feat(tvix/castore/fs): support extended attributes r/7912

This exposes `user.tvix.castore.{blob,directory}.digest` xattr keys for
files and directories:

```
❯ getfattr -d /tmp/tvix/06jrrv6wwp0nc1m7fr5bgdw012rfzfx2-nano-7.2-info
getfattr: Removing leading '/' from absolute path names
user.tvix.castore.directory.digest="b3:SuYDcUM9RpWcnA40tYB1BtYpR0xw72v3ymhKDQbBfe4="

❯ getfattr -d /tmp/tvix/156a89x10c3kaby9rgf3fi4k0p6r9wl1-etc-shells
getfattr: Removing leading '/' from absolute path names
user.tvix.castore.blob.digest="b3:pZkwZoHN+/VQ8wkaX0wYVXZ0tV/HhtKlSqiaWDK7uRs="
```

It's currently mostly used for debugging, though it might be useful for
tvix-castore-aware syncing programs using the filesystem too.

Change-Id: I26ac3cb9fe51ffbf7f880519f26741549cb5ab6a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11422
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: Brian Olsen <me@griff.name>

7 files changed, 227 insertions, 5 deletions

diff --git a/tvix/Cargo.lock b/tvix/Cargo.lock
index d53ced76ae..a15c71d26d 100644
--- a/tvix/Cargo.lock
+++ b/tvix/Cargo.lock
@@ -4380,6 +4380,7 @@ dependencies = [
  "vm-memory",
  "vmm-sys-util",
  "walkdir",
+ "xattr",
  "zstd",
 ]
 
diff --git a/tvix/Cargo.nix b/tvix/Cargo.nix
index 6886685b63..48b53a1943 100644
--- a/tvix/Cargo.nix
+++ b/tvix/Cargo.nix
@@ -13953,6 +13953,10 @@ rec {
             name = "tokio-retry";
             packageId = "tokio-retry";
           }
+          {
+            name = "xattr";
+            packageId = "xattr";
+          }
         ];
         features = {
           "cloud" = [ "dep:bigtable_rs" "object_store/aws" "object_store/azure" "object_store/gcp" ];
diff --git a/tvix/castore/Cargo.toml b/tvix/castore/Cargo.toml
index 6fc44f1574..447ddea1dc 100644
--- a/tvix/castore/Cargo.toml
+++ b/tvix/castore/Cargo.toml
@@ -86,6 +86,7 @@ tempfile = "3.3.0"
 tokio-retry = "0.3.0"
 hex-literal = "0.4.1"
 rstest_reuse = "0.6.0"
+xattr = "1.3.1"
 
 [features]
 default = []
diff --git a/tvix/castore/src/fs/mod.rs b/tvix/castore/src/fs/mod.rs
index 699598b520..8731c8374d 100644
--- a/tvix/castore/src/fs/mod.rs
+++ b/tvix/castore/src/fs/mod.rs
@@ -19,10 +19,14 @@ use crate::{
     proto::{node::Node, NamedNode},
     B3Digest,
 };
+use bstr::ByteVec;
 use fuse_backend_rs::abi::fuse_abi::stat64;
-use fuse_backend_rs::api::filesystem::{Context, FileSystem, FsOptions, ROOT_ID};
+use fuse_backend_rs::api::filesystem::{
+    Context, FileSystem, FsOptions, GetxattrReply, ListxattrReply, ROOT_ID,
+};
 use futures::StreamExt;
 use parking_lot::RwLock;
+use std::ffi::CStr;
 use std::{
     collections::HashMap,
     io,
@@ -83,6 +87,9 @@ pub struct TvixStoreFs<BS, DS, RN> {
     /// Whether to (try) listing elements in the root.
     list_root: bool,
 
+    /// Whether to expose blob and directory digests as extended attributes.
+    show_xattr: bool,
+
     /// This maps a given basename in the root to the inode we allocated for the node.
     root_nodes: RwLock<HashMap<Vec<u8>, u64>>,
 
@@ -109,6 +116,7 @@ where
         directory_service: DS,
         root_nodes_provider: RN,
         list_root: bool,
+        show_xattr: bool,
     ) -> Self {
         Self {
             blob_service,
@@ -116,6 +124,7 @@ where
             root_nodes_provider,
 
             list_root,
+            show_xattr,
 
             root_nodes: RwLock::new(HashMap::default()),
             inode_tracker: RwLock::new(Default::default()),
@@ -267,6 +276,9 @@ where
     }
 }
 
