Support SAML authentication (#25165)

Closes https://github.com/go-gitea/gitea/issues/5512

This PR adds basic SAML support
- Adds SAML 2.0 as an auth source
- Adds SAML configuration documentation
- Adds integration test:
- Use bare-bones SAML IdP to test protocol flow and test account is
linked successfully (only runs on Postgres by default)
- Adds documentation for configuring and running SAML integration test
locally

Future PRs:
- Support group mapping
- Support auto-registration (account linking)

Co-Authored-By: @jackHay22

---------

Co-authored-by: jackHay22 <jack@allspice.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: morphelinho <morphelinho@users.noreply.github.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: silverwind <me@silverwind.io>

models/auth/source.go

@@ -14,6 +14,7 @@ import (
 	"code.gitea.io/gitea/modules/timeutil"
 	"code.gitea.io/gitea/modules/util"
 
+	"github.com/markbates/goth"
 	"xorm.io/builder"
 	"xorm.io/xorm"
 	"xorm.io/xorm/convert"
@@ -32,6 +33,7 @@ const (
 	DLDAP       // 5
 	OAuth2      // 6
 	SSPI        // 7
+	SAML        // 8
 )
 
 // String returns the string name of the LoginType
@@ -52,6 +54,7 @@ var Names = map[Type]string{
 	PAM:    "PAM",
 	OAuth2: "OAuth2",
 	SSPI:   "SPNEGO with SSPI",
+	SAML:   "SAML",
 }
 
 // Config represents login config as far as the db is concerned
@@ -121,6 +124,12 @@ type Source struct {
 	UpdatedUnix timeutil.TimeStamp `xorm:"INDEX updated"`
 }
 
+// LinkAccountUser is used to link an external user with a local user
+type LinkAccountUser struct {
+	Type     Type
+	GothUser goth.User
+}
+
 // TableName xorm will read the table name from this method
 func (Source) TableName() string {
 	return "login_source"
@@ -180,6 +189,11 @@ func (source *Source) IsSSPI() bool {
 	return source.Type == SSPI
 }
 
+// IsSAML returns true of this source is of the SAML type.
+func (source *Source) IsSAML() bool {
+	return source.Type == SAML
+}
+
 // HasTLS returns true of this source supports TLS.
 func (source *Source) HasTLS() bool {
 	hasTLSer, ok := source.Cfg.(HasTLSer)
@@ -392,3 +406,27 @@ func IsErrSourceInUse(err error) bool {
 func (err ErrSourceInUse) Error() string {
 	return fmt.Sprintf("login source is still used by some users [id: %d]", err.ID)
 }
+
+// GetActiveAuthProviderSources returns all activated sources
+func GetActiveAuthProviderSources(ctx context.Context, authType Type) ([]*Source, error) {
+	sources := make([]*Source, 0, 1)
+	if err := db.GetEngine(ctx).Where("is_active = ? and type = ?", true, authType).Find(&sources); err != nil {
+		return nil, err
+	}
+	return sources, nil
+}
+
+// GetActiveAuthSourceByName returns an AuthSource based on the given name and type
+func GetActiveAuthSourceByName(ctx context.Context, name string, authType Type) (*Source, error) {
+	authSource := new(Source)
+	has, err := db.GetEngine(ctx).Where("name = ? and type = ? and is_active = ?", name, authType, true).Get(authSource)
+	if err != nil {
+		return nil, err
+	}
+
+	if !has {
+		return nil, fmt.Errorf("auth source not found, name: %q", name)
+	}
+
+	return authSource, nil
+}