Server cannot process large json request

[denprog]

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Please create a new issue only if you think you have found a bug or if have a feature request/enhancement.

Describe the bug
I have a method which receives json documents:

void ServiceController::SaveDocument(const HttpRequestPtr& req, std::function<void (const HttpResponsePtr &)>&& callback)
{
    std::string session_id;
    if (!GetSessionId(req, callback, session_id))
        return;

    GetLogger(session_id)->Debug("SaveDocument request");
    for (const auto &h : req->getHeaders())
        GetLogger(session_id)->Debug("{}:{}", h.first, h.second);
    GetLogger(session_id)->Debug("Body length: {}", req->getBody().size());
    GetLogger(session_id)->Debug("Body: {}", req->getBody());
    auto json = req->getJsonObject();
    if (!json || !json->isObject())
    {
        GetLogger(session_id)->Error("SaveDocument error: Wrong request: Json not found in the request");
        SendError(k400BadRequest, "Json not found in the request", session_id, callback);
        return;
    }
...
}

I log the received request in it.
I send a large json - the method works fine. I send many large documents - the method works fine. After I send the same document - getJsonObject() returns null.

I dig into the problem, wrote a request with a little json file, which works fine:

curl -v -X POST http://127.0.0.1:9001/service/save-document -H "Content-Type: application/json" -H "access_token:eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJhY2Nlc3MtdXVpZCI6IjJhNjc5NTY4LWU3Y2QtNDM0ZC1hNTJkLTJiMTI5YWVkYzcwMyIsImV4cCI6MTc2MTc0NjU4OSwiaWF0IjoxNzYxNzQ2NDY5LCJpc3MiOiJhdXRoMCIsImp0aSI6Inl1dG92by1zZXJ2ZXIiLCJsb2dpbiI6InVzZXIyIn0.CO3XzGEThd4V1IB-Wpy_gHBeKzIc4Rwamy7eX-QQj_uoBmMzvSluY3Kv56T7mhd3nk8BcF_dTooU2FEH1Fim_TvmWj6ZQvb_BvsqXEBEzw_Ng5NN4xMkjVEckXbiODbEtATjIqtLBm7MSo3KBovSWIVHLIVj0QcVSaY_A5r2RPNKKshQ-X87iot8-bwg-VGmVK64eQAEaXXY8psolVNKJ0z5VoVkg1xEoVv8sfmNgKHdWAfsrusa2bhjsEuDp36IMBXr6Le3MGcGaoPE736-gWdc3Ocjl5OgX3BV6Tkm-hi6r-W0QsWmFrEH3JShpTvvp7B9c_PtU_mqLK5UFBkwRQ" -H "Cookie:session_id=e264b882-1f03-4297-94d5-cd898b06d642;JSESSIONID=36bd6810-0af8-4c24-9ef6-90b18ff03502" --upload-file /home/denis/programs/Math/yutovo/yutovo-desktop/build/debug/22
*   Trying 127.0.0.1:9001...
* Connected to 127.0.0.1 (127.0.0.1) port 9001
> POST /service/save-document HTTP/1.1
> Host: 127.0.0.1:9001
> User-Agent: curl/8.5.0
> Accept: */*
> Content-Type: application/json
> access_token:eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJhY2Nlc3MtdXVpZCI6IjJhNjc5NTY4LWU3Y2QtNDM0ZC1hNTJkLTJiMTI5YWVkYzcwMyIsImV4cCI6MTc2MTc0NjU4OSwiaWF0IjoxNzYxNzQ2NDY5LCJpc3MiOiJhdXRoMCIsImp0aSI6Inl1dG92by1zZXJ2ZXIiLCJsb2dpbiI6InVzZXIyIn0.CO3XzGEThd4V1IB-Wpy_gHBeKzIc4Rwamy7eX-QQj_uoBmMzvSluY3Kv56T7mhd3nk8BcF_dTooU2FEH1Fim_TvmWj6ZQvb_BvsqXEBEzw_Ng5NN4xMkjVEckXbiODbEtATjIqtLBm7MSo3KBovSWIVHLIVj0QcVSaY_A5r2RPNKKshQ-X87iot8-bwg-VGmVK64eQAEaXXY8psolVNKJ0z5VoVkg1xEoVv8sfmNgKHdWAfsrusa2bhjsEuDp36IMBXr6Le3MGcGaoPE736-gWdc3Ocjl5OgX3BV6Tkm-hi6r-W0QsWmFrEH3JShpTvvp7B9c_PtU_mqLK5UFBkwRQ
> Cookie:session_id=e264b882-1f03-4297-94d5-cd898b06d642;JSESSIONID=36bd6810-0af8-4c24-9ef6-90b18ff03502
> Content-Length: 23400
> 
* We are completely uploaded and fine
< HTTP/1.1 200 OK
< content-length: 31
< content-type: application/json; charset=utf-8
< server: drogon/1.9.10
< date: Wed, 29 Oct 2025 18:35:00 GMT
< 
* Connection #0 to host 127.0.0.1 left intact

