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I found this exception in the logs:

2018-01-08 10:13:23,734 WARN o.e.j.s.Response:565 Committed before 500 null
2018-01-08 10:13:23,734 WARN o.e.j.s.ServletHandler:628 /stats/clients.csv
java.lang.IllegalStateException: Committed
        at org.eclipse.jetty.server.Response.resetBuffer(Response.java:1242)
        at org.eclipse.jetty.server.Response.sendError(Response.java:567)
        at org.eclipse.jetty.server.Response.sendError(Response.java:544)
        at org.torproject.metrics.web.research.ResearchStatsServlet.doGet(ResearchStatsServlet.java:68)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:649)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
        at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:812)
        at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:587)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
        at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
        at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
        at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
        at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
        at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
        at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
        at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:215)
        at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
        at org.eclipse.jetty.server.Server.handle(Server.java:499)
        at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311)
        at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:258)
        at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)
        at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
        at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
        at java.lang.Thread.run(Thread.java:748)
2018-01-08 10:13:23,734 WARN o.e.j.s.HttpChannel:396 /stats/clients.csv
java.lang.IllegalStateException: Committed
        at org.eclipse.jetty.server.Response.resetBuffer(Response.java:1242)
        at org.eclipse.jetty.server.Response.sendError(Response.java:567)
        at org.eclipse.jetty.server.Response.sendError(Response.java:544)
        at org.torproject.metrics.web.research.ResearchStatsServlet.doGet(ResearchStatsServlet.java:68)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:649)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
        at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:812)
        at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:587)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
        at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
        at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
        at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
        at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
        at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
        at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
        at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:215)
        at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
        at org.eclipse.jetty.server.Server.handle(Server.java:499)
        at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311)
        at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:258)
        at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)
        at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
        at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
        at java.lang.Thread.run(Thread.java:748)
2018-01-08 10:13:23,734 WARN o.e.j.s.HttpChannel:482 Could not send response error 500: java.lang.IllegalStateException: Committed

I believe that it's the result of receiving a request for a (large) CSV file that is then not fully downloaded by the client. If that's true, it's harmless to have this exception in the logs, except that it may make us blind for discovering actual problems. We should avoid logging this exception, if possible. Low priority, though.