//! The files this node reads and writes for itself, and how much it did (`/admin/stats`,
//! `disk`): the state file (`swarm::state_file`), the document file
//! (`document::file`) and a backup to recover from (`swarm::bootstrap`). Not the
//! Mycelium daemon's files, configuration or logs.
//!
//! Writes replace a file atomically: a temporary file next to it, flushed to disk, then
//! renamed over it, so a crash leaves the old content or the new one. Appends add to the
//! end and are flushed to disk before they count as done; a crash may leave the last one
//! cut short, which the file's reader has to expect.

use crate::locks::MutexExt;
use std::io::Write;
use std::path::{Path, PathBuf};
use std::sync::{Mutex, MutexGuard};

/// What a file is, for the counts.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub(crate) enum FileKind {
    State,
    Document,
    Backup,
    /// The input of a genesis remake (`--genesis --remake`).
    Remake,
}

impl FileKind {
    pub(crate) const ALL: [FileKind; 4] = [
        FileKind::State,
        FileKind::Document,
        FileKind::Backup,
        FileKind::Remake,
    ];

    pub(crate) fn as_str(self) -> &'static str {
        match self {
            FileKind::State => "state",
            FileKind::Document => "document",
            FileKind::Backup => "backup",
            FileKind::Remake => "remake",
        }
    }

    fn index(self) -> usize {
        self as usize
    }
}

/// Counts for one kind of file, since the node started.
#[derive(Clone, Copy, Debug, Default)]
pub(crate) struct Counts {
    pub(crate) reads: u64,
    pub(crate) read_bytes: u64,
    /// Whole files written.
    pub(crate) writes: u64,
    pub(crate) written_bytes: u64,
    pub(crate) appends: u64,
    pub(crate) appended_bytes: u64,
    /// Reads, writes or appends that failed (a missing file read is no failure).
    pub(crate) errors: u64,
}

/// Reads and writes this node's files, counting per kind.
#[derive(Default)]
pub(crate) struct Files {
    // A std mutex: every access is short and never spans an await.
    counts: Mutex<[Counts; FileKind::ALL.len()]>,
}

impl Files {
    fn lock(&self) -> MutexGuard<'_, [Counts; FileKind::ALL.len()]> {
        self.counts.lock_or_recover()
    }

    fn count(&self, kind: FileKind, f: impl FnOnce(&mut Counts)) {
        f(&mut self.lock()[kind.index()]);
    }

    /// The file's content; None if it doesn't exist.
    pub(crate) fn read(&self, kind: FileKind, path: &Path) -> std::io::Result<Option<Vec<u8>>> {
        match std::fs::read(path) {
            Ok(bytes) => {
                self.count(kind, |counts| {
                    counts.reads += 1;
                    counts.read_bytes += bytes.len() as u64;
                });
                Ok(Some(bytes))
            }
            Err(error) if error.kind() == std::io::ErrorKind::NotFound => Ok(None),
            Err(error) => {
                self.count(kind, |counts| counts.errors += 1);
                Err(error)
            }
        }
    }

    /// Replaces the file's content atomically (see the module docs).
    pub(crate) fn write_atomic(
        &self,
        kind: FileKind,
        path: &Path,
        bytes: &[u8],
    ) -> std::io::Result<()> {
        let result = write_atomic(path, bytes);
        self.count(kind, |counts| match &result {
            Ok(()) => {
                counts.writes += 1;
                counts.written_bytes += bytes.len() as u64;
            }
            Err(_) => counts.errors += 1,
        });
        result
    }

    /// Adds `bytes` to the end of the file (which must exist), flushed to disk.
    pub(crate) fn append(&self, kind: FileKind, path: &Path, bytes: &[u8]) -> std::io::Result<()> {
        let result = (|| {
            let mut file = std::fs::OpenOptions::new().append(true).open(path)?;
            file.write_all(bytes)?;
            file.sync_data()
        })();
        self.count(kind, |counts| match &result {
            Ok(()) => {
                counts.appends += 1;
                counts.appended_bytes += bytes.len() as u64;
            }
            Err(_) => counts.errors += 1,
        });
        result
    }

    /// Moves an unusable file aside (`<path><suffix>`), returning where to.
    pub(crate) fn move_aside(&self, path: &Path, suffix: &str) -> std::io::Result<PathBuf> {
        let aside = with_suffix(path, suffix);
        std::fs::rename(path, &aside)?;
        Ok(aside)
    }

    /// The counts per kind.
    pub(crate) fn counts(&self) -> Vec<(FileKind, Counts)> {
        let counts = *self.lock();
        FileKind::ALL
            .into_iter()
            .map(|kind| (kind, counts[kind.index()]))
            .collect()
    }
}

fn write_atomic(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
    let temporary = with_suffix(path, ".tmp");
    {
        let mut file = std::fs::File::create(&temporary)?;
        file.write_all(bytes)?;
        file.sync_all()?;
    }
    std::fs::rename(&temporary, path)?;
    sync_parent(path);
    Ok(())
}

