//! `git clone` over HTTP, using git's "smart" protocol.
//!
//! A clone is two HTTP requests:
//!
//! 1. `GET /{repo}/info/refs?service=git-upload-pack`
//!    The server lists its branches/tags ("ref advertisement").
//! 2. `POST /{repo}/git-upload-pack`
//!    The client says which commits it wants; the server replies with a
//!    packfile containing them.
//!
//! Rather than re-implementing the protocol we run the real
//! `git upload-pack --stateless-rpc` (exactly what git's own
//! `git-http-backend` CGI does) and pipe the HTTP bodies through it.
//! upload-pack is the *read* side of git, so this can never modify a repo.
//! Pushing (`git-receive-pack`) is refused.

use axum::body::{Body, Bytes};
use axum::extract::{Path, Query, State};
use axum::http::{header, HeaderMap, HeaderValue, StatusCode};
use axum::response::{IntoResponse, Response};
use serde::Deserialize;
use std::io::Read;
use std::process::Stdio;
use tokio::io::AsyncWriteExt;
use tokio::process::Command;
use tokio_util::io::ReaderStream;

use crate::web::Shared;

/// Largest request body (the client's "want/have" list) we accept, before and
/// after gzip decompression.
pub const MAX_REQUEST_BYTES: usize = 32 * 1024 * 1024;

#[derive(Deserialize)]
pub struct ServiceQuery {
    service: Option<String>,
}

fn plain(status: StatusCode, msg: &str) -> Response {
    (status, [(header::CONTENT_TYPE, "text/plain; charset=utf-8")], format!("{msg}\n")).into_response()
}

/// Encode one line in git's "pkt-line" format: 4 hex digits of length
/// (including the 4 digits themselves) followed by the data.
pub fn pkt_line(data: &str) -> String {
    format!("{:04x}{data}", data.len() + 4)
}

/// The client tells us which protocol version it speaks via the `Git-Protocol`
/// header (e.g. "version=2"); upload-pack reads it from the GIT_PROTOCOL env var.
fn git_protocol(headers: &HeaderMap) -> Option<String> {
    let v = headers.get("git-protocol")?.to_str().ok()?;
    let ok = v.len() < 256 && v.chars().all(|c| c.is_ascii_alphanumeric() || "=:._-".contains(c));
    ok.then(|| v.to_string())
}

/// Step 1: `GET /{repo}/info/refs?service=git-upload-pack`
pub async fn info_refs(
    State(st): State<Shared>,
    Path(repo): Path<String>,
    Query(q): Query<ServiceQuery>,
    headers: HeaderMap,
) -> Response {
    let Ok(rc) = st.repo(&repo) else {
        return plain(StatusCode::NOT_FOUND, "repository not found");
    };
    match q.service.as_deref() {
        Some("git-upload-pack") => {}
        Some("git-receive-pack") => return plain(StatusCode::FORBIDDEN, "push is not supported"),
        _ => {
            return plain(
                StatusCode::FORBIDDEN,
                "only the smart HTTP protocol is supported (use git >= 1.6.6)",
            )
        }
    }

    let protocol = git_protocol(&headers);
    let mut cmd = Command::new("git");
    cmd.args(["upload-pack", "--stateless-rpc", "--advertise-refs"])
        .arg(&rc.path)
        .stdin(Stdio::null())
        .kill_on_drop(true);
    if let Some(p) = &protocol {
        cmd.env("GIT_PROTOCOL", p);
    }
    let out = match cmd.output().await {
        Ok(o) if o.status.success() => o,
        Ok(o) => {
            eprintln!("upload-pack --advertise-refs failed: {}", String::from_utf8_lossy(&o.stderr));
            return plain(StatusCode::INTERNAL_SERVER_ERROR, "git upload-pack failed");
        }
        Err(e) => {
            eprintln!("could not run git: {e}");
            return plain(StatusCode::INTERNAL_SERVER_ERROR, "could not run git");
        }
    };

