//! Git shellout helpers for the per-agent proposed/applied repos: run
//! `git` with the hive-c0re identity, resolve/plant refs and tags, and
//! fetch proposal commits into the applied repo.

use std::path::Path;
use std::process::{Output, Stdio};
use std::time::Duration;

use anyhow::{Context, Result, bail};
use tokio::io::{AsyncRead, AsyncReadExt};
use tokio::process::Command;

/// Deadline for a child that talks to a remote (forge fetch/push): a healthy
/// one takes seconds, so ten minutes only ever fires on a hang.
pub const FETCH_DEADLINE: Duration = Duration::from_mins(10);

/// Deadline for a git child that touches only the local repo.
pub const GIT_LOCAL_DEADLINE: Duration = Duration::from_mins(2);

/// Run `cmd` to completion and capture its output, failing once `deadline`
/// passes. `what` names the command in the error. The child leads its own
/// process group and the whole group is `SIGKILL`ed on timeout, so a
/// grandchild (a `git` or credential helper it spawned) dies with it instead
/// of outliving the lock.
pub async fn run_with_deadline(
    cmd: &mut Command,
    deadline: Duration,
    what: &str,
) -> Result<Output> {
    // `kill_on_drop` covers the caller dropping this future; `process_group(0)`
    // makes the child's pid the group id the timeout path kills.
    let mut child = cmd
        .stdin(Stdio::null())
        .stdout(Stdio::piped())
        .stderr(Stdio::piped())
        .kill_on_drop(true)
        .process_group(0)
        .spawn()
        .with_context(|| what.to_owned())?;
    // Read before waiting: a reaped child has no id.
    let pgid = child.id().and_then(|pid| i32::try_from(pid).ok());
    let (stdout, stderr) = (child.stdout.take(), child.stderr.take());
    let io = async { tokio::join!(child.wait(), drain(stdout), drain(stderr)) };
    if let Ok((status, stdout, stderr)) = tokio::time::timeout(deadline, io).await {
        return Ok(Output {
            status: status.with_context(|| what.to_owned())?,
            stdout: stdout.with_context(|| what.to_owned())?,
            stderr: stderr.with_context(|| what.to_owned())?,
        });
    }
    if let Some(pgid) = pgid {
        // SAFETY: plain `kill(2)`; `-pgid` targets the child's own process
        // group, which `process_group(0)` made it lead.
        unsafe {
            libc::kill(-pgid, libc::SIGKILL);
        }
    }
    child.wait().await.with_context(|| what.to_owned())?;
    bail!("{what} timed out after {}s", deadline.as_secs_f64())
}

/// Everything `pipe` yields until EOF.
async fn drain<R: AsyncRead + Unpin>(pipe: Option<R>) -> std::io::Result<Vec<u8>> {
    let mut buf = Vec::new();
    if let Some(mut p) = pipe {
        p.read_to_end(&mut buf).await?;
    }
    Ok(buf)
}

const GIT_NAME: &str = "c0re";
const GIT_EMAIL: &str = "c0re@hyperhive.local";

/// Return the SHA of the root (oldest, no-parent) commit in a repo.
/// Used to seed the applied repo at the template baseline rather than at
/// `main`, so `deployed/0` records the template, not the manager's first commit.
pub(super) async fn git_root_commit(dir: &Path) -> Result<String> {
    let out = git_command()
        .current_dir(dir)
        .args(["rev-list", "--max-parents=0", "HEAD"])
        .output()
        .await
        .with_context(|| format!("git rev-list --max-parents=0 HEAD in {}", dir.display()))?;
    if !out.status.success() {
        anyhow::bail!(
            "git rev-list --max-parents=0 failed: {}",
            String::from_utf8_lossy(&out.stderr).trim()
        );
    }
    Ok(String::from_utf8_lossy(&out.stdout).trim().to_owned())
}

pub(super) async fn git_commit(dir: &Path, message: &str) -> Result<()> {
    git(
        dir,
        &[
            "-c",
            &format!("user.name={GIT_NAME}"),
            "-c",
            &format!("user.email={GIT_EMAIL}"),
            "commit",
            "-m",
            message,
        ],
    )
    .await
}

/// Spawn `git` honoring the `HYPERHIVE_GIT` env var (absolute path baked in
/// by the NixOS module), falling back to bare `git` (PATH lookup) otherwise.
///
/// `kill_on_drop(true)`: if the caller's future is dropped before the child
/// exits — e.g. a `tokio::time::timeout` around startup migration fires — the
/// git child is killed instead of orphaned (left retrying an unreachable
/// forge). No-op on normal completion, where the child has already exited.
#[must_use]
pub fn git_command() -> Command {
    let exe = std::env::var("HYPERHIVE_GIT").unwrap_or_else(|_| "git".into());
    let mut cmd = Command::new(exe);
    cmd.kill_on_drop(true);
    cmd
}

/// [`git_command`] carrying HTTP credentials **in its environment**.
///
/// `auth_header` is an `Authorization:` line (see
/// `forge::basic_auth_header`) handed to git as `http.extraHeader`
/// through `GIT_CONFIG_*`, which git reads exactly like a config file. The
/// alternative — userinfo in the remote URL — puts the secret in `argv`, and
/// `/proc/<pid>/cmdline` is world-readable while `/proc/<pid>/environ` is
/// owner-only. Same credential, materially smaller audience.
///
/// `GIT_CONFIG_COUNT` needs git >= 2.31; `HYPERHIVE_GIT` points at a pinned
/// nixpkgs git well past that.
#[must_use]
pub fn git_command_authed(auth_header: &str) -> Command {
    let mut cmd = git_command();
    cmd.env("GIT_CONFIG_COUNT", "1")
        .env("GIT_CONFIG_KEY_0", "http.extraHeader")
        .env("GIT_CONFIG_VALUE_0", auth_header);
    cmd
}

