mod file_attr;
mod inode_tracker;
mod inodes;

#[cfg(feature = "fuse")]
pub mod fuse;

#[cfg(feature = "virtiofs")]
pub mod virtiofs;

#[cfg(test)]
mod tests;

use crate::pathinfoservice::PathInfoService;

use fuse_backend_rs::abi::fuse_abi::stat64;
use fuse_backend_rs::api::filesystem::{Context, FileSystem, FsOptions, ROOT_ID};
use futures::StreamExt;
use nix_compat::store_path::StorePath;
use parking_lot::RwLock;
use std::{
    collections::HashMap,
    io,
    sync::atomic::AtomicU64,
    sync::{atomic::Ordering, Arc},
    time::Duration,
};
use tokio::{
    io::{AsyncBufReadExt, AsyncSeekExt},
    sync::mpsc,
};
use tracing::{debug, info_span, instrument, warn};
use tvix_castore::proto as castorepb;
use tvix_castore::{
    blobservice::{BlobReader, BlobService},
    directoryservice::DirectoryService,
    proto::{node::Node, NamedNode},
    B3Digest,
};

use self::{
    file_attr::{gen_file_attr, ROOT_FILE_ATTR},
    inode_tracker::InodeTracker,
    inodes::{DirectoryInodeData, InodeData},
};

/// This implements a read-only FUSE filesystem for a tvix-store
/// with the passed [BlobService], [DirectoryService] and [PathInfoService].
///
/// We don't allow listing on the root mountpoint (inode 0).
/// In the future, this might be made configurable once a listing method is
/// added to [self.path_info_service], and then show all store paths in that
/// store.
///
/// Linux uses inodes in filesystems. When implementing FUSE, most calls are
/// *for* a given inode.
///
/// This means, we need to have a stable mapping of inode numbers to the
/// corresponding store nodes.
///
/// We internally delegate all inode allocation and state keeping to the
/// inode tracker, and store the currently "explored" store paths together with
/// root inode of the root.
///
/// There's some places where inodes are allocated / data inserted into
/// the inode tracker, if not allocated before already:
///  - Processing a `lookup` request, either in the mount root, or somewhere
///    deeper
///  - Processing a `readdir` request
///
///  Things pointing to the same contents get the same inodes, irrespective of
///  their own location.
///  This means:
///  - Symlinks with the same target will get the same inode.
///  - Regular/executable files with the same contents will get the same inode
///  - Directories with the same contents will get the same inode.
///
/// Due to the above being valid across the whole store, and considering the
/// merkle structure is a DAG, not a tree, this also means we can't do "bucketed
/// allocation", aka reserve Directory.size inodes for each PathInfo.
pub struct TvixStoreFs {
    blob_service: Arc<dyn BlobService>,
    directory_service: Arc<dyn DirectoryService>,
    path_info_service: Arc<dyn PathInfoService>,

    /// Whether to (try) listing elements in the root.
    list_root: bool,

    /// This maps a given StorePath to the inode we allocated for the root inode.
    store_paths: RwLock<HashMap<StorePath, u64>>,

    /// This keeps track of inodes and data alongside them.
    inode_tracker: RwLock<InodeTracker>,

    /// This holds all open file handles
    #[allow(clippy::type_complexity)]
    file_handles: RwLock<HashMap<u64, Arc<tokio::sync::Mutex<Box<dyn BlobReader>>>>>,

    next_file_handle: AtomicU64,

    tokio_handle: tokio::runtime::Handle,
}

impl TvixStoreFs {
    pub fn new(
        blob_service: Arc<dyn BlobService>,
        directory_service: Arc<dyn DirectoryService>,
        path_info_service: Arc<dyn PathInfoService>,
        list_root: bool,
    ) -> Self {
        Self {
            blob_service,
            directory_service,
            path_info_service,

            list_root,

            store_paths: RwLock::new(HashMap::default()),
            inode_tracker: RwLock::new(Default::default()),

            file_handles: RwLock::new(Default::default()),
            next_file_handle: AtomicU64::new(1),
            tokio_handle: tokio::runtime::Handle::current(),
        }
    }

    /// Retrieves the inode for a given StorePath, if present.
    /// This obtains a read lock on self.store_paths.
    fn get_inode_for_store_path(&self, store_path: &StorePath) -> Option<u64> {
        let store_paths = self.store_paths.read();
        store_paths.get(store_path).cloned()
    }

