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Adding "content-length" header to response causes closed connection. #862
Description
This is adapted from the streaming example from here
https://github.com/awslabs/aws-lambda-rust-runtime/blob/main/examples/basic-streaming-response/src/main.rs
use lambda_runtime::{
Error, LambdaEvent, service_fn,
streaming::{Body, Response},
tracing,
};
use serde_json::Value;
async fn func(_event: LambdaEvent<Value>) -> Result<Response<Body>, Error> {
let body = "hello\n";
let len = body.as_bytes().len();
let mut resp = Response::from(Body::from(body));
// resp.metadata_prelude.headers.append("content-type", "text/plain".parse().unwrap());
resp.metadata_prelude.headers.append("content-length", len.into());
Ok(resp)
}
#[tokio::main]
async fn main() -> Result<(), Error> {
// required to enable CloudWatch error logging by the runtime
tracing::init_default_subscriber();
lambda_runtime::run(service_fn(func)).await?;
Ok(())
}run it with cargo lambda watch, then curl -v
>curl -v http://127.0.0.1:9000
* Trying 127.0.0.1:9000...
* Connected to 127.0.0.1 (127.0.0.1) port 9000
> GET / HTTP/1.1
> Host: 127.0.0.1:9000
> User-Agent: curl/8.9.1
> Accept: */*
>
* Empty reply from server
* shutting down connection #0
curl: (52) Empty reply from server
remove the header will fix it, but I wonder why?
upload the binary on aws, it works just fine, except the header becomes "x-amzn-remapped-content-length", so I guess it's not lambda-runtime's fault.
Activity
calavera commentedon Jun 26, 2025
It's probably some modification AWS does at the proxy level. Which service are you integrating your Lambda function with?
Jimmy-Z commentedon Jun 26, 2025
Nothing, maybe I'm too new to AWS to understand the question, I deployed it on AWS, and call it as a function url.
calavera commentedon Jun 26, 2025
This is what I was looking for. There are usually a few ways to call the Lambda function, and unfortunately, they sometimes differ in behavior.
I think it'd be possible to remove the header from the response payload to prevent the server from closing the connection, I'll have to investigate it further.
github-actions commentedon Jul 17, 2025
This issue has been automatically locked. Create a new discussion if you'd like to continue the conversation. https://github.com/cargo-lambda/cargo-lambda/discussions/new?category=general
calavera commentedon Jul 18, 2025
Version 1.8.6 released with a fix for this issue: https://github.com/cargo-lambda/cargo-lambda/releases/tag/v1.8.6