diff --git a/mercurial/commandserver.py b/mercurial/commandserver.py
--- a/mercurial/commandserver.py
+++ b/mercurial/commandserver.py
@@ -355,7 +355,18 @@
         )
 
         try:
-            ret = self._dispatchcommand(req) & 255
+            err = None
+            try:
+                status = self._dispatchcommand(req)
+            except error.StdioError as e:
+                status = -1
+                err = e
+
+            retval = dispatch.closestdio(req.ui, err)
+            if retval:
+                status = retval
+
+            ret = status & 255
             # If shutdown-on-interrupt is off, it's important to write the
             # result code *after* SIGINT handler removed. If the result code
             # were lost, the client wouldn't be able to continue processing.
diff --git a/mercurial/dispatch.py b/mercurial/dispatch.py
--- a/mercurial/dispatch.py
+++ b/mercurial/dispatch.py
@@ -104,6 +104,35 @@
                 raise exc
 
 
+def closestdio(ui, err):
+    status = None
+    # In all cases we try to flush stdio streams.
+    if util.safehasattr(ui, b'fout'):
+        assert ui is not None  # help pytype
+        assert ui.fout is not None  # help pytype
+        try:
+            ui.fout.flush()
+        except IOError as e:
+            err = e
+            status = -1
+
+    if util.safehasattr(ui, b'ferr'):
+        assert ui is not None  # help pytype
+        assert ui.ferr is not None  # help pytype
+        try:
+            if err is not None and err.errno != errno.EPIPE:
+                ui.ferr.write(
+                    b'abort: %s\n' % encoding.strtolocal(err.strerror)
+                )
+            ui.ferr.flush()
+        # There's not much we can do about an I/O error here. So (possibly)
+        # change the status code and move on.
+        except IOError:
+            status = -1
+
+    return status
+
+
 def run():
     """run the command in sys.argv"""
     try:
@@ -117,30 +146,9 @@
             err = e
             status = -1
 
-        # In all cases we try to flush stdio streams.
-        if util.safehasattr(req.ui, b'fout'):
-            assert req.ui is not None  # help pytype
-            assert req.ui.fout is not None  # help pytype
-            try:
-                req.ui.fout.flush()
-            except IOError as e:
-                err = e
-                status = -1
-
-        if util.safehasattr(req.ui, b'ferr'):
-            assert req.ui is not None  # help pytype
-            assert req.ui.ferr is not None  # help pytype
-            try:
-                if err is not None and err.errno != errno.EPIPE:
-                    req.ui.ferr.write(
-                        b'abort: %s\n' % encoding.strtolocal(err.strerror)
-                    )
-                req.ui.ferr.flush()
-            # There's not much we can do about an I/O error here. So (possibly)
-            # change the status code and move on.
-            except IOError:
-                status = -1
-
+        ret = closestdio(req.ui, err)
+        if ret:
+            status = ret
         _silencestdio()
     except KeyboardInterrupt:
         # Catch early/late KeyboardInterrupt as last ditch. Here nothing will