    /// For a given inode, look up the given directory behind it (from
    /// self.inode_tracker), and return its children.
    /// The inode_tracker MUST know about this inode already, and it MUST point
    /// to a [InodeData::Directory].
    /// It is ok if it's a [DirectoryInodeData::Sparse] - in that case, a lookup
    /// in self.directory_service is performed, and self.inode_tracker is updated with the
    /// [DirectoryInodeData::Populated].
    #[instrument(skip(self), err)]
    fn get_directory_children(&self, ino: u64) -> io::Result<(B3Digest, Vec<(u64, Node)>)> {
        let data = self.inode_tracker.read().get(ino).unwrap();
        match *data {
            // if it's populated already, return children.
            InodeData::Directory(DirectoryInodeData::Populated(
                ref parent_digest,
                ref children,
            )) => Ok((parent_digest.clone(), children.clone())),
            // if it's sparse, fetch data using directory_service, populate child nodes
            // and update it in [self.inode_tracker].
            InodeData::Directory(DirectoryInodeData::Sparse(ref parent_digest, _)) => {
                let directory = self
                    .tokio_handle
                    .block_on(self.tokio_handle.spawn({
                        let directory_service = self.directory_service.clone();
                        let parent_digest = parent_digest.to_owned();
                        async move { directory_service.get(&parent_digest).await }
                    }))
                    .unwrap()?
                    .ok_or_else(|| {
                        warn!(directory.digest=%parent_digest, "directory not found");
                        // If the Directory can't be found, this is a hole, bail out.
                        io::Error::from_raw_os_error(libc::EIO)
                    })?;

                // Turn the retrieved directory into a InodeData::Directory(DirectoryInodeData::Populated(..)),
                // allocating inodes for the children on the way.
                let children = {
                    let mut inode_tracker = self.inode_tracker.write();

                    let children: Vec<(u64, castorepb::node::Node)> = directory
                        .nodes()
                        .map(|child_node| {
                            let child_ino = inode_tracker.put((&child_node).into());
                            (child_ino, child_node)
                        })
                        .collect();

                    // replace.
                    inode_tracker.replace(
                        ino,
                        Arc::new(InodeData::Directory(DirectoryInodeData::Populated(
                            parent_digest.clone(),
                            children.clone(),
                        ))),
                    );

                    children
                };

                Ok((parent_digest.clone(), children))
            }
            // if the parent inode was not a directory, this doesn't make sense
            InodeData::Regular(..) | InodeData::Symlink(_) => {
                Err(io::Error::from_raw_os_error(libc::ENOTDIR))
            }
        }
    }

    /// This will turn a lookup request for a name in the root to a ino and
    /// [InodeData].
    /// It will peek in [self.store_paths], and then either look it up from
    /// [self.inode_tracker],
    /// or otherwise fetch from [self.path_info_service], and then insert into
    /// [self.inode_tracker].
    /// In the case the name can't be found, a libc::ENOENT is returned.
    fn name_in_root_to_ino_and_data(
        &self,
        name: &std::ffi::CStr,
    ) -> io::Result<(u64, Arc<InodeData>)> {
        // parse the name into a [StorePath].
        let store_path = StorePath::from_bytes(name.to_bytes()).map_err(|e| {
            debug!(e=?e, "unable to parse as store path");
            // This is not an error, but a "ENOENT", as someone can stat
            // a file inside the root that's no valid store path
            io::Error::from_raw_os_error(libc::ENOENT)
        })?;

        // Look up the inode for that store path.
        // If there's one, [self.inode_tracker] MUST also contain the data,
        // which we can then return.
        if let Some(inode) = self.get_inode_for_store_path(&store_path) {
            return Ok((
                inode,
                self.inode_tracker
                    .read()
                    .get(inode)
                    .expect("must exist")
                    .to_owned(),
            ));
        }

        // We don't have it yet, look it up in [self.path_info_service].
        match self
            .tokio_handle
            .block_on({
                let path_info_service = self.path_info_service.clone();
                let digest = *store_path.digest();
                async move { path_info_service.get(digest).await }
            })
            .unwrap()
        {
            // the pathinfo doesn't exist, so the file doesn't exist.
            None => Err(io::Error::from_raw_os_error(libc::ENOENT)),
            // The pathinfo does exist
            Some(path_info) => {
                // There must be a root node (ensured by the validation happening inside clients)
                let root_node = path_info.node.unwrap().node.unwrap();

