Fix HeaderWriterFilter Race Condition

Closes gh-9175

Signed-off-by: Dmitrii Bocharov <bdshadow@gmail.com>

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

@@ -18,6 +18,7 @@ package org.springframework.security.web.header;
 
 import java.io.IOException;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import jakarta.servlet.FilterChain;
 import jakarta.servlet.RequestDispatcher;
@@ -114,6 +115,8 @@ public class HeaderWriterFilter extends OncePerRequestFilter {
 
 		private final HttpServletRequest request;
 
+		private final AtomicBoolean headersWritten = new AtomicBoolean(false);
+
 		HeaderWriterResponse(HttpServletRequest request, HttpServletResponse response) {
 			super(response);
 			this.request = request;
@@ -129,7 +132,9 @@ public class HeaderWriterFilter extends OncePerRequestFilter {
 			if (isDisableOnResponseCommitted()) {
 				return;
 			}
-			HeaderWriterFilter.this.writeHeaders(this.request, getHttpResponse());
+			if (this.headersWritten.compareAndSet(false, true)) {
+				HeaderWriterFilter.this.writeHeaders(this.request, getHttpResponse());
+			}
 		}
 
 		private HttpServletResponse getHttpResponse() {

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

@@ -113,6 +113,26 @@ public class HeaderWriterFilterTests {
 		verifyNoMoreInteractions(this.writer1);
 	}
 
+	// gh-9175
+	@Test
+	public void doFilterWhenWriteHeadersCalledConcurrentlyThenHeadersWrittenOnlyOnce() throws Exception {
+		List<HeaderWriter> headerWriters = new ArrayList<>();
+		headerWriters.add(this.writer1);
+		HeaderWriterFilter filter = new HeaderWriterFilter(headerWriters);
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		MockHttpServletResponse response = new MockHttpServletResponse();
+		filter.doFilter(request, response, (req, resp) -> {
+			// Calling writeHeaders() directly simulates the race window where an
+			// async thread enters writeHeaders() via onResponseCommitted() but has
+			// not yet called disableOnResponseCommitted().
+			((HeaderWriterFilter.HeaderWriterResponse) resp).writeHeaders();
+		});
+		// The finally block in doHeadersAfter also calls writeHeaders().
+		// Without the fix, the header writers would be invoked twice.
+		verify(this.writer1).writeHeaders(any(HttpServletRequest.class), any(HttpServletResponse.class));
+		verifyNoMoreInteractions(this.writer1);
+	}
+
 	@Test
 	public void headersWrittenAtBeginningOfRequest() throws Exception {
 		HeaderWriterFilter filter = new HeaderWriterFilter(Collections.singletonList(this.writer1));