Check if response is committed before tryying to forward

It' sthe best we can do, but the container seems to still use our error page
(it just can't have the status code that we asked for).

Fixes gh-748

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

@@ -30,6 +30,8 @@ import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 import javax.servlet.http.HttpServletResponseWrapper;
 
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import org.springframework.boot.context.embedded.AbstractConfigurableEmbeddedServletContainer;
 import org.springframework.boot.context.embedded.EmbeddedServletContainerCustomizer;
 import org.springframework.boot.context.embedded.ErrorPage;
@@ -55,6 +57,8 @@ import org.springframework.stereotype.Component;
 class ErrorPageFilter extends AbstractConfigurableEmbeddedServletContainer implements
 		Filter, NonEmbeddedServletContainerFactory {
 
+	private static Log logger = LogFactory.getLog(ErrorPageFilter.class);
+
 	// From RequestDispatcher but not referenced to remain compatible with Servlet 2.5
 
 	private static final String ERROR_EXCEPTION = "javax.servlet.error.exception";
@@ -132,7 +136,18 @@ class ErrorPageFilter extends AbstractConfigurableEmbeddedServletContainer imple
 		request.setAttribute(ERROR_EXCEPTION, ex);
 		request.setAttribute(ERROR_EXCEPTION_TYPE, type.getName());
 		wrapped.sendError(500, ex.getMessage());
-		request.getRequestDispatcher(errorPath).forward(request, response);
+		if (!wrapped.isCommitted()) {
+			wrapped.reset();
+			request.getRequestDispatcher(errorPath).forward(request, wrapped);
+		}
+		else {
+			String message = "Cannot forward to error page for"
+					+ request.getRequestURI()
+					+ " (response is committed), so this response may have the wrong status code";
+			logger.error(message);
+			// User might see the error page without all the data here but the exception
+			// isn't going to help anyone (and it's already been logged)
+		}
 	}
 
 	private String getErrorPath(Map<Integer, String> map, Integer status) {