Fix request buffering with post_copy_size=0

src/proxy/http/HttpTransact.cc

@@ -1573,7 +1573,7 @@ HttpTransact::HandleRequest(State *s)
         }
       }
     }
-    if (s->txn_conf->request_buffer_enabled &&
+    if (s->txn_conf->request_buffer_enabled && s->http_config_param->post_copy_size > 0 &&
         s->state_machine->get_ua_txn()->has_request_body(s->hdr_info.request_content_length,
                                                          s->client_info.transfer_encoding == TransferEncoding_t::CHUNKED)) {
       TRANSACT_RETURN(StateMachineAction_t::WAIT_FOR_FULL_BODY, nullptr);

tests/gold_tests/pluginTest/test_hooks/body_buffer.test.py

@@ -130,5 +130,94 @@ def run(self):
         tr.Processes.Default.Streams.stderr = "200.gold"
 
 
+class ZeroPostCopySizeTest:
+    '''
+    Test for issue #6900: Verify that post_copy_size=0 with request_buffer_enabled=1
+    does not cause 403 errors. When post_copy_size is 0, request buffering should be
+    disabled and POST requests should proceed normally.
+    '''
+
+    def __init__(self):
+        self.setupOriginServer()
+        self.setupTS()
+
+    def setupOriginServer(self):
+        self._server = Test.MakeOriginServer("server_zero_copy")
+        # Empty body POST
+        request_header_empty = {
+            "headers": "POST /empty HTTP/1.1\r\n"
+                       "Host: www.example.com\r\n"
+                       "Content-Length: 0\r\n\r\n",
+            "timestamp": "1469733493.993",
+            "body": ""
+        }
+        response_header_empty = {
+            "headers": "HTTP/1.1 200 OK\r\n"
+                       "Server: microserver\r\n"
+                       "Content-Length: 2\r\n"
+                       "Connection: close\r\n\r\n",
+            "timestamp": "1469733493.993",
+            "body": "OK"
+        }
+        self._server.addResponse("sessionlog.json", request_header_empty, response_header_empty)
+
+        # Small body POST
+        self.small_body = "small"
+        request_header_small = {
+            "headers": "POST /small HTTP/1.1\r\n"
+                       "Host: www.example.com\r\n"
+                       f"Content-Length: {len(self.small_body)}\r\n\r\n",
+            "timestamp": "1469733493.993",
+            "body": self.small_body
+        }
+        response_header_small = {
+            "headers": "HTTP/1.1 200 OK\r\n"
+                       "Server: microserver\r\n"
+                       "Content-Length: 2\r\n"
+                       "Connection: close\r\n\r\n",
+            "timestamp": "1469733493.993",
+            "body": "OK"
+        }
+        self._server.addResponse("sessionlog.json", request_header_small, response_header_small)
+
+    def setupTS(self):
+        # Use default port selection (not select_ports=False) to avoid port conflicts
+        self._ts = Test.MakeATSProcess("ts_zero_copy")
+        self._ts.Disk.remap_config.AddLine(f'map / http://127.0.0.1:{self._server.Variables.Port}')
+        # Install the request_buffer plugin to enable per-request buffering
+        Test.PrepareInstalledPlugin('request_buffer.so', self._ts)
+        self._ts.Disk.records_config.update(
+            {
+                'proxy.config.diags.debug.enabled': 1,
+                'proxy.config.diags.debug.tags': 'http|request_buffer',
+                # The key configuration: post_copy_size=0 with request_buffer_enabled=1
+                # This should NOT cause 403 errors (issue #6900 fix)
+                # The plugin will enable buffering per-request, but with post_copy_size=0
+                # buffering should be disabled and the request should proceed normally
+                'proxy.config.http.post_copy_size': 0,
+                'proxy.config.http.request_buffer_enabled': 1,
+            })
+
+    def run(self):
+        tr = Test.AddTestRun()
+        # Send POST with empty body and small body.
+        # These should return 200, not 403, verifying the fix for issue #6900.
+        tr.MakeCurlCommand(
+            f'-v -X POST -H "Content-Length: 0" http://127.0.0.1:{self._ts.Variables.port}/empty --next '
+            f'-v http://127.0.0.1:{self._ts.Variables.port}/small -d {self.small_body}',
+            ts=self._ts)
+        tr.Processes.Default.ReturnCode = 0
+        tr.Processes.Default.StartBefore(self._server)
+        tr.Processes.Default.StartBefore(self._ts)
+        # Verify we get 200 OK, not 403 Forbidden
+        tr.Processes.Default.Streams.stderr += Testers.ContainsExpression(
+            "< HTTP/1.1 200 OK", "Verify POST with empty body returns 200")
+        tr.Processes.Default.Streams.stderr += Testers.ExcludesExpression("< HTTP/1.1 403", "Verify no 403 Forbidden errors")
+
+
 bodyBufferTest = BodyBufferTest("Test request body buffering.")
 bodyBufferTest.run()
+
+# Test for issue #6900: post_copy_size=0 should not cause 403 errors
+zeroCopySizeTest = ZeroPostCopySizeTest()
+zeroCopySizeTest.run()