+const XATTR_NAME_DIRECTORY_DIGEST: &[u8] = b"user.tvix.castore.directory.digest";
+const XATTR_NAME_BLOB_DIGEST: &[u8] = b"user.tvix.castore.blob.digest";
+
 impl<BS, DS, RN> FileSystem for TvixStoreFs<BS, DS, RN>
 where
     BS: AsRef<dyn BlobService> + Clone + Send + 'static,
@@ -628,4 +640,85 @@ where
             InodeData::Symlink(ref target) => Ok(target.to_vec()),
         }
     }
+
+    #[tracing::instrument(skip_all, fields(rq.inode = inode, name=?name))]
+    fn getxattr(
+        &self,
+        _ctx: &Context,
+        inode: Self::Inode,
+        name: &CStr,
+        size: u32,
+    ) -> io::Result<GetxattrReply> {
+        if !self.show_xattr {
+            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
+        }
+
+        // Peek at the inode requested, and construct the response.
+        let digest_str = match *self
+            .inode_tracker
+            .read()
+            .get(inode)
+            .ok_or_else(|| io::Error::from_raw_os_error(libc::ENODATA))?
+        {
+            InodeData::Directory(DirectoryInodeData::Sparse(ref digest, _))
+            | InodeData::Directory(DirectoryInodeData::Populated(ref digest, _))
+                if name.to_bytes() == XATTR_NAME_DIRECTORY_DIGEST =>
+            {
+                digest.to_string()
+            }
+            InodeData::Regular(ref digest, _, _) if name.to_bytes() == XATTR_NAME_BLOB_DIGEST => {
+                digest.to_string()
+            }
+            _ => {
+                return Err(io::Error::from_raw_os_error(libc::ENODATA));
+            }
+        };
+
+        if size == 0 {
+            Ok(GetxattrReply::Count(digest_str.len() as u32))
+        } else if size < digest_str.len() as u32 {
+            Err(io::Error::from_raw_os_error(libc::ERANGE))
+        } else {
+            Ok(GetxattrReply::Value(digest_str.into_bytes()))
+        }
+    }
+
+    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
+    fn listxattr(
+        &self,
+        _ctx: &Context,
+        inode: Self::Inode,
+        size: u32,
+    ) -> io::Result<ListxattrReply> {
+        if !self.show_xattr {
+            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
+        }
+
+        // determine the (\0-terminated list) to of xattr keys present, depending on the type of the inode.
+        let xattrs_names = {
+            let mut out = Vec::new();
+            if let Some(inode_data) = self.inode_tracker.read().get(inode) {
+                match *inode_data {
+                    InodeData::Directory(_) => {
+                        out.extend_from_slice(XATTR_NAME_DIRECTORY_DIGEST);
+                        out.push_byte(b'\x00');
+                    }
+                    InodeData::Regular(..) => {
+                        out.extend_from_slice(XATTR_NAME_BLOB_DIGEST);
+                        out.push_byte(b'\x00');
+                    }
+                    _ => {}
+                }
+            }
+            out
+        };
+
+        if size == 0 {
+            Ok(ListxattrReply::Count(xattrs_names.len() as u32))
+        } else if size < xattrs_names.len() as u32 {
+            Err(io::Error::from_raw_os_error(libc::ERANGE))
+        } else {
+            Ok(ListxattrReply::Names(xattrs_names.to_vec()))
+        }
+    }
 }
diff --git a/tvix/castore/src/fs/tests.rs b/tvix/castore/src/fs/tests.rs
index 924454caa6..226c9975d5 100644
--- a/tvix/castore/src/fs/tests.rs
+++ b/tvix/castore/src/fs/tests.rs
@@ -1,18 +1,19 @@
+use bstr::ByteSlice;
+use bytes::Bytes;
 use std::{
     collections::BTreeMap,
+    ffi::{OsStr, OsString},
     io::{self, Cursor},
-    os::unix::fs::MetadataExt,
+    os::unix::{ffi::OsStrExt, fs::MetadataExt},
     path::Path,
     sync::Arc,
 };
-
-use bytes::Bytes;
 use tempfile::TempDir;
 use tokio_stream::{wrappers::ReadDirStream, StreamExt};
 
 use super::{fuse::FuseDaemon, TvixStoreFs};
+use crate::proto as castorepb;
 use crate::proto::node::Node;
-use crate::proto::{self as castorepb};
 use crate::{
     blobservice::{BlobService, MemoryBlobService},
     directoryservice::{DirectoryService, MemoryDirectoryService},
@@ -40,6 +41,7 @@ fn do_mount<P: AsRef<Path>, BS, DS>(
     root_nodes: BTreeMap<bytes::Bytes, Node>,
     mountpoint: P,
     list_root: bool,
+    show_xattr: bool,
 ) -> io::Result<FuseDaemon>
 where
     BS: AsRef<dyn BlobService> + Send + Sync + Clone + 'static,
@@ -50,6 +52,7 @@ where
         directory_service,
         Arc::new(root_nodes),
         list_root,
+        show_xattr,
     );
     FuseDaemon::new(Arc::new(fs), mountpoint.as_ref(), 4, false)
 }
@@ -250,6 +253,7 @@ async fn mount() {
         BTreeMap::default(),
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -272,6 +276,7 @@ async fn root() {
         BTreeMap::default(),
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -311,6 +316,7 @@ async fn root_with_listing() {
         root_nodes,
         tmpdir.path(),
         true, /* allow listing */
+        false,
     )
     .expect("must succeed");
 