                // The name must match what's passed in the lookup, otherwise this is also a ENOENT.
                if root_node.get_name() != store_path.to_string().as_bytes() {
                    debug!(root_node.name=?root_node.get_name(), store_path.name=%store_path.to_string(), "store path mismatch");
                    return Err(io::Error::from_raw_os_error(libc::ENOENT));
                }

                // Let's check if someone else beat us to updating the inode tracker and
                // store_paths map. This avoids locking inode_tracker for writing.
                if let Some(ino) = self.store_paths.read().get(&store_path) {
                    return Ok((
                        *ino,
                        self.inode_tracker.read().get(*ino).expect("must exist"),
                    ));
                }

                // Only in case it doesn't, lock [self.store_paths] and
                // [self.inode_tracker] for writing.
                let mut store_paths = self.store_paths.write();
                let mut inode_tracker = self.inode_tracker.write();

                // insert the (sparse) inode data and register in
                // self.store_paths.
                let inode_data: InodeData = (&root_node).into();
                let ino = inode_tracker.put(inode_data.clone());
                store_paths.insert(store_path, ino);

                Ok((ino, Arc::new(inode_data)))
            }
        }
    }
}

impl FileSystem for TvixStoreFs {
    type Handle = u64;
    type Inode = u64;

    fn init(&self, _capable: FsOptions) -> io::Result<FsOptions> {
        Ok(FsOptions::empty())
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
    fn getattr(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        _handle: Option<Self::Handle>,
    ) -> io::Result<(stat64, Duration)> {
        if inode == ROOT_ID {
            return Ok((ROOT_FILE_ATTR.into(), Duration::MAX));
        }

        match self.inode_tracker.read().get(inode) {
            None => Err(io::Error::from_raw_os_error(libc::ENOENT)),
            Some(node) => {
                debug!(node = ?node, "found node");
                Ok((gen_file_attr(&node, inode).into(), Duration::MAX))
            }
        }
    }

    #[tracing::instrument(skip_all, fields(rq.parent_inode = parent, rq.name = ?name))]
    fn lookup(
        &self,
        _ctx: &Context,
        parent: Self::Inode,
        name: &std::ffi::CStr,
    ) -> io::Result<fuse_backend_rs::api::filesystem::Entry> {
        debug!("lookup");

        // This goes from a parent inode to a node.
        // - If the parent is [ROOT_ID], we need to check
        //   [self.store_paths] (fetching from PathInfoService if needed)
        // - Otherwise, lookup the parent in [self.inode_tracker] (which must be
        //   a [InodeData::Directory]), and find the child with that name.
        if parent == ROOT_ID {
            let (ino, inode_data) = self.name_in_root_to_ino_and_data(name)?;

            debug!(inode_data=?&inode_data, ino=ino, "Some");
            return Ok(fuse_backend_rs::api::filesystem::Entry {
                inode: ino,
                attr: gen_file_attr(&inode_data, ino).into(),
                attr_timeout: Duration::MAX,
                entry_timeout: Duration::MAX,
                ..Default::default()
            });
        }
        // This is the "lookup for "a" inside inode 42.
        // We already know that inode 42 must be a directory.
        let (parent_digest, children) = self.get_directory_children(parent)?;

        let span = info_span!("lookup", directory.digest = %parent_digest);
        let _enter = span.enter();

        // Search for that name in the list of children and return the FileAttrs.

        // in the children, find the one with the desired name.
        if let Some((child_ino, _)) = children.iter().find(|e| e.1.get_name() == name.to_bytes()) {
            // lookup the child [InodeData] in [self.inode_tracker].
            // We know the inodes for children have already been allocated.
            let child_inode_data = self.inode_tracker.read().get(*child_ino).unwrap();

            // Reply with the file attributes for the child.
            // For child directories, we still have all data we need to reply.
            Ok(fuse_backend_rs::api::filesystem::Entry {
                inode: *child_ino,
                attr: gen_file_attr(&child_inode_data, *child_ino).into(),
                attr_timeout: Duration::MAX,
                entry_timeout: Duration::MAX,
                ..Default::default()
            })
        } else {
            // Child not found, return ENOENT.
            Err(io::Error::from_raw_os_error(libc::ENOENT))
        }
    }

    // TODO: readdirplus?