/// Makes a rename in `path`'s directory durable (best effort: not every platform syncs
/// directories).
fn sync_parent(path: &Path) {
    if let Some(dir) = path.parent().filter(|dir| !dir.as_os_str().is_empty())
        && let Ok(dir) = std::fs::File::open(dir)
    {
        let _ = dir.sync_all();
    }
}

/// A key file's bytes: read from `path`; or with `create`, if there is no file there,
/// `len` random bytes in a new one only its owner may read (then also `true`). Without
/// `create`, a missing file is an error saying to give `create_option` (`what`: which
/// key; `hint`: what a new one means). Never created where a file is: a mistyped path
/// stays an error unless asked.
pub(crate) fn key_file(
    path: &Path,
    what: &str,
    create: bool,
    create_option: &str,
    hint: &str,
    len: usize,
) -> anyhow::Result<(Vec<u8>, bool)> {
    use anyhow::Context;
    use std::io::Write;
    match std::fs::read(path) {
        Ok(bytes) => {
            warn_if_readable_by_others(path, what);
            Ok((bytes, false))
        }
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {
            if !create {
                anyhow::bail!(
                    "The {what} file {} doesn't exist; add --{create_option} to create a new {what} there ({hint})",
                    path.display()
                );
            }
            let mut bytes = vec![0u8; len];
            getrandom::fill(&mut bytes)
                .map_err(|error| anyhow::anyhow!("No randomness: {error}"))?;
            let mut options = std::fs::OpenOptions::new();
            options.write(true).create_new(true);
            #[cfg(unix)]
            {
                use std::os::unix::fs::OpenOptionsExt;
                options.mode(0o600);
            }
            let written = (|| {
                let mut file = options.open(path)?;
                file.write_all(&bytes)?;
                file.sync_all()
            })();
            written.with_context(|| format!("Creating the {what} file {}", path.display()))?;
            Ok((bytes, true))
        }
        Err(error) => {
            Err(error).with_context(|| format!("Reading the {what} from {}", path.display()))
        }
    }
}

/// A key is a secret: a file others may read is worth a warning (not an error: a
/// group may be deliberate, e.g. shared with a deployment tool). `what`: which key.
pub(crate) fn warn_if_readable_by_others(path: &Path, what: &str) {
    #[cfg(unix)]
    {
        use std::os::unix::fs::PermissionsExt;
        if let Ok(metadata) = std::fs::metadata(path) {
            let mode = metadata.permissions().mode();
            if mode & 0o077 != 0 {
                tracing::warn!(
                    path = %path.display(),
                    mode = format!("{:o}", mode & 0o777),
                    "The {what} file is readable by others than its owner; chmod 600 it"
                );
            }
        }
    }
    #[cfg(not(unix))]
    let _ = (path, what);
}

fn with_suffix(path: &Path, suffix: &str) -> PathBuf {
    let mut name = path.as_os_str().to_owned();
    name.push(suffix);
    PathBuf::from(name)
}

/// An empty directory for the test `name`: `continuum-<name>-<process ID>` in the
/// temporary directory, emptied first.
#[cfg(test)]
pub(crate) fn scratch_dir(name: &str) -> PathBuf {
    let dir = std::env::temp_dir().join(format!("continuum-{name}-{}", std::process::id()));
    let _ = std::fs::remove_dir_all(&dir);
    std::fs::create_dir_all(&dir).unwrap();
    dir
}

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

    #[test]
    fn writes_reads_and_counts() {
        let dir = scratch_dir("files");
        let path = dir.join("state.json");
        let files = Files::default();

        assert_eq!(files.read(FileKind::State, &path).unwrap(), None);
        files.write_atomic(FileKind::State, &path, b"one").unwrap();
        files
            .write_atomic(FileKind::State, &path, b"three")
            .unwrap();
        assert_eq!(
            files.read(FileKind::State, &path).unwrap().as_deref(),
            Some(&b"three"[..])
        );
        assert!(!with_suffix(&path, ".tmp").exists());

        let (kind, state) = files.counts()[0];
        assert_eq!(kind, FileKind::State);
        assert_eq!((state.writes, state.written_bytes), (2, 8));
        assert_eq!((state.reads, state.read_bytes, state.errors), (1, 5, 0));
        files.append(FileKind::State, &path, b"!").unwrap();
        assert_eq!(
            files.read(FileKind::State, &path).unwrap().as_deref(),
            Some(&b"three!"[..])
        );
        assert_eq!(files.counts()[0].1.appended_bytes, 1);
        assert_eq!(files.counts()[1].1.reads, 0);

        std::fs::remove_dir_all(&dir).unwrap();
    }
}