//! `$FLOW_HOME` (default `~/.local/share/flow`): the pinned parts of each
//! isolation tier, rootfs images by hash, scratch space and the runner's
//! configuration, all described by `flow.toml`.
//!
//! ```toml
//! [microvm]
//! firecracker = { path = "bin/firecracker", blake3 = "…" }
//! kernel = { path = "kernel/vmlinux", blake3 = "…" }
//! init = { path = "bin/flow-init", blake3 = "…" }
//!
//! [runner]                       # only for `flow runner`
//! hub = "https://hub.example:7343"
//! token = "…"                    # a runner token from the hub's [flow] runner_tokens
//! ca = "/path/to/hub-ca.pem"     # when the hub's certificate is self-signed
//! name = "earth-1"
//! ```

use anyhow::{bail, Context, Result};
use serde::Deserialize;
use std::collections::HashMap;
use std::path::{Path, PathBuf};
use std::sync::Mutex;

#[derive(Clone, Debug, Deserialize)]
pub struct Pinned {
    pub path: PathBuf,
    pub blake3: String,
}

#[derive(Clone, Debug, Deserialize)]
pub struct MicrovmConfig {
    pub firecracker: Pinned,
    pub kernel: Pinned,
    pub init: Pinned,
}

#[derive(Clone, Debug, Deserialize)]
pub struct RunnerConfig {
    pub hub: String,
    pub token: String,
    pub ca: Option<PathBuf>,
    pub name: Option<String>,
}

#[derive(Clone, Debug, Default, Deserialize)]
pub struct Config {
    pub microvm: Option<MicrovmConfig>,
    pub runner: Option<RunnerConfig>,
}

pub struct Home {
    pub root: PathBuf,
    pub config: Config,
    /// Files already verified, by path, with the (len, mtime) they had then.
    verified: Mutex<HashMap<PathBuf, (u64, std::time::SystemTime)>>,
}

impl Home {
    pub fn locate(explicit: Option<&Path>) -> Result<Home> {
        let root = match explicit {
            Some(p) => p.to_path_buf(),
            None => match std::env::var_os("FLOW_HOME") {
                Some(p) => PathBuf::from(p),
                None => PathBuf::from(std::env::var_os("HOME").context("HOME is not set")?).join(".local/share/flow"),
            },
        };
        let file = root.join("flow.toml");
        let config = if file.is_file() {
            let text = std::fs::read_to_string(&file).with_context(|| format!("read {}", file.display()))?;
            toml::from_str(&text).with_context(|| format!("parse {}", file.display()))?
        } else {
            Config::default()
        };
        for d in ["cas", "work", "repos"] {
            std::fs::create_dir_all(root.join(d)).with_context(|| format!("create {}", root.join(d).display()))?;
        }
        Ok(Home { root, config, verified: Mutex::new(HashMap::new()) })
    }

    pub fn resolve(&self, p: &Path) -> PathBuf {
        if p.is_absolute() {
            p.to_path_buf()
        } else {
            self.root.join(p)
        }
    }

    /// Where a rootfs image with this hash lives.
    pub fn rootfs(&self, hash: &str) -> PathBuf {
        self.root.join("cas").join(format!("{hash}.ext4"))
    }

    /// A pinned file, checked against its BLAKE3 before use (again only
    /// when its size or modification time changed since).
    pub fn verified(&self, path: &Path, blake3_hex: &str) -> Result<PathBuf> {
        let meta = std::fs::metadata(path).with_context(|| format!("{} is missing", path.display()))?;
        let stamp = (meta.len(), meta.modified()?);
        if self.verified.lock().unwrap().get(path) == Some(&stamp) {
            return Ok(path.to_path_buf());
        }
        let mut got = hash_file(path)?;
        if got != blake3_hex {
            // The read may have come from bad cached pages rather than the
            // disk (oxygenflow#22: a RAM fault on Earth failed every job for
            // hours while the file was intact): drop the file's pages and
            // read it again from the disk before giving up.
            drop_cached(path);
            let again = hash_file(path)?;
            if again == blake3_hex {
                eprintln!(
                    "warning: {} hashed {got} from the page cache but {again} from the disk: the cached copy was bad (check this machine's memory)",
                    path.display()
                );
            }
            got = again;
        }
        if got != blake3_hex {
            bail!("{} has BLAKE3 {got}, but {blake3_hex} is pinned", path.display());
        }
        self.verified.lock().unwrap().insert(path.to_path_buf(), stamp);
        Ok(path.to_path_buf())
    }
}

/// Ask the kernel to forget a file's cached pages, so the next read comes
/// from the disk. Best effort: dirty pages and other platforms stay.
fn drop_cached(path: &Path) {
    #[cfg(target_os = "linux")]
    if let Ok(f) = std::fs::File::open(path) {
        use std::os::fd::AsRawFd;
        // SAFETY: a valid descriptor; the advice only affects caching.
        unsafe {
            libc::posix_fadvise(f.as_raw_fd(), 0, 0, libc::POSIX_FADV_DONTNEED);
        }
    }
    #[cfg(not(target_os = "linux"))]
    let _ = path;
}

pub fn hash_file(path: &Path) -> Result<String> {
    let mut h = blake3::Hasher::new();
    h.update_reader(std::fs::File::open(path).with_context(|| format!("open {}", path.display()))?)?;
    Ok(h.finalize().to_hex().to_string())
}