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Headers allow the same entry multiple times with different case

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
38/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
kotlin
Domain
networking

Research direction

Start with the Headers APIs used in the Kotlin example, especially headersOf and toHeaders, and trace how Request.Builder.headers handles names that differ only by case. Add a focused test based on the reproduction; done means the test establishes the intended behavior for repeated case-insensitive header names, which the issue does not specify fully.

Written by the indexing model from the issue text.

Description

bug

HTTP header names are case-insensitive according to the HTTP specification (RFC 2616, RFC 7230).
So "User-Agent" and "user-agent" should be treated as the same key.

I don't know if this is the intended behavior, but okHttp is sending multiple times the same entry.

import okhttp3.OkHttpClient
import okhttp3.Request
import okhttp3.Headers.Companion.toHeaders
import okhttp3.Headers.Companion.headersOf
import okhttp3.mockwebserver.MockResponse
import okhttp3.mockwebserver.MockWebServer

private val client = OkHttpClient()
private val server = MockWebServer()

fun main() {
    val defHeaders = mapOf(
        "User-Agent" to "my-user-agent1",
        "Referer" to "my-referer1"
    )
    server.enqueue(MockResponse())

    println("### Headers from MockWebServer ###")
    val myHeaders = mapOf(
        "user-agent" to "my-user-agent2",
        "User-Agent" to "my-user-agent3",
        "referer" to "my-referer2"
    )
    val request1 = Request.Builder()
        .url(server.url("/"))
        .headers((defHeaders + myHeaders).toHeaders())
        .build()

    client.newCall(request1).execute()

    val recordedRequest = server.takeRequest()
    println(recordedRequest.headers)
    server.close()


    println("### Headers from httpbin ###")
    val request2 = Request.Builder()
        .url("https://httpbin.org/get")
        .headers(headersOf(
            "User-Agent", "my-user-agent1",
            "user-agent", "my-user-agent2",
            "User-Agent", "my-user-agent3",
            "Referer", "my-referer1",
            "referer", "my-referer2",
            "Cookie", "key1=va1",
            "cookie", "key2=val2"))
        .build()

    client.newCall(request2).execute().use { response ->
        println(response.body?.string())
    }
}

Terminal output:

### Headers from MockWebServer ###
User-Agent: my-user-agent1
Referer: my-referer1
user-agent: my-user-agent2
Host: localhost:63575
Connection: Keep-Alive
Accept-Encoding: gzip

### Headers from httpbin ###
{
  "args": {},
  "headers": {
    "Accept-Encoding": "gzip",
    "Cookie": "key1=va1; key2=val2",
    "Host": "httpbin.org",
    "Referer": "my-referer1,my-referer2",
    "User-Agent": "my-user-agent1,my-user-agent2,my-user-agent3,my-user-agent4",
    "X-Amzn-Trace-Id": "Root=########################"
  },
  "origin": "###.###.###.###",
  "url": "https://httpbin.org/get"
}
Dominant language
Kotlin
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Forks
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Avg merge
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Merged PRs (30d)
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