fix: prevent postprocessor/errorHandler from overriding HTTP_STATUS_UNFINISHED (#828)

* Initial plan

* fix: prevent postprocessor/errorHandler from overriding HTTP_STATUS_UNFINISHED

When a handler returns HTTP_STATUS_UNFINISHED (e.g., sendLargeFile), the
postprocessor's return value was overwriting status_code, causing the
connection to be treated as complete. This resulted in a premature empty
response being sent and the connection being reset/closed.

The fix uses local variables for the postprocessor and errorHandler return
values, only checking for HTTP_STATUS_CLOSE without modifying status_code.
This preserves the UNFINISHED status while still supporting the
HTTP_STATUS_CLOSE feature added in commit 62cd137.

Agent-Logs-Url: https://github.com/ithewei/libhv/sessions/aa478496-9e54-4c62-b0f7-810817b3867f

Co-authored-by: ithewei <26049660+ithewei@users.noreply.github.com>

* Refactor postprocessor variable name for clarity

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ithewei <26049660+ithewei@users.noreply.github.com>
Co-authored-by: ithewei <ithewei@163.com>

Author: Copilot

http/server/HttpHandler.cpp

@@ -482,21 +482,25 @@ int HttpHandler::HandleHttpRequest() {
     }
 
 postprocessor:
+    // Handle HTTP_STATUS_CLOSE: close connection without response
+    if (status_code == HTTP_STATUS_CLOSE) {
+        state = WANT_CLOSE;
+        return HTTP_STATUS_CLOSE;
+    }
     if (status_code >= 100 && status_code < 600) {
         pResp->status_code = (http_status)status_code;
         if (pResp->status_code >= 400 && pResp->body.size() == 0 && pReq->method != HTTP_HEAD) {
             if (service->errorHandler) {
-                status_code = customHttpHandler(service->errorHandler);
+                int err_status_code = customHttpHandler(service->errorHandler);
+                if (err_status_code == HTTP_STATUS_CLOSE) {
+                    state = WANT_CLOSE;
+                    return HTTP_STATUS_CLOSE;
+                }
             } else {
                 defaultErrorHandler();
             }
         }
     }
-    // Handle HTTP_STATUS_CLOSE: close connection without response
-    if (status_code == HTTP_STATUS_CLOSE) {
-        state = WANT_CLOSE;
-        return HTTP_STATUS_CLOSE;
-    }
     if (fc) {
         pResp->content = fc->filebuf.base;
         pResp->content_length = fc->filebuf.len;
@@ -505,8 +509,8 @@ int HttpHandler::HandleHttpRequest() {
         pResp->headers["Etag"] = fc->etag;
     }
     if (service->postprocessor) {
-        status_code = customHttpHandler(service->postprocessor);
-        if (status_code == HTTP_STATUS_CLOSE) {
+        int post_status_code = customHttpHandler(service->postprocessor);
+        if (post_status_code == HTTP_STATUS_CLOSE) {
             state = WANT_CLOSE;
             return HTTP_STATUS_CLOSE;
         }