/// Run `cmd` as `git <args>` in `dir` within `deadline`, erroring on a
/// non-zero exit.
async fn run(mut cmd: Command, dir: &Path, args: &[&str], deadline: Duration) -> Result<()> {
    let what = format!("git {} in {}", args.join(" "), dir.display());
    let out = run_with_deadline(cmd.current_dir(dir).args(args), deadline, &what).await?;
    if !out.status.success() {
        bail!(
            "git {} failed ({}): {}",
            args.join(" "),
            out.status,
            String::from_utf8_lossy(&out.stderr).trim()
        );
    }
    Ok(())
}

pub async fn git(dir: &Path, args: &[&str]) -> Result<()> {
    run(git_command(), dir, args, GIT_LOCAL_DEADLINE).await
}

/// [`git`] against an authenticated remote — for the paths that talk to the
/// forge. The credential rides the environment, so `args` (and therefore the
/// error above) stay free of it.
pub async fn git_authed(dir: &Path, args: &[&str], auth_header: &str) -> Result<()> {
    run(git_command_authed(auth_header), dir, args, FETCH_DEADLINE).await
}

/// Resolve `refname` (a tag, branch, or sha) in `dir` to its full sha.
pub async fn git_rev_parse(dir: &Path, refname: &str) -> Result<String> {
    let out = run_with_deadline(
        git_command().current_dir(dir).args(["rev-parse", refname]),
        GIT_LOCAL_DEADLINE,
        &format!("git rev-parse {refname} in {}", dir.display()),
    )
    .await?;
    if !out.status.success() {
        bail!(
            "git rev-parse {refname} failed ({}): {}",
            out.status,
            String::from_utf8_lossy(&out.stderr).trim()
        );
    }
    Ok(String::from_utf8_lossy(&out.stdout).trim().to_owned())
}

/// Plant a lightweight tag at `target`. Errors if the tag already
/// exists — we want loud failures on id reuse, not silent
/// overwrites.
pub async fn git_tag(dir: &Path, name: &str, target: &str) -> Result<()> {
    git(dir, &["tag", name, target]).await
}

/// Replace working tree + index with the tree at `target` without
/// moving HEAD. `applied/main` stays pointing at the last known-good
/// `deployed/*` while we let `nixos-container update` evaluate the
/// candidate. On build failure callers reset back to HEAD; on
/// success they fast-forward main to `target`.
pub async fn git_read_tree_reset(dir: &Path, target: &str) -> Result<()> {
    git(dir, &["read-tree", "--reset", "-u", target]).await
}

/// Hard-set a ref to `target` with `update-ref`, so it works regardless of
/// where HEAD currently sits. Test fixtures only: production moves refs with
/// [`git_update_ref_cas`].
#[cfg(test)]
pub async fn git_update_ref(dir: &Path, refname: &str, target: &str) -> Result<()> {
    git(dir, &["update-ref", refname, target]).await
}

/// Compare-and-swap a ref: move `refname` to `new` only if it currently points
/// at `old`. `git update-ref <ref> <new> <old>` refuses — leaving the ref
/// untouched — when the current value differs. That refusal is the whole
/// difference between "advance this branch" and "overwrite whatever is there".
///
/// Prefer this over a plain `update-ref` whenever the caller already knows the
/// value it believes the ref holds: a plain `update-ref` that raced another
/// writer discards the other writer's commits without a word.
///
/// # Errors
///
/// Returns an error if the ref does not currently point at `old` (the CAS lost)
/// or if the `git` invocation itself fails.
pub async fn git_update_ref_cas(dir: &Path, refname: &str, new: &str, old: &str) -> Result<()> {
    git(dir, &["update-ref", refname, new, old]).await
}

/// True when `ancestor` is reachable from `descendant` — i.e. moving a branch
/// from `ancestor` to `descendant` is a genuine fast-forward that discards
/// nothing.
///
/// Not the same question as "did this branch land upstream": a squash-merge
/// rewrites the commit, so `--is-ancestor` correctly answers `false` for a
/// branch whose *contents* were merged. This helper compares two commits with
/// real shared ancestry inside one repo, which is precisely what it decides.
///
/// # Errors
///
/// Returns an error if `git` fails to run or either revision can't be resolved.
/// A clean "no, not an ancestor" is `Ok(false)`, not an error.
pub async fn git_is_ancestor(dir: &Path, ancestor: &str, descendant: &str) -> Result<bool> {
    let out = git_command()
        .current_dir(dir)
        .args(["merge-base", "--is-ancestor", ancestor, descendant])
        .output()
        .await
        .with_context(|| {
            format!(
                "git merge-base --is-ancestor {ancestor} {descendant} in {}",
                dir.display()
            )
        })?;
    match out.status.code() {
        Some(0) => Ok(true),
        Some(1) => Ok(false),
        _ => bail!(
            "git merge-base --is-ancestor {ancestor} {descendant} failed ({}): {}",
            out.status,
            String::from_utf8_lossy(&out.stderr).trim()
        ),
    }
}