[feature] overhaul the oidc system (#961)

* [feature] overhaul the oidc system

this allows for more flexible username handling and prevents account
takeover using old email addresses

* [feature] add migration path for old OIDC users

* [feature] nicer error reporting for users

* [docs] document the new OIDC flow

* [fix] return early on oidc error

* [docs]: add comments on the finalization logic

internal/db/bundb/admin.go

@@ -90,7 +90,7 @@ func (a *adminDB) IsEmailAvailable(ctx context.Context, email string) (bool, db.
 	return a.conn.NotExists(ctx, q)
 }
 
-func (a *adminDB) NewSignup(ctx context.Context, username string, reason string, requireApproval bool, email string, password string, signUpIP net.IP, locale string, appID string, emailVerified bool, admin bool) (*gtsmodel.User, db.Error) {
+func (a *adminDB) NewSignup(ctx context.Context, username string, reason string, requireApproval bool, email string, password string, signUpIP net.IP, locale string, appID string, emailVerified bool, externalID string, admin bool) (*gtsmodel.User, db.Error) {
 	key, err := rsa.GenerateKey(rand.Reader, rsaKeyBits)
 	if err != nil {
 		log.Errorf("error creating new rsa key: %s", err)
@@ -169,6 +169,7 @@ func (a *adminDB) NewSignup(ctx context.Context, username string, reason string,
 		UnconfirmedEmail:       email,
 		CreatedByApplicationID: appID,
 		Approved:               &approved,
+		ExternalID:             externalID,
 	}
 
 	if emailVerified {

internal/db/bundb/migrations/20221103203553_add_external_id.go

@@ -0,0 +1,46 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package migrations
+
+import (
+	"context"
+	"strings"
+
+	"github.com/uptrace/bun"
+)
+
+func init() {
+	up := func(ctx context.Context, db *bun.DB) error {
+		_, err := db.ExecContext(ctx, "ALTER TABLE ? ADD COLUMN ? TEXT", bun.Ident("users"), bun.Ident("external_id"))
+		if err != nil && !(strings.Contains(err.Error(), "already exists") || strings.Contains(err.Error(), "duplicate column name") || strings.Contains(err.Error(), "SQLSTATE 42701")) {
+			return err
+		}
+		return nil
+	}
+
+	down := func(ctx context.Context, db *bun.DB) error {
+		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
+			return nil
+		})
+	}
+
+	if err := Migrations.Register(up, down); err != nil {
+		panic(err)
+	}
+}

internal/db/bundb/user.go

@@ -40,6 +40,7 @@ func (u *userDB) init() {
 		{Name: "AccountID"},
 		{Name: "Email"},
 		{Name: "ConfirmationToken"},
+		{Name: "ExternalID"},
 	}, func(u1 *gtsmodel.User) *gtsmodel.User {
 		u2 := new(gtsmodel.User)
 		*u2 = *u1
@@ -104,6 +105,24 @@ func (u *userDB) GetUserByEmailAddress(ctx context.Context, emailAddress string)
 		return &user, nil
 	}, emailAddress)
 }
+func (u *userDB) GetUserByExternalID(ctx context.Context, id string) (*gtsmodel.User, db.Error) {
+
+	return u.cache.Load("ExternalID", func() (*gtsmodel.User, error) {
+		var user gtsmodel.User
+
+		q := u.conn.
+			NewSelect().
+			Model(&user).
+			Relation("Account").
+			Where("? = ?", bun.Ident("user.external_id"), id)
+
+		if err := q.Scan(ctx); err != nil {
+			return nil, u.conn.ProcessError(err)
+		}
+
+		return &user, nil
+	}, id)
+}
 
 func (u *userDB) GetUserByConfirmationToken(ctx context.Context, confirmationToken string) (*gtsmodel.User, db.Error) {
 	return u.cache.Load("ConfirmationToken", func() (*gtsmodel.User, error) {