feat(tvix/store/fuse): initial implementation

This is a first implementation of a FUSE filesystem, mounting tvix-store
to a given location.

This is mostly meant as one additional lens into a store, and could be
used for builds. It's not meant to be used as a general-purpose thing.

It still has some rough edges:

 - It doesn't implement open/close, so it doesn't use file handles.
   Which means, we need to open blobs for partial reads over and over
   again.
 - It doesn't implement seek, as BlobReader doesn't implement seek yet.
 - It doesn't track "lifetimes" of inodes by listening on forget,
   meaning it might hold more data in memory than necessary.
 - As we don't have store composition (and a caching layer) yet,
   operations might be slow.

Change-Id: Ib1812ed761dfaf6aeb548443ae939c87530b7be8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8667
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/fuse/inodes.rs

@@ -0,0 +1,78 @@
+///! This module contains all the data structures used to track information
+///! about inodes, which present tvix-store nodes in a filesystem.
+use crate::{proto, B3Digest};
+
+#[derive(Clone, Debug)]
+pub enum InodeData {
+    Regular(B3Digest, u32, bool),  // digest, size, executable
+    Symlink(String),               // target
+    Directory(DirectoryInodeData), // either [DirectoryInodeData:Sparse] or [DirectoryInodeData:Populated]
+}
+
+/// This encodes the two different states of [InodeData::Directory].
+/// Either the data still is sparse (we only saw a [proto::DirectoryNode], but
+/// didn't fetch the [proto::Directory] struct yet,
+/// or we processed a lookup and did fetch the data.
+#[derive(Clone, Debug)]
+pub enum DirectoryInodeData {
+    Sparse(B3Digest, u32),                              // digest, size
+    Populated(B3Digest, Vec<(u64, proto::node::Node)>), // [(child_inode, node)]
+}
+
+impl From<&proto::node::Node> for InodeData {
+    fn from(value: &proto::node::Node) -> Self {
+        match value {
+            proto::node::Node::Directory(directory_node) => directory_node.into(),
+            proto::node::Node::File(file_node) => file_node.into(),
+            proto::node::Node::Symlink(symlink_node) => symlink_node.into(),
+        }
+    }
+}
+
+impl From<&proto::SymlinkNode> for InodeData {
+    fn from(value: &proto::SymlinkNode) -> Self {
+        InodeData::Symlink(value.target.clone())
+    }
+}
+
+impl From<&proto::FileNode> for InodeData {
+    fn from(value: &proto::FileNode) -> Self {
+        InodeData::Regular(
+            B3Digest::from_vec(value.digest.clone()).unwrap(),
+            value.size,
+            value.executable,
+        )
+    }
+}
+
+/// Converts a DirectoryNode to a sparsely populated InodeData::Directory.
+impl From<&proto::DirectoryNode> for InodeData {
+    fn from(value: &proto::DirectoryNode) -> Self {
+        InodeData::Directory(DirectoryInodeData::Sparse(
+            B3Digest::from_vec(value.digest.clone()).unwrap(),
+            value.size,
+        ))
+    }
+}
+
+/// converts a proto::Directory to a InodeData::Directory(DirectoryInodeData::Populated(..)).
+/// The inodes for each child are 0, because it's up to the InodeTracker to allocate them.
+impl From<proto::Directory> for InodeData {
+    fn from(value: proto::Directory) -> Self {
+        let digest = value.digest();
+
+        let children: Vec<(u64, proto::node::Node)> = value.nodes().map(|node| (0, node)).collect();
+
+        InodeData::Directory(DirectoryInodeData::Populated(digest, children))
+    }
+}
+
+impl From<&InodeData> for fuser::FileType {
+    fn from(val: &InodeData) -> Self {
+        match val {
+            InodeData::Regular(..) => fuser::FileType::RegularFile,
+            InodeData::Symlink(_) => fuser::FileType::Symlink,
+            InodeData::Directory(..) => fuser::FileType::Directory,
+        }
+    }
+}