Attempting to use libcurl to do a HTTP POST but en...

# kotlin-native

[napperley]

Attempting to use libcurl to do a HTTP POST but end up with this response from the Prometheus server:

{
  "status": "error",
  "errorType": "bad_data",
  "error": "invalid parameter 'query': parse error at char 1: no expression found in input"
}

Can someone please confirm if I have correctly set the headers/body for the HTTP request. Below is the Kotlin code I am using:

fun doHttpPost(url: String, headers: Map<String, String>, body: Map<String, String>): Unit = memScoped {
    val headersList = alloc<curl_slist>()
    headers.forEach { (k, v) -> curl_slist_append(headersList.ptr, "$k: $v") }
    curl_easy_setopt(curlPost, CURLOPT_URL, url)
    curl_easy_setopt(curlPost, CURLOPT_HTTPHEADER, headersList.next)
    curl_easy_setopt(curlPost, CURLOPT_POSTFIELDS, createPostBody(body))
    // Do HTTP POST.
    curl_easy_perform(curlPost)
}

fun createPostBody(args: Map<String, String>) = buildString {
    args.forEach { (k,v) -> append("&$k=$v") }
}.replaceFirst("&", "")

Originally headersList.ptr was passed as an argument to curl_easy_setopt however that caused a seg fault. Fixed the seg fault by passing through headersList.next instead, although I am not sure if that is the correct way to do it.

[napperley]

URL used in the request: http://xx.xx.xx.xx:xxxx/api/v1/query

Trying to use the Prometheus Query API: https://prometheus.io/docs/prometheus/latest/querying/api/#instant-queries

[napperley]

Below is the function call being made to doHttpPost :

doHttpPost(
        url = "http://$host:$port/api/v1/query",
        headers = mapOf("Content-Type" to "application/x-www-form-urlencoded"),
        body = mapOf("query" to "up")
    )

[napperley]

Based on the debug output from libcurl a HTTP POST call has been made, and multiple headers are being included in the request:

*   Trying xx.xx.xx.xx...
* TCP_NODELAY set
* Connected to xx.xx.xx.xx (xx.xx.xx.xx) port xxxx (#0)
> POST /api/v1/query HTTP/1.1
Host: xx.xx.xx.xx:xxxx
Accept: */*
Content-Type: application/x-www-form-urlencoded
Size: 20
Content-Length: 6

* upload completely sent off: 6 out of 6 bytes
< HTTP/1.1 400 Bad Request
< Content-Type: application/json
< Date: Fri, 06 Mar 2020 02:04:00 GMT
< Content-Length: 130
< 
* Failed writing body (140695112576384 != 130)
* stopped the pause stream!
* Closing connection 0

What is strange is the error message about a HTTP body that can't be written (for the HTTP response?).

[napperley]

Wondering if there is a way for libcurl to print out the body of the HTTP request.

[napperley]

The linuxX64 version of Ktor Client uses libcurl under the hood. How does Ktor Client handle setting the body of a HTTP request (eg for a POST request) using libcurl ❓