After I take a file with more size, the method doesn't work:

curl -v -X POST http://127.0.0.1:9001/service/save-document -H "Content-Type: application/json" -H "access_token:eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJhY2Nlc3MtdXVpZCI6IjJhNjc5NTY4LWU3Y2QtNDM0ZC1hNTJkLTJiMTI5YWVkYzcwMyIsImV4cCI6MTc2MTc0NjU4OSwiaWF0IjoxNzYxNzQ2NDY5LCJpc3MiOiJhdXRoMCIsImp0aSI6Inl1dG92by1zZXJ2ZXIiLCJsb2dpbiI6InVzZXIyIn0.CO3XzGEThd4V1IB-Wpy_gHBeKzIc4Rwamy7eX-QQj_uoBmMzvSluY3Kv56T7mhd3nk8BcF_dTooU2FEH1Fim_TvmWj6ZQvb_BvsqXEBEzw_Ng5NN4xMkjVEckXbiODbEtATjIqtLBm7MSo3KBovSWIVHLIVj0QcVSaY_A5r2RPNKKshQ-X87iot8-bwg-VGmVK64eQAEaXXY8psolVNKJ0z5VoVkg1xEoVv8sfmNgKHdWAfsrusa2bhjsEuDp36IMBXr6Le3MGcGaoPE736-gWdc3Ocjl5OgX3BV6Tkm-hi6r-W0QsWmFrEH3JShpTvvp7B9c_PtU_mqLK5UFBkwRQ" -H "Cookie:session_id=e264b882-1f03-4297-94d5-cd898b06d642;JSESSIONID=36bd6810-0af8-4c24-9ef6-90b18ff03502" --upload-file /home/denis/programs/Math/yutovo/yutovo-desktop/build/debug/33
*   Trying 127.0.0.1:9001...
* Connected to 127.0.0.1 (127.0.0.1) port 9001
> POST /service/save-document HTTP/1.1
> Host: 127.0.0.1:9001
> User-Agent: curl/8.5.0
> Accept: */*
> Content-Type: application/json
> access_token:eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJhY2Nlc3MtdXVpZCI6IjJhNjc5NTY4LWU3Y2QtNDM0ZC1hNTJkLTJiMTI5YWVkYzcwMyIsImV4cCI6MTc2MTc0NjU4OSwiaWF0IjoxNzYxNzQ2NDY5LCJpc3MiOiJhdXRoMCIsImp0aSI6Inl1dG92by1zZXJ2ZXIiLCJsb2dpbiI6InVzZXIyIn0.CO3XzGEThd4V1IB-Wpy_gHBeKzIc4Rwamy7eX-QQj_uoBmMzvSluY3Kv56T7mhd3nk8BcF_dTooU2FEH1Fim_TvmWj6ZQvb_BvsqXEBEzw_Ng5NN4xMkjVEckXbiODbEtATjIqtLBm7MSo3KBovSWIVHLIVj0QcVSaY_A5r2RPNKKshQ-X87iot8-bwg-VGmVK64eQAEaXXY8psolVNKJ0z5VoVkg1xEoVv8sfmNgKHdWAfsrusa2bhjsEuDp36IMBXr6Le3MGcGaoPE736-gWdc3Ocjl5OgX3BV6Tkm-hi6r-W0QsWmFrEH3JShpTvvp7B9c_PtU_mqLK5UFBkwRQ
> Cookie:session_id=e264b882-1f03-4297-94d5-cd898b06d642;JSESSIONID=36bd6810-0af8-4c24-9ef6-90b18ff03502
> Content-Length: 201447
> 
* We are completely uploaded and fine
< HTTP/1.1 400 Bad Request
< content-length: 41
< content-type: application/json; charset=utf-8
< server: drogon/1.9.10
< date: Wed, 29 Oct 2025 18:34:55 GMT
< 
* Connection #0 to host 127.0.0.1 left intact
{"error":"Json not found in the request"}%              

I found the setClientMaxBodySize() method, and will check it. The question is: why large json documents were recognized in the beginning, but after some iterations were not? Is it well behavior? Is there a cache which accumulates?
To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
The server processes large json requests without errors and regardless their quantity.

Desktop (please complete the following information):

• OS: Ubuntu 24.04
• Browser curl

Additional context
Add any other context about the problem here.

[an-tao]

1. Compile drogon in debug mode;
2. setLogLevel to Trace;
3. Check the log output;
   @denprog