    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.offset = offset))]
    fn readdir(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        _handle: Self::Handle,
        _size: u32,
        offset: u64,
        add_entry: &mut dyn FnMut(fuse_backend_rs::api::filesystem::DirEntry) -> io::Result<usize>,
    ) -> io::Result<()> {
        debug!("readdir");

        if inode == ROOT_ID {
            if !self.list_root {
                return Err(io::Error::from_raw_os_error(libc::EPERM)); // same error code as ipfs/kubo
            } else {
                let path_info_service = self.path_info_service.clone();
                let (tx, mut rx) = mpsc::channel(16);

                // This task will run in the background immediately and will exit
                // after the stream ends or if we no longer want any more entries.
                self.tokio_handle.spawn(async move {
                    let mut stream = path_info_service.list().skip(offset as usize).enumerate();
                    while let Some(path_info) = stream.next().await {
                        if tx.send(path_info).await.is_err() {
                            // If we get a send error, it means the sync code
                            // doesn't want any more entries.
                            break;
                        }
                    }
                });

                while let Some((i, path_info)) = rx.blocking_recv() {
                    let path_info = match path_info {
                        Err(e) => {
                            warn!("failed to retrieve pathinfo: {}", e);
                            return Err(io::Error::from_raw_os_error(libc::EPERM));
                        }
                        Ok(path_info) => path_info,
                    };

                    // We know the root node exists and the store_path can be parsed because clients MUST validate.
                    let root_node = path_info.node.unwrap().node.unwrap();
                    let store_path = StorePath::from_bytes(root_node.get_name()).unwrap();

                    // obtain the inode, or allocate a new one.
                    let ino = self
                        .get_inode_for_store_path(&store_path)
                        .unwrap_or_else(|| {
                            // insert the (sparse) inode data and register in
                            // self.store_paths.
                            let ino = self.inode_tracker.write().put((&root_node).into());
                            self.store_paths.write().insert(store_path.clone(), ino);
                            ino
                        });

                    let ty = match root_node {
                        Node::Directory(_) => libc::S_IFDIR,
                        Node::File(_) => libc::S_IFREG,
                        Node::Symlink(_) => libc::S_IFLNK,
                    };

                    let written = add_entry(fuse_backend_rs::api::filesystem::DirEntry {
                        ino,
                        offset: offset + i as u64 + 1,
                        type_: ty,
                        name: store_path.to_string().as_bytes(),
                    })?;
                    // If the buffer is full, add_entry will return `Ok(0)`.
                    if written == 0 {
                        break;
                    }
                }

                return Ok(());
            }
        }

        // lookup the children, or return an error if it's not a directory.
        let (parent_digest, children) = self.get_directory_children(inode)?;

        let span = info_span!("lookup", directory.digest = %parent_digest);
        let _enter = span.enter();

        for (i, (ino, child_node)) in children.iter().skip(offset as usize).enumerate() {
            // the second parameter will become the "offset" parameter on the next call.
            let written = add_entry(fuse_backend_rs::api::filesystem::DirEntry {
                ino: *ino,
                offset: offset + i as u64 + 1,
                type_: match child_node {
                    #[allow(clippy::unnecessary_cast)]
                    // libc::S_IFDIR is u32 on Linux and u16 on MacOS
                    Node::Directory(_) => libc::S_IFDIR as u32,
                    #[allow(clippy::unnecessary_cast)]
                    // libc::S_IFDIR is u32 on Linux and u16 on MacOS
                    Node::File(_) => libc::S_IFREG as u32,
                    #[allow(clippy::unnecessary_cast)]
                    // libc::S_IFDIR is u32 on Linux and u16 on MacOS
                    Node::Symlink(_) => libc::S_IFLNK as u32,
                },
                name: child_node.get_name(),
            })?;
            // If the buffer is full, add_entry will return `Ok(0)`.
            if written == 0 {
                break;
            }
        }

        Ok(())
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
    fn open(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        _flags: u32,
        _fuse_flags: u32,
    ) -> io::Result<(
        Option<Self::Handle>,
        fuse_backend_rs::api::filesystem::OpenOptions,
    )> {
        if inode == ROOT_ID {
            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
        }

        // lookup the inode
        match *self.inode_tracker.read().get(inode).unwrap() {
            // read is invalid on non-files.
            InodeData::Directory(..) | InodeData::Symlink(_) => {
                warn!("is directory");
                Err(io::Error::from_raw_os_error(libc::EISDIR))
            }
            InodeData::Regular(ref blob_digest, _blob_size, _) => {
                let span = info_span!("read", blob.digest = %blob_digest);
                let _enter = span.enter();

                let blob_service = self.blob_service.clone();
                let blob_digest = blob_digest.clone();

                let task = self
                    .tokio_handle
                    .spawn(async move { blob_service.open_read(&blob_digest).await });

