Add Support for Clear Site Data on Logout

Added an implementation of HeaderWriter for Clear-Site-Data HTTP
response header as welll as an implementation of LogoutHanlder
that accepts an implementation of HeaderWriter to write headers.

- Added ClearSiteDataHeaderWriter and HeaderWriterLogoutHandler
that implements HeaderWriter and LogoutHandler respectively
- Added unit tests for both implementations's behaviours
- Integration tests for HeaderWriterLogoutHandler that uses
ClearSiteDataHeaderWriter
- Updated the documentation to include link to
HeaderWriterLogoutHandler

Fixes gh-4187

web/src/main/java/org/springframework/security/web/authentication/logout/HeaderWriterLogoutHandler.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2002-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.security.web.authentication.logout;
+
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+import org.springframework.security.core.Authentication;
+import org.springframework.security.web.header.HeaderWriter;
+import org.springframework.util.Assert;
+
+/**
+ *
+ * @author Rafiullah Hamedy
+ * @since 5.2
+ */
+public final class HeaderWriterLogoutHandler implements LogoutHandler {
+	private final HeaderWriter headerWriter;
+
+	/**
+	 * Constructs a new instance using the passed {@link HeaderWriter} implementation
+	 *
+	 * @param headerWriter
+	 * @throws {@link IllegalArgumentException} if headerWriter is null.
+	 */
+	public HeaderWriterLogoutHandler(HeaderWriter headerWriter) {
+		Assert.notNull(headerWriter, "headerWriter cannot be null.");
+		this.headerWriter = headerWriter;
+	}
+
+	@Override
+	public void logout(HttpServletRequest request, HttpServletResponse response,
+			Authentication authentication) {
+		this.headerWriter.writeHeaders(request, response);
+	}
+}

web/src/main/java/org/springframework/security/web/header/writers/ClearSiteDataHeaderWriter.java

@@ -0,0 +1,101 @@
+/*
+ * Copyright 2002-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.security.web.header.writers;
+
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import org.springframework.security.web.header.HeaderWriter;
+import org.springframework.security.web.util.matcher.RequestMatcher;
+import org.springframework.util.Assert;
+
+/**
+ * Provides support for <a href="https://w3c.github.io/webappsec-clear-site-data/">Clear
+ * Site Data</a>.
+ *
+ * <p>
+ * Developers may instruct a user agent to clear various types of relevant data by delivering
+ * a Clear-Site-Data HTTP response header in response to a request.
+ * <p>
+ *
+ * <p>
+ * Due to <a href="https://w3c.github.io/webappsec-clear-site-data/#incomplete">Incomplete Clearing</a>
+ * section the header is only applied if the request is secure.
+ * </p>
+ *
+ * @author Rafiullah Hamedy
+ * @since 5.2
+ */
+public final class ClearSiteDataHeaderWriter implements HeaderWriter {
+
+	private static final String CLEAR_SITE_DATA_HEADER = "Clear-Site-Data";
+
+	private final Log logger = LogFactory.getLog(getClass());
+
+	private final RequestMatcher requestMatcher;
+
+	private String headerValue;
+
+	/**
+	 * <p>
+	 * Creates a new instance of {@link ClearSiteDataHeaderWriter} with given sources.
+	 * The constructor also initializes <b>requestMatcher</b> with a new instance of
+	 * <b>SecureRequestMatcher</b> to ensure that header is only applied if and when
+	 * the request is secure as per the <b>Incomplete Clearing</b> section.
+	 * </p>
+	 *
+	 * @param sources (i.e. "cache", "cookies", "storage", "executionContexts" or "*")
+	 * @throws {@link IllegalArgumentException} if sources is null or empty.
+	 */
+	public ClearSiteDataHeaderWriter(String ...sources) {
+		Assert.notEmpty(sources, "Sources cannot be empty or null.");
+		this.requestMatcher = new SecureRequestMatcher();
+		this.headerValue = Stream.of(sources).map(this::quote).collect(Collectors.joining(", "));
+	}
+
+	@Override
+	public void writeHeaders(HttpServletRequest request, HttpServletResponse response) {
+		if (this.requestMatcher.matches(request)) {
+			if (!response.containsHeader(CLEAR_SITE_DATA_HEADER)) {
+				response.setHeader(CLEAR_SITE_DATA_HEADER, this.headerValue);
+			}
+		} else if (logger.isDebugEnabled()) {
+			logger.debug("Not injecting Clear-Site-Data header since it did not match the "
+						+ "requestMatcher " + this.requestMatcher);
+		}
+	}
+
+	private static final class SecureRequestMatcher implements RequestMatcher {
+		public boolean matches(HttpServletRequest request) {
+			return request.isSecure();
+		}
+	}
+
+	private String quote(String source) {
+		return "\"" + source + "\"";
+	}
+
+	@Override
+	public String toString() {
+		return getClass().getName() + " [headerValue=" + this.headerValue + "]";
+	}
+}