Improve oauth2 client "preferred username field" logic and the error handling (#30622)

Follow #30454
And fix #24957

When using "preferred_username", if no such field,
`extractUserNameFromOAuth2` (old `getUserName`) shouldn't return an
error. All other USERNAME options do not return such error.

And fine tune some logic and error messages, make code more stable and
more friendly to end users.

modules/session/mock.go

@@ -0,0 +1,26 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package session
+
+import (
+	"net/http"
+
+	"gitea.com/go-chi/session"
+)
+
+type MockStore struct {
+	*session.MemStore
+}
+
+func (m *MockStore) Destroy(writer http.ResponseWriter, request *http.Request) error {
+	return nil
+}
+
+type mockStoreContextKeyStruct struct{}
+
+var MockStoreContextKey = mockStoreContextKeyStruct{}
+
+func NewMockStore(sid string) *MockStore {
+	return &MockStore{session.NewMemStore(sid)}
+}

modules/session/store.go

@@ -6,6 +6,8 @@ package session
 import (
 	"net/http"
 
+	"code.gitea.io/gitea/modules/setting"
+
 	"gitea.com/go-chi/session"
 )
 
@@ -14,6 +16,10 @@ type Store interface {
 	Get(any) any
 	Set(any, any) error
 	Delete(any) error
+	ID() string
+	Release() error
+	Flush() error
+	Destroy(http.ResponseWriter, *http.Request) error
 }
 
 // RegenerateSession regenerates the underlying session and returns the new store
@@ -21,8 +27,21 @@ func RegenerateSession(resp http.ResponseWriter, req *http.Request) (Store, erro
 	for _, f := range BeforeRegenerateSession {
 		f(resp, req)
 	}
-	s, err := session.RegenerateSession(resp, req)
-	return s, err
+	if setting.IsInTesting {
+		if store, ok := req.Context().Value(MockStoreContextKey).(*MockStore); ok {
+			return store, nil
+		}
+	}
+	return session.RegenerateSession(resp, req)
+}
+
+func GetContextSession(req *http.Request) Store {
+	if setting.IsInTesting {
+		if store, ok := req.Context().Value(MockStoreContextKey).(*MockStore); ok {
+			return store
+		}
+	}
+	return session.GetSession(req)
 }
 
 // BeforeRegenerateSession is a list of functions that are called before a session is regenerated.

modules/setting/oauth2.go

@@ -16,14 +16,10 @@ import (
 type OAuth2UsernameType string
 
 const (
-	// OAuth2UsernameUserid oauth2 userid field will be used as gitea name
-	OAuth2UsernameUserid OAuth2UsernameType = "userid"
-	// OAuth2UsernameNickname oauth2 nickname field will be used as gitea name
-	OAuth2UsernameNickname OAuth2UsernameType = "nickname"
-	// OAuth2UsernameEmail username of oauth2 email field will be used as gitea name
-	OAuth2UsernameEmail OAuth2UsernameType = "email"
-	// OAuth2UsernameEmail username of oauth2 preferred_username field will be used as gitea name
-	OAuth2UsernamePreferredUsername OAuth2UsernameType = "preferred_username"
+	OAuth2UsernameUserid            OAuth2UsernameType = "userid"             // use user id (sub) field as gitea's username
+	OAuth2UsernameNickname          OAuth2UsernameType = "nickname"           // use nickname field
+	OAuth2UsernameEmail             OAuth2UsernameType = "email"              // use email field
+	OAuth2UsernamePreferredUsername OAuth2UsernameType = "preferred_username" // use preferred_username field
 )
 
 func (username OAuth2UsernameType) isValid() bool {
@@ -71,8 +67,8 @@ func loadOAuth2ClientFrom(rootCfg ConfigProvider) {
 	OAuth2Client.EnableAutoRegistration = sec.Key("ENABLE_AUTO_REGISTRATION").MustBool()
 	OAuth2Client.Username = OAuth2UsernameType(sec.Key("USERNAME").MustString(string(OAuth2UsernameNickname)))
 	if !OAuth2Client.Username.isValid() {
-		log.Warn("Username setting is not valid: '%s', will fallback to '%s'", OAuth2Client.Username, OAuth2UsernameNickname)
 		OAuth2Client.Username = OAuth2UsernameNickname
+		log.Warn("[oauth2_client].USERNAME setting is invalid, falls back to %q", OAuth2Client.Username)
 	}
 	OAuth2Client.UpdateAvatar = sec.Key("UPDATE_AVATAR").MustBool()
 	OAuth2Client.AccountLinking = OAuth2AccountLinkingType(sec.Key("ACCOUNT_LINKING").MustString(string(OAuth2AccountLinkingLogin)))