used as public key,
+// it is OK to use same key as deploy key for multiple repositories/users.
+func checkKeyFingerprint(e Engine, fingerprint string) error {
+	has, err := e.Get(&PublicKey{
+		Fingerprint: fingerprint,
+	})
+	if err != nil {
+		return err
+	} else if has {
+		return ErrKeyAlreadyExist{0, fingerprint, ""}
+	}
+	return nil
+}
+
+func calcFingerprintSSHKeygen(publicKeyContent string) (string, error) {
+	// Calculate fingerprint.
+	tmpPath, err := writeTmpKeyFile(publicKeyContent)
+	if err != nil {
+		return "", err
+	}
+	defer func() {
+		if err := util.Remove(tmpPath); err != nil {
+			log.Warn("Unable to remove temporary key file: %s: Error: %v", tmpPath, err)
+		}
+	}()
+	stdout, stderr, err := process.GetManager().Exec("AddPublicKey", "ssh-keygen", "-lf", tmpPath)
+	if err != nil {
+		if strings.Contains(stderr, "is not a public key file") {
+			return "", ErrKeyUnableVerify{stderr}
+		}
+		return "", fmt.Errorf("'ssh-keygen -lf %s' failed with error '%s': %s", tmpPath, err, stderr)
+	} else if len(stdout) < 2 {
+		return "", errors.New("not enough output for calculating fingerprint: " + stdout)
+	}
+	return strings.Split(stdout, " ")[1], nil
+}
+
+func calcFingerprintNative(publicKeyContent string) (string, error) {
+	// Calculate fingerprint.
+	pk, _, _, _, err := ssh.ParseAuthorizedKey([]byte(publicKeyContent))
+	if err != nil {
+		return "", err
+	}
+	return ssh.FingerprintSHA256(pk), nil
+}
+
+func calcFingerprint(publicKeyContent string) (string, error) {
+	// Call the method based on configuration
+	var (
+		fnName, fp string
+		err        error
+	)
+	if setting.SSH.StartBuiltinServer {
+		fnName = "calcFingerprintNative"
+		fp, err = calcFingerprintNative(publicKeyContent)
+	} else {
+		fnName = "calcFingerprintSSHKeygen"
+		fp, err = calcFingerprintSSHKeygen(publicKeyContent)
+	}
+	if err != nil {
+		if IsErrKeyUnableVerify(err) {
+			log.Info("%s", publicKeyContent)
+			return "", err
+		}
+		return "", fmt.Errorf("%s: %v", fnName, err)
+	}
+	return fp, nil
+}