Don't flush in ErrorPageFilter for < 400 response that's committed

Previously, for a non-async response with a successful status (< 400),
ErrorPageFilter would always call flushBuffer. This triggers an
exception in Tomcat if the client has closed the connection before the
response has been fully sent. In this case, Tomcat treats the response
as successful and commits it before control returns to the filter.

This commit updates ErrorPageFilter to only perform the flush if the
response has not already been committed, leaving any further flushing
that may be necessary to be handled by the servlet container.

Fixes gh-1938

spring-boot/src/main/java/org/springframework/boot/context/web/ErrorPageFilter.java

@@ -112,7 +112,7 @@ class ErrorPageFilter extends AbstractConfigurableEmbeddedServletContainer imple
 				handleErrorStatus(request, response, status, wrapped.getMessage());
 				response.flushBuffer();
 			}
-			else if (!request.isAsyncStarted()) {
+			else if (!request.isAsyncStarted() && !response.isCommitted()) {
 				response.flushBuffer();
 			}
 		}

spring-boot/src/test/java/org/springframework/boot/context/web/ErrorPageFilterTests.java

@@ -40,6 +40,10 @@ import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
 
 /**
  * Tests for {@link ErrorPageFilter}.
@@ -343,4 +347,16 @@ public class ErrorPageFilterTests {
 		assertTrue(this.response.isCommitted());
 	}
 
+	@Test
+	public void responseIsNotFlushedIfStatusIsLessThan400AndItHasAlreadyBeenCommitted()
+			throws Exception {
+		HttpServletResponse committedResponse = mock(HttpServletResponse.class);
+		given(committedResponse.isCommitted()).willReturn(true);
+		given(committedResponse.getStatus()).willReturn(200);
+
+		this.filter.doFilter(this.request, committedResponse, this.chain);
+
+		verify(committedResponse, times(0)).flushBuffer();
+	}
+
 }