HeaderWriterFilter writes headers at beginning

Add support for HeaderWriterFilter to write headers at the beginning of the request

Fixes: gh-6501

web/src/main/java/org/springframework/security/web/header/HeaderWriterFilter.java

@@ -51,6 +51,11 @@ public class HeaderWriterFilter extends OncePerRequestFilter {
 	 */
 	private final HeaderWriter headerWriter;
 
+	/**
+	 * Indicates whether to write the headers at the beginning of the request.
+	 */
+	private boolean shouldWriteHeadersEagerly = false;
+
 	/**
 	 * Creates a new instance.
 	 *
@@ -67,11 +72,23 @@ public class HeaderWriterFilter extends OncePerRequestFilter {
 			HttpServletResponse response, FilterChain filterChain)
 			throws ServletException, IOException {
 
+		if (this.shouldWriteHeadersEagerly) {
+			doHeadersBefore(request, response, filterChain);
+		} else {
+			doHeadersAfter(request, response, filterChain);
+		}
+	}
+
+	private void doHeadersBefore(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws IOException, ServletException {
+		this.headerWriter.writeHeaders(request, response);
+		filterChain.doFilter(request, response);
+	}
+
+	private void doHeadersAfter(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws IOException, ServletException {
 		HeaderWriterResponse headerWriterResponse = new HeaderWriterResponse(request,
 				response, this.headerWriter);
 		HeaderWriterRequest headerWriterRequest = new HeaderWriterRequest(request,
 				headerWriterResponse);
-
 		try {
 			filterChain.doFilter(headerWriterRequest, headerWriterResponse);
 		} finally {
@@ -79,6 +96,17 @@ public class HeaderWriterFilter extends OncePerRequestFilter {
 		}
 	}
 
+	/**
+	 * Allow writing headers at the beginning of the request.
+	 *
+	 * @param shouldWriteHeadersEagerly boolean to allow writing headers at the beginning of the request.
+	 * @author Ankur Pathak
+	 * @since 5.2
+	 */
+	public void setShouldWriteHeadersEagerly(boolean shouldWriteHeadersEagerly) {
+		this.shouldWriteHeadersEagerly = shouldWriteHeadersEagerly;
+	}
+
 	static class HeaderWriterResponse extends OnCommittedResponseWrapper {
 		private final HttpServletRequest request;
 		private final HeaderWriter headerWriter;

web/src/test/java/org/springframework/security/web/header/HeaderWriterFilterTests.java

@@ -136,4 +136,25 @@ public class HeaderWriterFilterTests {
 
 		verifyNoMoreInteractions(this.writer1);
 	}
+
+	@Test
+	public void headersWrittenAtBeginningOfRequest() throws Exception {
+		HeaderWriterFilter filter = new HeaderWriterFilter(
+				Collections.singletonList(this.writer1));
+		filter.setShouldWriteHeadersEagerly(true);
+
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		MockHttpServletResponse response = new MockHttpServletResponse();
+
+		filter.doFilter(request, response, new FilterChain() {
+			@Override
+			public void doFilter(ServletRequest request, ServletResponse response)
+					throws IOException, ServletException {
+				verify(HeaderWriterFilterTests.this.writer1).writeHeaders(
+						any(HttpServletRequest.class), any(HttpServletResponse.class));
+			}
+		});
+
+		verifyNoMoreInteractions(this.writer1);
+	}
 }