curl responses mixed up with pcnt_fork

[mrsuh]

Description

Hi!
I have a PHP daemon with two forks. Each fork makes an HTTP requests to a remote server.
Sometimes(often) responses in forks are mixed up.

How to reproduce

Remote server

php -v
PHP 8.1.16 (cli) (built: Feb 14 2023 18:59:41) (NTS gcc x86_64)
Copyright (c) The PHP Group
Zend Engine v4.1.16, Copyright (c) Zend Technologies

server/index.php

<?php

switch ($_SERVER['REQUEST_URI']) {
    case '/200':
        http_response_code(200);
        break;
    case '/404':
        http_response_code(404);
        break;
    default:
        http_response_code(500);
}

symfony serve --no-tls --port=9999 --dir=server
 [OK] Web server listening
      The Web server is using PHP CGI 8.1.16
      http://127.0.0.1:9999
[Web Server ] Feb 15 15:02:25 |DEBUG  | PHP    Reloading PHP versions
[Web Server ] Feb 15 15:02:25 |DEBUG  | PHP    Using PHP version 8.1.16 (from default version in $PATH)
[Web Server ] Feb 15 15:02:25 |INFO   | PHP    listening path="/usr/bin/php-cgi" php="8.1.16" port=11238
[Web Server ] Feb 15 15:02:28 |INFO   | SERVER GET  (200) /200 ip=""
[Web Server ] Feb 15 15:02:28 |INFO   | SERVER GET  (200) /200
[Web Server ] Feb 15 15:02:28 |WARN   | SERVER GET  (404) /404
[Web Server ] Feb 15 15:02:29 |INFO   | SERVER GET  (200) /200
[Web Server ] Feb 15 15:02:29 |WARN   | SERVER GET  (404) /404
[Web Server ] Feb 15 15:02:30 |WARN   | SERVER GET  (404) /404
[Web Server ] Feb 15 15:02:30 |INFO   | SERVER GET  (200) /200
[Web Server ] Feb 15 15:02:31 |WARN   | SERVER GET  (404) /404
[Web Server ] Feb 15 15:02:31 |INFO   | SERVER GET  (200) /200

curl 'http://{remote_server}:9999/200' -I
HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
X-Powered-By: PHP/8.1.16

curl 'http://{remote_server}:9999/404' -I
HTTP/1.1 404 Not Found
Content-Type: text/html; charset=UTF-8
X-Powered-By: PHP/8.1.16

Local server

php -v
PHP 8.1.0 (cli) (built: Dec  2 2021 12:37:35) (ZTS)
Copyright (c) The PHP Group
Zend Engine v4.1.0, Copyright (c) Zend Technologies

client/test.php

<?php

$url200 = 'http://{remote_server}:9999/200';
$url404 = 'http://{remote_server}:9999/404';

function request(\CurlMultiHandle $multi, string $url): int
{
    $ch = curl_init();

    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_TIMEOUT, 5);
    curl_setopt($ch, CURLOPT_HEADER, true);
    curl_setopt($ch, CURLOPT_NOBODY, true);

    curl_multi_add_handle($multi, $ch);
    do {
        $status = curl_multi_exec($multi, $active);
        if ($active) {
            curl_multi_select($multi);
        }
    } while ($active && $status === CURLM_OK);
    $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
    curl_multi_remove_handle($multi, $ch);

    return (int)$httpCode;
}

$multi = curl_multi_init();

request($multi, $url200);// the request before pcntl_fork is important to reproduce error

$pid = pcntl_fork();
if ($pid === -1) {
    exit(1);
}

if ($pid) {
    $status = 0;
    while (pcntl_wait($status, WNOHANG | WUNTRACED) === 0) {
        $httpCode = request($multi, $url200);
        if ($httpCode !== 200) {
            printf("GET /200 - %d\n", $httpCode);
            break;
        }
        sleep(1);
    }
    posix_kill($pid, SIGKILL);
} else {
    while (true) {
        $httpCode = request($multi, $url404);
        if ($httpCode !== 404) {
            printf("GET /404 - %d\n", $httpCode);
            break;
        }

        sleep(1);
    }
}

Resulted in this output:

GET /404 - 200
GET /200 - 0 //timeout

But I expected this output instead:

GET /404 - 404
GET /200 - 200

Error only reproduces with a remote(not local) server.
I tested it with two different remote servers.

PHP Version

8.1.0

Operating System

macOs BigSur 11.7.4

[KapitanOczywisty]

I wonder If curl is trying to reuse the same connection in different forks: can you check verbose logs?

curl_setopt($ch, CURLOPT_VERBOSE, true);
$streamVerboseHandle = fopen('php://temp', 'w+');
curl_setopt($ch, CURLOPT_STDERR, $streamVerboseHandle);

And after exec:

rewind($streamVerboseHandle);
echo stream_get_contents($streamVerboseHandle);

[mrsuh]

I changed the request function to this:

function request(\CurlMultiHandle $multi, string $url): int
{
    $ch = curl_init();

    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_TIMEOUT, 5);
    curl_setopt($ch, CURLOPT_HEADER, true);
    curl_setopt($ch, CURLOPT_NOBODY, true);
    curl_setopt($ch, CURLOPT_VERBOSE, true);
    $streamVerboseHandle = fopen('php://temp', 'w+');
    curl_setopt($ch, CURLOPT_STDERR, $streamVerboseHandle);

    curl_multi_add_handle($multi, $ch);
    do {
        $status = curl_multi_exec($multi, $active);
        if ($active) {
            curl_multi_select($multi);
        }
    } while ($active && $status === CURLM_OK);
    $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
    rewind($streamVerboseHandle);
    echo stream_get_contents($streamVerboseHandle);
    curl_multi_remove_handle($multi, $ch);

    return (int)$httpCode;
}

output:

HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
X-Powered-By: PHP/8.1.16
Date: Mon, 20 Feb 2023 10:49:26 GMT

*   Trying {remote_server}:9999...
* Connected to {remote_server} ({remote_server}) port 9999 (#0)
> HEAD /200 HTTP/1.1
Host: {remote_server}:9999
Accept: */*

* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Content-Type: text/html; charset=UTF-8
< X-Powered-By: PHP/8.1.16
< Date: Mon, 20 Feb 2023 10:49:26 GMT
<
* Connection #0 to host {remote_server} left intact
HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
X-Powered-By: PHP/8.1.16
Date: Mon, 20 Feb 2023 10:49:26 GMT

* Found bundle for host {remote_server}: 0x56478f9ac6d0 [serially]
* Can not multiplex, even if we wanted to!
* Re-using existing connection! (#0) with host {remote_server}
* Connected to {remote_server} ({remote_server}) port 9999 (#0)
> HEAD /404 HTTP/1.1
Host: {remote_server}:9999
Accept: */*

* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Content-Type: text/html; charset=UTF-8
< X-Powered-By: PHP/8.1.16
< Date: Mon, 20 Feb 2023 10:49:26 GMT
<
* Excess found: excess = 129 url = /404 (zero-length body)
* Connection #0 to host {remote_server} left intact
GET /404 - 200
* Found bundle for host {remote_server}: 0x56478f9ac6d0 [serially]
* Can not multiplex, even if we wanted to!
* Re-using existing connection! (#0) with host {remote_server}
* Connected to {remote_server} ({remote_server}) port 9999 (#0)
> HEAD /200 HTTP/1.1
Host: {remote_server}:9999
Accept: */*

* Operation timed out after 5001 milliseconds with 0 bytes received
* Closing connection 0
GET /200 - 0

[KapitanOczywisty]

Yup, that exactly what is happening, one instance is consuming both responses.

I don't know enough about curl to say if it's a bug or misuse or won't fix. Probably curl connection bundles should be dropped while forking.

[iluuu1994]

@adoy Can you help here?

[adoy]

@iluuu1994 I'll try to have a look and see what I can find later this week.

[mrsuh]

Hi guys.
I reproduce the error in C and with a local server!

server.js

const http = require('http');

http.createServer(function(request, response) {
    let url = new URL(request.url, 'http://0.0.0.0:8888');
    switch (true) {
        case url.pathname.startsWith('/200'):
            response.statusCode = 200;
            response.end();
            break;
        case url.pathname.startsWith('/404'):
            response.statusCode = 404;
            response.end();
            break
        default:
            response.statusCode = 500;
            response.end();
    }

}).listen(8888, (err) => {
    if (err) {
        return console.log('something bad happened', err);
    }
    console.log(`server is listening on 8888`);
});

main.c

#include <stdlib.h>
#include <stdio.h>
#include <curl/curl.h>
#include <curl/multi.h>
#include <unistd.h>

size_t curl_write_function(void *ptr, size_t size, size_t nmemb, void *stream)
{
    return size * nmemb;
}

int request(CURLM *multi, const char *url)
{
    CURL *curl = curl_easy_init();

    curl_easy_setopt(curl, CURLOPT_URL, url);
    curl_easy_setopt(curl, CURLOPT_TIMEOUT, 1);
    curl_easy_setopt(curl, CURLOPT_HEADER, 1L);
    curl_easy_setopt(curl, CURLOPT_NOBODY, 1L);
    curl_easy_setopt(curl, CURLOPT_HTTPGET, 1L);
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, curl_write_function);
    curl_multi_add_handle(multi, curl);
    int still_running;
    do {
        CURLMcode mc = curl_multi_perform(multi, &still_running);
        if(!mc && still_running) {
            curl_multi_poll(multi, NULL, 0, 200, NULL);
        }
    } while (still_running);
    long response_code;
    curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &response_code);
    curl_multi_remove_handle(multi, curl);
    curl_easy_cleanup(curl);

    return (int)response_code;
}

int main()
{
    const char* url200 = "http://127.0.0.1:8888/200";
    const char* url404 = "http://127.0.0.1:8888/404";

    CURLM *multi = curl_multi_init();

    request(multi, url200);// the request before fork() is important to reproduce the error

    pid_t pid = fork();
    if(pid == -1) {
        return 1;
    }

    if (pid == 0) {
        while(1) {
            int response_code = request(multi, url200);
            if(response_code != 200) {
                printf("GET /200 - %d\n", response_code);
                break;
            }
        }
    } else {
        while(1) {
            int response_code = request(multi, url404);
            if(response_code != 404) {
                printf("GET /404 - %d\n", response_code);
                break;
            }
            sleep(1);
        }
    }

    return 0;
}

start the server:

node server.js

reproduce the error:

gcc main.c -o main -lcurl
./main
GET /200 - 404
GET /404 - 200

I hope this helps you.

[cmb69]

As this can apparently be reproduced in C without PHP being involved, this doesn't look like a PHP issue (and I don't think we should work-around). Actually, I'm not even sure this is a cURL issue; sharing a multi handle between a parent and (several) forked children is probably just not being supported ("undefined behavior").

Anyway, I'm not likely the best address to ask about fork(2).

[bukka]

Agreed this should not be shared and doesn't look like PHP issue to me either so closing.