    // Protocol v0/v1 responses start with a "# service=..." banner and a flush
    // packet; protocol v2 responses don't (same as git-http-backend).
    let mut body = Vec::new();
    let is_v2 = protocol.as_deref().is_some_and(|p| p.contains("version=2"));
    if !is_v2 {
        body.extend_from_slice(pkt_line("# service=git-upload-pack\n").as_bytes());
        body.extend_from_slice(b"0000");
    }
    body.extend_from_slice(&out.stdout);

    let mut res = (StatusCode::OK, body).into_response();
    let h = res.headers_mut();
    h.insert(header::CONTENT_TYPE, HeaderValue::from_static("application/x-git-upload-pack-advertisement"));
    h.insert(header::CACHE_CONTROL, HeaderValue::from_static("no-cache"));
    res
}

/// Step 2: `POST /{repo}/git-upload-pack` – stream a packfile back.
pub async fn upload_pack(
    State(st): State<Shared>,
    Path(repo): Path<String>,
    headers: HeaderMap,
    body: Bytes,
) -> Response {
    let Ok(rc) = st.repo(&repo) else {
        return plain(StatusCode::NOT_FOUND, "repository not found");
    };
    let ctype = headers.get(header::CONTENT_TYPE).and_then(|v| v.to_str().ok());
    if ctype != Some("application/x-git-upload-pack-request") {
        return plain(StatusCode::UNSUPPORTED_MEDIA_TYPE, "expected application/x-git-upload-pack-request");
    }

    // git gzips large requests.
    let gzipped = headers
        .get(header::CONTENT_ENCODING)
        .and_then(|v| v.to_str().ok())
        .is_some_and(|v| v.eq_ignore_ascii_case("gzip") || v.eq_ignore_ascii_case("x-gzip"));
    let input: Vec<u8> = if gzipped {
        let mut out = Vec::new();
        let decoder = flate2::read::GzDecoder::new(&body[..]);
        if decoder.take(MAX_REQUEST_BYTES as u64 + 1).read_to_end(&mut out).is_err()
            || out.len() > MAX_REQUEST_BYTES
        {
            return plain(StatusCode::BAD_REQUEST, "bad or oversized gzip body");
        }
        out
    } else {
        body.to_vec()
    };

    let mut cmd = Command::new("git");
    cmd.args(["upload-pack", "--stateless-rpc"])
        .arg(&rc.path)
        .stdin(Stdio::piped())
        .stdout(Stdio::piped())
        .stderr(Stdio::inherit())
        // If the client disconnects, the response body (and with it the
        // child's stdout) is dropped; kill_on_drop makes sure git exits too.
        .kill_on_drop(true);
    if let Some(p) = git_protocol(&headers) {
        cmd.env("GIT_PROTOCOL", p);
    }
    let mut child = match cmd.spawn() {
        Ok(c) => c,
        Err(e) => {
            eprintln!("could not run git: {e}");
            return plain(StatusCode::INTERNAL_SERVER_ERROR, "could not run git");
        }
    };
    let mut stdin = child.stdin.take().expect("stdin was piped");
    let stdout = child.stdout.take().expect("stdout was piped");

    // Feed the request to git in the background, then wait for it to exit,
    // while we stream its output to the client.
    tokio::spawn(async move {
        let _ = stdin.write_all(&input).await;
        drop(stdin); // closing stdin tells git the request is complete
        let _ = child.wait().await;
    });

    let mut res = Response::new(Body::from_stream(ReaderStream::new(stdout)));
    let h = res.headers_mut();
    h.insert(header::CONTENT_TYPE, HeaderValue::from_static("application/x-git-upload-pack-result"));
    h.insert(header::CACHE_CONTROL, HeaderValue::from_static("no-cache"));
    res
}

/// `POST /{repo}/git-receive-pack` – pushing is not supported in v0.1.
pub async fn receive_pack_refused() -> Response {
    plain(StatusCode::FORBIDDEN, "push is not supported")
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn pkt_line_format() {
        assert_eq!(pkt_line("# service=git-upload-pack\n"), "001e# service=git-upload-pack\n");
        assert_eq!(pkt_line(""), "0004");
    }

    #[test]
    fn protocol_header_is_filtered() {
        let mut h = HeaderMap::new();
        h.insert("git-protocol", HeaderValue::from_static("version=2"));
        assert_eq!(git_protocol(&h).as_deref(), Some("version=2"));
        h.insert("git-protocol", HeaderValue::from_static("version=2; rm -rf /"));
        assert_eq!(git_protocol(&h), None);
    }
}