Ktor Curl engine hangs/freezes after multiple requests on Linux x64 (Kotlin Native)

[orchestr7]

Environment

• Ktor version: 3.3.2
• Kotlin version: 2.2.21
• Platform: Linux x64 (Kotlin Native)
• Engine: Curl
• OS: Linux
• FROM --platform=linux/amd64 alpine:latest
  RUN apk add --no-cache gcompat libcurl libgcc libstdc++

Description

The Ktor Curl engine appears to have a resource leak or connection pool issue that causes requests to hang indefinitely after a certain number of requests (typically around 80-100 requests in my testing). This happens flaky and causes serious issues while connecting to paginated pages.

Reproduction Steps

1. Create a Kotlin Multiplatform project targeting linuxX64()
2. Use the Curl engine with timeout and retry configurations
3. Make repeated HTTP requests in a loop
4. Observe that after ~80-100 requests, the application hangs

Complete Minimal Reproduction

Click to expand full code

// build.gradle.kts
val ktorVersion: String = "3.3.2"

plugins {
    kotlin("multiplatform") version "2.2.21"
    kotlin("plugin.serialization") version "2.2.21"
}

repositories {
    mavenCentral()
}

kotlin {
    linuxX64().apply {
        binaries {
            executable {
                entryPoint = "test.main"
            }
        }
    }

    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation("io.ktor:ktor-client-core:$ktorVersion")
                implementation("io.ktor:ktor-client-curl:$ktorVersion")
            }
        }
    }
}

// Main.kt
package test

import io.ktor.client.*
import io.ktor.client.engine.curl.Curl
import io.ktor.client.plugins.HttpTimeout
import io.ktor.client.request.*
import io.ktor.client.statement.*
import kotlinx.coroutines.*

fun main() = runBlocking {
    val client = HttpClient(Curl) {
        install(HttpTimeout) {
            requestTimeoutMillis = 10000
            connectTimeoutMillis = 5000
            socketTimeoutMillis = 6000
        }
    }

    var pageCount = 0
    while(true) {
        pageCount++
        println("Page $pageCount")
        
        try {
            val result = withTimeoutOrNull(20000) {
                val response = client.get("https://google.com")
                response.bodyAsText()
            }
            
            if (result == null) {
                println("REQUEST TIMED OUT - HANGING DETECTED")
                break
            }
        } catch (e: Exception) {
            println("Exception: ${e.message}")
            break
        }
    }
    
    client.close()
}

Logs

The application runs successfully for multiple requests, then hangs:

Page 1
Page 2
Page 3
Page 4
Page 5
Page 6
Page 7
Page 8
Page 9
Page 10
Page 11
Page 12
Page 13
Page 14
Page 15
Page 16
Page 17
<FREEZE>

Analysis

1. Consistent failure point: The hang occurs consistently , but randomly. Everything works on MAC.
2. Timeout ignored: The configured HTTP timeouts (5s/6s/10s) and even the coroutine timeout (20s) are not respected
3. Platform specific: This issue appears specific to the Curl engine on Linux x64. Everything works on MAC.

Potential Root Causes

• Connection pool not properly releasing connections?
• File descriptor leak in the native Curl integration?
• Memory leak in the Kotlin Native <-> libcurl bridge?
• Some problem with retry mechanism?

Additional Information

This issue makes the Curl engine unreliable for long-running applications or applications that need to make many HTTP requests. The problem appears to be at the native layer since Kotlin/JVM timeouts are not respected.