//! Read-only Git smart-HTTP support backed directly by stateless
//! `git upload-pack` processes.

use std::io::Read as _;
use std::path::Path;
use std::process::Stdio;

use axum::body::{Body, Bytes};
use axum::http::{HeaderMap, StatusCode, header};
use axum::response::Response;
use flate2::read::GzDecoder;
use tokio::io::AsyncWriteExt as _;
use tokio::sync::OwnedSemaphorePermit;
use tokio_util::io::ReaderStream;

const MAX_INFLATED_REQUEST: u64 = 64 * 1024 * 1024;
const SERVICE_PREAMBLE: &[u8] = b"001e# service=git-upload-pack\n0000";
type WebError = (StatusCode, String);

pub fn advertise(
    repo_dir: &Path,
    headers: &HeaderMap,
    permit: OwnedSemaphorePermit,
) -> Result<Response, WebError> {
    let protocol = git_protocol(headers);
    let stdout = spawn(repo_dir, "--http-backend-info-refs", protocol, None, permit)?;
    let preamble = if protocol.is_some_and(|p| p.split(':').any(|v| v == "version=2")) {
        &[][..]
    } else {
        SERVICE_PREAMBLE
    };
    response(
        "application/x-git-upload-pack-advertisement",
        tokio::io::AsyncReadExt::chain(std::io::Cursor::new(preamble), stdout),
    )
}

pub fn upload(
    repo_dir: &Path,
    headers: &HeaderMap,
    body: Bytes,
    permit: OwnedSemaphorePermit,
) -> Result<Response, WebError> {
    if headers
        .get(header::CONTENT_TYPE)
        .and_then(|v| v.to_str().ok())
        != Some("application/x-git-upload-pack-request")
    {
        return Err((
            StatusCode::UNSUPPORTED_MEDIA_TYPE,
            "expected a git-upload-pack request".into(),
        ));
    }

    let body = decode_body(headers, body)?;
    let stdout = spawn(
        repo_dir,
        "--stateless-rpc",
        git_protocol(headers),
        Some(body),
        permit,
    )?;
    response("application/x-git-upload-pack-result", stdout)
}

fn spawn(
    repo_dir: &Path,
    mode: &str,
    protocol: Option<&str>,
    input: Option<Bytes>,
    permit: OwnedSemaphorePermit,
) -> Result<tokio::process::ChildStdout, WebError> {
    let mut command = tokio::process::Command::new("git");
    command
        .arg("upload-pack")
        .arg(mode)
        .arg(repo_dir)
        .stdin(if input.is_some() {
            Stdio::piped()
        } else {
            Stdio::null()
        })
        .stdout(Stdio::piped())
        .stderr(Stdio::inherit())
        .kill_on_drop(true);
    if let Some(protocol) = protocol {
        command.env("GIT_PROTOCOL", protocol);
    }

    let mut child = command
        .spawn()
        .map_err(|e| bad_gateway(format!("spawning git upload-pack: {e}")))?;
    let stdin = child.stdin.take();
    let stdout = child.stdout.take().expect("stdout was piped");
    tokio::spawn(async move {
        if let Some(input) = input {
            let mut stdin = stdin.expect("stdin was piped");
            let _ = stdin.write_all(&input).await;
        }
        match child.wait().await {
            Ok(status) if !status.success() => {
                eprintln!("sorceryd: git upload-pack exited with {status}")
            }
            Err(error) => eprintln!("sorceryd: waiting for git upload-pack: {error}"),
            _ => {}
        }
        drop(permit);
    });
    Ok(stdout)
}

fn decode_body(headers: &HeaderMap, body: Bytes) -> Result<Bytes, WebError> {
    let Some(encoding) = headers.get(header::CONTENT_ENCODING) else {
        return Ok(body);
    };
    let encoding = encoding.to_str().map_err(|_| {
        (
            StatusCode::UNSUPPORTED_MEDIA_TYPE,
            "unsupported content encoding".into(),
        )
    })?;
    if encoding.eq_ignore_ascii_case("identity") {
        return Ok(body);
    }
    if !encoding.eq_ignore_ascii_case("gzip") && !encoding.eq_ignore_ascii_case("x-gzip") {
        return Err((
            StatusCode::UNSUPPORTED_MEDIA_TYPE,
            "unsupported content encoding".into(),
        ));
    }