                let blob_reader = self.tokio_handle.block_on(task).unwrap();

                match blob_reader {
                    Ok(None) => {
                        warn!("blob not found");
                        Err(io::Error::from_raw_os_error(libc::EIO))
                    }
                    Err(e) => {
                        warn!(e=?e, "error opening blob");
                        Err(io::Error::from_raw_os_error(libc::EIO))
                    }
                    Ok(Some(blob_reader)) => {
                        // get a new file handle
                        // TODO: this will overflow after 2**64 operations,
                        // which is fine for now.
                        // See https://cl.tvl.fyi/c/depot/+/8834/comment/a6684ce0_d72469d1
                        // for the discussion on alternatives.
                        let fh = self.next_file_handle.fetch_add(1, Ordering::SeqCst);

                        debug!("add file handle {}", fh);
                        self.file_handles
                            .write()
                            .insert(fh, Arc::new(tokio::sync::Mutex::new(blob_reader)));

                        Ok((
                            Some(fh),
                            fuse_backend_rs::api::filesystem::OpenOptions::empty(),
                        ))
                    }
                }
            }
        }
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode, fh = handle))]
    fn release(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        _flags: u32,
        handle: Self::Handle,
        _flush: bool,
        _flock_release: bool,
        _lock_owner: Option<u64>,
    ) -> io::Result<()> {
        // remove and get ownership on the blob reader
        match self.file_handles.write().remove(&handle) {
            // drop it, which will close it.
            Some(blob_reader) => drop(blob_reader),
            None => {
                // These might already be dropped if a read error occured.
                debug!("file_handle {} not found", handle);
            }
        }

        Ok(())
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.offset = offset, rq.size = size))]
    fn read(
        &self,
        _ctx: &Context,
        inode: Self::Inode,
        handle: Self::Handle,
        w: &mut dyn fuse_backend_rs::api::filesystem::ZeroCopyWriter,
        size: u32,
        offset: u64,
        _lock_owner: Option<u64>,
        _flags: u32,
    ) -> io::Result<usize> {
        debug!("read");

        // We need to take out the blob reader from self.file_handles, so we can
        // interact with it in the separate task.
        // On success, we pass it back out of the task, so we can put it back in self.file_handles.
        let blob_reader = match self.file_handles.read().get(&handle) {
            Some(blob_reader) => blob_reader.clone(),
            None => {
                warn!("file handle {} unknown", handle);
                return Err(io::Error::from_raw_os_error(libc::EIO));
            }
        };

        let task = self.tokio_handle.spawn(async move {
            let mut blob_reader = blob_reader.lock().await;

            // seek to the offset specified, which is relative to the start of the file.
            let resp = blob_reader.seek(io::SeekFrom::Start(offset)).await;

            match resp {
                Ok(pos) => {
                    debug_assert_eq!(offset, pos);
                }
                Err(e) => {
                    warn!("failed to seek to offset {}: {}", offset, e);
                    return Err(io::Error::from_raw_os_error(libc::EIO));
                }
            }

            // As written in the fuse docs, read should send exactly the number
            // of bytes requested except on EOF or error.

            let mut buf: Vec<u8> = Vec::with_capacity(size as usize);

            while (buf.len() as u64) < size as u64 {
                let int_buf = blob_reader.fill_buf().await?;
                // copy things from the internal buffer into buf to fill it till up until size

                // an empty buffer signals we reached EOF.
                if int_buf.is_empty() {
                    break;
                }

                // calculate how many bytes we can read from int_buf.
                // It's either all of int_buf, or the number of bytes missing in buf to reach size.
                let len_to_copy = std::cmp::min(int_buf.len(), size as usize - buf.len());

                // copy these bytes into our buffer
                buf.extend_from_slice(&int_buf[..len_to_copy]);
                // and consume them in the buffered reader.
                blob_reader.consume(len_to_copy);
            }

            Ok(buf)
        });

        let buf = self.tokio_handle.block_on(task).unwrap()?;

        w.write(&buf)
    }

    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
    fn readlink(&self, _ctx: &Context, inode: Self::Inode) -> io::Result<Vec<u8>> {
        if inode == ROOT_ID {
            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
        }

        // lookup the inode
        match *self.inode_tracker.read().get(inode).unwrap() {
            InodeData::Directory(..) | InodeData::Regular(..) => {
                Err(io::Error::from_raw_os_error(libc::EINVAL))
            }
            InodeData::Symlink(ref target) => Ok(target.to_vec()),
        }
    }
}