diff --git a/tests/testlib/badserverext.py b/tests/testlib/badserverext.py
--- a/tests/testlib/badserverext.py
+++ b/tests/testlib/badserverext.py
@@ -31,10 +31,16 @@
    If defined, close the client socket after sending this many bytes.
    (The value is a list, multiple value can use used to close a series of
    request)
+
+close-after-send-patterns
+   If defined, close the client socket after the configured regexp is seen.
+   (The value is a list, multiple value can use used to close a series of
+   request)
 """
 
 from __future__ import absolute_import
 
+import re
 import socket
 
 from mercurial import (
@@ -64,20 +70,33 @@
 )
 configitem(
     b'badserver',
+    b'close-after-send-patterns',
+    default=b'',
+)
+configitem(
+    b'badserver',
     b'close-before-accept',
     default=False,
 )
 
 
 class ConditionTracker(object):
-    def __init__(self, close_after_recv_bytes, close_after_send_bytes):
+    def __init__(
+        self,
+        close_after_recv_bytes,
+        close_after_send_bytes,
+        close_after_send_patterns,
+    ):
         self._all_close_after_recv_bytes = close_after_recv_bytes
         self._all_close_after_send_bytes = close_after_send_bytes
+        self._all_close_after_send_patterns = close_after_send_patterns
 
         self.target_recv_bytes = None
         self.remaining_recv_bytes = None
         self.target_send_bytes = None
         self.remaining_send_bytes = None
+        self.send_pattern = None
+        self.send_data = b''
 
     def start_next_request(self):
         """move to the next set of close condition"""
@@ -87,6 +106,7 @@
         else:
             self.target_recv_bytes = None
             self.remaining_recv_bytes = None
+
         if self._all_close_after_send_bytes:
             self.target_send_bytes = self._all_close_after_send_bytes.pop(0)
             self.remaining_send_bytes = self.target_send_bytes
@@ -94,12 +114,20 @@
             self.target_send_bytes = None
             self.remaining_send_bytes = None
 
+        self.send_data = b''
+        if self._all_close_after_send_patterns:
+            self.send_pattern = self._all_close_after_send_patterns.pop(0)
+        else:
+            self.send_pattern = None
+
     def might_close(self):
         """True, if any processing will be needed"""
         if self.remaining_recv_bytes is not None:
             return True
         if self.remaining_send_bytes is not None:
             return True
+        if self.send_pattern is not None:
+            return True
         return False
 
     def forward_write(self, obj, method, data, *args, **kwargs):
@@ -108,11 +136,19 @@
         When the condition are met the socket is closed
         """
         remaining = self.remaining_send_bytes
+        pattern = self.send_pattern
 
         orig = object.__getattribute__(obj, '_orig')
         bmethod = method.encode('ascii')
         func = getattr(orig, method)
 
+        if pattern:
+            self.send_data += data
+            pieces = pattern.split(self.send_data, maxsplit=1)
+            if len(pieces) > 1:
+                dropped = len(pieces[-1])
+                remaining = len(data) - dropped
+
         if remaining:
             remaining = max(0, remaining)
 
@@ -131,16 +167,9 @@
         if remaining is None:
             obj._writelog(b'%s(%d) -> %s' % (bmethod, len(data), data))
         else:
-            obj._writelog(
-                b'%s(%d from %d) -> (%d) %s'
-                % (
-                    bmethod,
-                    len(newdata),
-                    len(data),
-                    remaining,
-                    newdata,
-                )
-            )
+            msg = b'%s(%d from %d) -> (%d) %s'
+            msg %= (bmethod, len(newdata), len(data), remaining, newdata)
+            obj._writelog(msg)
 
         if remaining is not None and remaining <= 0:
             obj._writelog(b'write limit reached; closing socket')
@@ -305,12 +334,23 @@
         self._close()
 
 
-def process_config(value):
+def process_bytes_config(value):
     parts = value.split(b',')
     integers = [int(v) for v in parts if v]
     return [v if v else None for v in integers]
 
 
+def process_pattern_config(value):
+    patterns = []
+    for p in value.split(b','):
+        if not p:
+            p = None
+        else:
+            p = re.compile(p, re.DOTALL | re.MULTILINE)
+        patterns.append(p)
+    return patterns
+
+
 def extsetup(ui):
     # Change the base HTTP server class so various events can be performed.
     # See SocketServer.BaseServer for how the specially named methods work.
@@ -322,12 +362,20 @@
             all_recv_bytes = self._ui.config(
                 b'badserver', b'close-after-recv-bytes'
             )
-            all_recv_bytes = process_config(all_recv_bytes)
+            all_recv_bytes = process_bytes_config(all_recv_bytes)
             all_send_bytes = self._ui.config(
                 b'badserver', b'close-after-send-bytes'
             )
-            all_send_bytes = process_config(all_send_bytes)
-            self._cond = ConditionTracker(all_recv_bytes, all_send_bytes)
+            all_send_bytes = process_bytes_config(all_send_bytes)
+            all_send_patterns = self._ui.config(
+                b'badserver', b'close-after-send-patterns'
+            )
+            all_send_patterns = process_pattern_config(all_send_patterns)
+            self._cond = ConditionTracker(
+                all_recv_bytes,
+                all_send_bytes,
+                all_send_patterns,
+            )
 
             # Need to inherit object so super() works.
             class badrequesthandler(self.RequestHandlerClass, object):