[v15.0/forgejo] refactor: clarify four different outputs that authentication methods provide (#12468)

**Backport:** https://codeberg.org/forgejo/forgejo/pulls/12231

#12202 began a refactor of Forgejo's authentication implementations by providing structured data on an authentication success.  However, error cases were maintained as-is in that refactor, leaving a complex situation: what does returning an error from an authentication method mean?; does it mean that the authentication failed, or that a server error occurred?  Can another authentication still be tried?

This PR changes authentication methods so that they can return one of four things:
- `AuthenticationSuccess` with an authentication result.
- `AuthenticationNotAttempted` which indicates that no credentials relevant for this authentication method were presented.  If every method returned `AuthenticationNotAttempted`, then you would have an unauthenticated access.
- `AuthenticationAttemptedIncorrectCredential` which indicates that credentials were present and failed validation -- a situation indicating a `401 Unauthorized`.
- `AuthenticationError` which indicates that an internal server error occurred and failed authentication -- indicating a `500 Internal Server Error`.

This paves the way for one more refactor coming next: `basic.go` and `oauth2.go` perform 3-4 different authentications each (access tokens, oauth JWTs, actions tokens, actions JWTs, and username/password).  With the capability to return these more precise responses, these authentication methods can be split up into separate logic that isn't intertwined together.

Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12468
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

services/auth/method/httpsign.go

@@ -35,10 +35,10 @@ type HTTPSign struct{}
 // Verify extracts and validates HTTPsign from the Signature header of the request and returns
 // the corresponding user object on successful validation.
 // Returns nil if header is empty or validation fails.
-func (h *HTTPSign) Verify(req *http.Request, w http.ResponseWriter, _ auth.SessionStore) (auth.AuthenticationResult, error) {
+func (h *HTTPSign) Verify(req *http.Request, w http.ResponseWriter, _ auth.SessionStore) auth.MethodOutput {
 	sigHead := req.Header.Get("Signature")
 	if len(sigHead) == 0 {
-		return &auth.UnauthenticatedResult{}, nil
+		return &auth.AuthenticationNotAttempted{}
 	}
 
 	var (
@@ -49,14 +49,14 @@ func (h *HTTPSign) Verify(req *http.Request, w http.ResponseWriter, _ auth.Sessi
 	if len(req.Header.Get("X-Ssh-Certificate")) != 0 {
 		// Handle Signature signed by SSH certificates
 		if len(setting.SSH.TrustedUserCAKeys) == 0 {
-			return &auth.UnauthenticatedResult{}, nil
+			return &auth.AuthenticationNotAttempted{}
 		}
 
 		publicKey, err = VerifyCert(req)
 		if err != nil {
 			log.Debug("VerifyCert on request from %s: failed: %v", req.RemoteAddr, err)
 			log.Warn("Failed authentication attempt from %s", req.RemoteAddr)
-			return &auth.UnauthenticatedResult{}, nil
+			return &auth.AuthenticationNotAttempted{} // 401 is not expected on signature validation miss; return not attempted
 		}
 	} else {
 		// Handle Signature signed by Public Key
@@ -64,18 +64,17 @@ func (h *HTTPSign) Verify(req *http.Request, w http.ResponseWriter, _ auth.Sessi
 		if err != nil {
 			log.Debug("VerifyPubKey on request from %s: failed: %v", req.RemoteAddr, err)
 			log.Warn("Failed authentication attempt from %s", req.RemoteAddr)
-			return &auth.UnauthenticatedResult{}, nil
+			return &auth.AuthenticationNotAttempted{} // 401 is not expected on signature validation miss; return not attempted
 		}
 	}
 
 	u, err := user_model.GetUserByID(req.Context(), publicKey.OwnerID)
 	if err != nil {
-		log.Error("GetUserByID:  %v", err)
-		return nil, err
+		return &auth.AuthenticationError{Error: fmt.Errorf("httpsign GetUserByID: %w", err)}
 	}
 
 	log.Trace("HTTP Sign: Logged in user %-v", u)
-	return &httpSignAuthenticationResult{user: u}, nil
+	return &auth.AuthenticationSuccess{Result: &httpSignAuthenticationResult{user: u}}
 }
 
 func VerifyPubKey(r *http.Request) (*asymkey_model.PublicKey, error) {