OAuth2LoginAuthenticationFilter ignores authenticated Users

This ensures that OAuth2 Client support works with the same log in URL as
oauth2 login.

Fixes: gh-5915

config/src/main/java/org/springframework/security/config/annotation/web/configurers/oauth2/client/OAuth2LoginConfigurer.java

@@ -26,6 +26,7 @@ import org.springframework.security.config.annotation.web.configurers.AbstractHt
 import org.springframework.security.core.Authentication;
 import org.springframework.security.core.AuthenticationException;
 import org.springframework.security.core.authority.mapping.GrantedAuthoritiesMapper;
+import org.springframework.security.core.context.SecurityContextHolder;
 import org.springframework.security.oauth2.client.OAuth2AuthorizedClientService;
 import org.springframework.security.oauth2.client.authentication.OAuth2LoginAuthenticationProvider;
 import org.springframework.security.oauth2.client.authentication.OAuth2LoginAuthenticationToken;
@@ -428,6 +429,9 @@ public final class OAuth2LoginConfigurer<B extends HttpSecurityBuilder<B>> exten
 				this.loginProcessingUrl);
 		this.setAuthenticationFilter(authenticationFilter);
 		super.loginProcessingUrl(this.loginProcessingUrl);
+		RequestMatcher authenticationNullMatcher = request -> SecurityContextHolder.getContext().getAuthentication() == null;
+		authenticationFilter.setRequiresAuthenticationRequestMatcher(new AndRequestMatcher(createLoginProcessingUrlMatcher(this.loginProcessingUrl),
+				authenticationNullMatcher));
 
 		if (this.loginPage != null) {
 			// Set custom login page

config/src/test/java/org/springframework/security/config/annotation/web/configurers/oauth2/client/OAuth2LoginConfigurerTests.java

@@ -27,6 +27,7 @@ import org.springframework.http.MediaType;
 import org.springframework.mock.web.MockFilterChain;
 import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.mock.web.MockHttpServletResponse;
+import org.springframework.security.authentication.TestingAuthenticationToken;
 import org.springframework.security.config.annotation.web.builders.HttpSecurity;
 import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;
 import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
@@ -35,6 +36,7 @@ import org.springframework.security.core.Authentication;
 import org.springframework.security.core.GrantedAuthority;
 import org.springframework.security.core.authority.SimpleGrantedAuthority;
 import org.springframework.security.core.authority.mapping.GrantedAuthoritiesMapper;
+import org.springframework.security.core.context.SecurityContextImpl;
 import org.springframework.security.oauth2.client.endpoint.OAuth2AccessTokenResponseClient;
 import org.springframework.security.oauth2.client.endpoint.OAuth2AuthorizationCodeGrantRequest;
 import org.springframework.security.oauth2.client.oidc.userinfo.OidcUserRequest;
@@ -154,6 +156,31 @@ public class OAuth2LoginConfigurerTests {
 				.isInstanceOf(OAuth2UserAuthority.class).hasToString("ROLE_USER");
 	}
 
+	@Test
+	public void oauth2LoginWhenAuthenticatedThenIgnored() throws Exception {
+		// setup application context
+		loadConfig(OAuth2LoginConfig.class);
+
+		// authenticate
+		TestingAuthenticationToken expectedAuthentication = new TestingAuthenticationToken("a",
+				"b", "ROLE_TEST");
+
+		this.request.getSession().setAttribute(HttpSessionSecurityContextRepository.SPRING_SECURITY_CONTEXT_KEY, new SecurityContextImpl(expectedAuthentication));
+
+		// setup authentication parameters
+		this.request.setParameter("code", "code123");
+		this.request.setParameter("state", "state");
+
+		// perform test
+		this.springSecurityFilterChain.doFilter(this.request, this.response, this.filterChain);
+
+		// assertions
+		Authentication authentication = this.securityContextRepository
+				.loadContext(new HttpRequestResponseHolder(this.request, this.response))
+				.getAuthentication();
+		assertThat(authentication).isEqualTo(expectedAuthentication);
+	}
+
 	@Test
 	public void oauth2LoginCustomWithConfigurer() throws Exception {
 		// setup application context