    let mut decoded = Vec::new();
    GzDecoder::new(body.as_ref())
        .take(MAX_INFLATED_REQUEST + 1)
        .read_to_end(&mut decoded)
        .map_err(|_| (StatusCode::BAD_REQUEST, "invalid gzip request body".into()))?;
    if decoded.len() as u64 > MAX_INFLATED_REQUEST {
        return Err((
            StatusCode::PAYLOAD_TOO_LARGE,
            "git request body is too large".into(),
        ));
    }
    Ok(decoded.into())
}

fn git_protocol(headers: &HeaderMap) -> Option<&str> {
    headers.get("git-protocol").and_then(|v| v.to_str().ok())
}

fn response(
    reader_type: &'static str,
    reader: impl tokio::io::AsyncRead + Send + 'static,
) -> Result<Response, WebError> {
    Response::builder()
        .header(header::CONTENT_TYPE, reader_type)
        .header(header::EXPIRES, "Fri, 01 Jan 1980 00:00:00 GMT")
        .header(header::PRAGMA, "no-cache")
        .header(
            header::CACHE_CONTROL,
            "no-cache, max-age=0, must-revalidate",
        )
        .body(Body::from_stream(ReaderStream::new(reader)))
        .map_err(|e| bad_gateway(format!("building response: {e}")))
}

fn bad_gateway(message: String) -> WebError {
    eprintln!("sorceryd: {message}");
    (StatusCode::BAD_GATEWAY, "git backend error".into())
}

#[cfg(test)]
mod tests {
    use std::fs;
    use std::path::PathBuf;
    use std::sync::Arc;

    use anyhow::Result;
    use axum::Router;
    use axum::body::Bytes;
    use axum::extract::State;
    use axum::http::HeaderMap;
    use axum::response::Response;
    use axum::routing::{get, post};
    use tokio::sync::Semaphore;

    use crate::testutil::{TempDir, commit, git, init_sha256_repo};

    use super::{WebError, advertise, upload};

    #[derive(Clone)]
    struct GitState {
        repo: PathBuf,
        permits: Arc<Semaphore>,
    }

    async fn info_refs(
        State(state): State<GitState>,
        headers: HeaderMap,
    ) -> Result<Response, WebError> {
        advertise(
            &state.repo,
            &headers,
            state.permits.try_acquire_owned().unwrap(),
        )
    }

    async fn upload_pack(
        State(state): State<GitState>,
        headers: HeaderMap,
        body: Bytes,
    ) -> Result<Response, WebError> {
        upload(
            &state.repo,
            &headers,
            body,
            state.permits.try_acquire_owned().unwrap(),
        )
    }

    #[tokio::test]
    async fn clones_sha256_over_smart_http() -> Result<()> {
        let root = TempDir::new("sha256-clone");
        let repo = root.join("repo");
        init_sha256_repo(&repo)?;
        fs::write(repo.join("file"), "contents")?;
        let head = commit(&repo, "initial")?;

        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await?;
        let address = listener.local_addr()?;
        let app = Router::new()
            .route("/repo/info/refs", get(info_refs))
            .route("/repo/git-upload-pack", post(upload_pack))
            .with_state(GitState {
                repo,
                permits: Arc::new(Semaphore::new(8)),
            });
        let server = tokio::spawn(async move { axum::serve(listener, app).await });

        let root = root.to_path_buf();
        tokio::task::spawn_blocking(move || {
            git(
                &root,
                &[
                    "-c",
                    "protocol.version=2",
                    "clone",
                    "-q",
                    &format!("http://{address}/repo"),
                    "clone",
                ],
            )?;
            assert_eq!(git(&root.join("clone"), &["rev-parse", "HEAD"])?, head);
            assert_eq!(
                git(&root.join("clone"), &["rev-parse", "--show-object-format"])?,
                "sha256"
            );
            Ok::<(), anyhow::Error>(())
        })
        .await??;
        server.abort();
        Ok(())
    }
}