shrpx: Check the length of output buffer in write callback

Possibly because of deferred callback, we may get this callback when
the output buffer is not empty.

src/shrpx_client_handler.cc

@@ -54,6 +54,11 @@ void upstream_writecb(bufferevent *bev, void *arg)
 {
   ClientHandler *handler = reinterpret_cast<ClientHandler*>(arg);
   // We actually depend on write low-warter mark == 0.
+  if(evbuffer_get_length(bufferevent_get_output(bev)) > 0) {
+    // Possibly because of deferred callback, we may get this callback
+    // when the output buffer is not empty.
+    return;
+  }
   if(handler->get_should_close_after_write()) {
     delete handler;
   } else {

src/shrpx_https_upstream.cc

@@ -417,6 +417,9 @@ void https_downstream_readcb(bufferevent *bev, void *ptr)
 namespace {
 void https_downstream_writecb(bufferevent *bev, void *ptr)
 {
+  if(evbuffer_get_length(bufferevent_get_output(bev)) > 0) {
+    return;
+  }
   DownstreamConnection *dconn = reinterpret_cast<DownstreamConnection*>(ptr);
   Downstream *downstream = dconn->get_downstream();
   HttpsUpstream *upstream;

src/shrpx_spdy_session.cc

@@ -194,6 +194,9 @@ void readcb(bufferevent *bev, void *ptr)
 namespace {
 void writecb(bufferevent *bev, void *ptr)
 {
+  if(evbuffer_get_length(bufferevent_get_output(bev)) > 0) {
+    return;
+  }
   int rv;
   SpdySession *spdy = reinterpret_cast<SpdySession*>(ptr);
   rv = spdy->on_write();

src/shrpx_spdy_upstream.cc

@@ -491,6 +491,9 @@ void spdy_downstream_readcb(bufferevent *bev, void *ptr)
 namespace {
 void spdy_downstream_writecb(bufferevent *bev, void *ptr)
 {
+  if(evbuffer_get_length(bufferevent_get_output(bev)) > 0) {
+    return;
+  }
   DownstreamConnection *dconn = reinterpret_cast<DownstreamConnection*>(ptr);
   Downstream *downstream = dconn->get_downstream();
   SpdyUpstream *upstream;