@@ -354,6 +360,7 @@ async fn stat_file_at_root() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -390,6 +397,7 @@ async fn read_file_at_root() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -426,6 +434,7 @@ async fn read_large_file_at_root() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -470,6 +479,7 @@ async fn symlink_readlink() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -516,6 +526,7 @@ async fn read_stat_through_symlink() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -560,6 +571,7 @@ async fn read_stat_directory() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -573,6 +585,91 @@ async fn read_stat_directory() {
     fuse_daemon.unmount().expect("unmount");
 }
 
+/// Read a directory and file in the root, and ensure the xattrs expose blob or
+/// directory digests.
+#[tokio::test]
+async fn xattr() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service) = gen_svcs();
+    let mut root_nodes = BTreeMap::default();
+
+    populate_directory_with_keep(&blob_service, &directory_service, &mut root_nodes).await;
+    populate_blob_a(&blob_service, &mut root_nodes).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        root_nodes,
+        tmpdir.path(),
+        false,
+        true, /* support xattr */
+    )
+    .expect("must succeed");
+
+    // peek at the directory
+    {
+        let p = tmpdir.path().join(DIRECTORY_WITH_KEEP_NAME);
+
+        let xattr_names: Vec<OsString> = xattr::list(&p).expect("must succeed").collect();
+        // There should be 1 key, XATTR_NAME_DIRECTORY_DIGEST.
+        assert_eq!(1, xattr_names.len(), "there should be 1 xattr name");
+        assert_eq!(
+            super::XATTR_NAME_DIRECTORY_DIGEST,
+            xattr_names.first().unwrap().as_encoded_bytes()
+        );
+
+        // The key should equal to the string-formatted b3 digest.
+        let val = xattr::get(&p, OsStr::from_bytes(super::XATTR_NAME_DIRECTORY_DIGEST))
+            .expect("must succeed")
+            .expect("must be some");
+        assert_eq!(
+            fixtures::DIRECTORY_WITH_KEEP
+                .digest()
+                .to_string()
+                .as_bytes()
+                .as_bstr(),
+            val.as_bstr()
+        );
+
+        // Reading another xattr key is gonna return None.
+        let val = xattr::get(&p, OsStr::from_bytes(b"user.cheesecake")).expect("must succeed");
+        assert_eq!(None, val);
+    }
+    // peek at the file
+    {
+        let p = tmpdir.path().join(BLOB_A_NAME);
+
+        let xattr_names: Vec<OsString> = xattr::list(&p).expect("must succeed").collect();
+        // There should be 1 key, XATTR_NAME_BLOB_DIGEST.
+        assert_eq!(1, xattr_names.len(), "there should be 1 xattr name");
+        assert_eq!(
+            super::XATTR_NAME_BLOB_DIGEST,
+            xattr_names.first().unwrap().as_encoded_bytes()
+        );
+
+        // The key should equal to the string-formatted b3 digest.
+        let val = xattr::get(&p, OsStr::from_bytes(super::XATTR_NAME_BLOB_DIGEST))
+            .expect("must succeed")
+            .expect("must be some");
+        assert_eq!(
+            fixtures::BLOB_A_DIGEST.to_string().as_bytes().as_bstr(),
+            val.as_bstr()
+        );
+
+        // Reading another xattr key is gonna return None.
+        let val = xattr::get(&p, OsStr::from_bytes(b"user.cheesecake")).expect("must succeed");
+        assert_eq!(None, val);
+    }
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 /// Read a blob inside a directory. This ensures we successfully populate directory data.
 async fn read_blob_inside_dir() {
@@ -594,6 +691,7 @@ async fn read_blob_inside_dir() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -633,6 +731,7 @@ async fn read_blob_deep_inside_dir() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -675,6 +774,7 @@ async fn readdir() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -734,6 +834,7 @@ async fn readdir_deep() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -783,6 +884,7 @@ async fn check_attributes() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -857,6 +959,7 @@ async fn compare_inodes_directories() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -900,6 +1003,7 @@ async fn compare_inodes_files() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -948,6 +1052,7 @@ async fn compare_inodes_symlinks() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -990,6 +1095,7 @@ async fn read_wrong_paths_in_root() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -1044,6 +1150,7 @@ async fn disallow_writes() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -1075,6 +1182,7 @@ async fn missing_directory() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
@@ -1122,6 +1230,7 @@ async fn missing_blob() {
         root_nodes,
         tmpdir.path(),
         false,
+        false,
     )
     .expect("must succeed");
 
diff --git a/tvix/store/src/bin/tvix-store.rs b/tvix/store/src/bin/tvix-store.rs
index a5845c74ae..15f37d301f 100644
--- a/tvix/store/src/bin/tvix-store.rs
+++ b/tvix/store/src/bin/tvix-store.rs
@@ -161,6 +161,10 @@ enum Commands {
         /// (exhaustive) listing.
         #[clap(long, short, action)]
         list_root: bool,
+
+        #[arg(long, default_value_t = true)]
+        /// Whether to expose blob and directory digests as extended attributes.
+        show_xattr: bool,
     },
     /// Starts a tvix-store virtiofs daemon at the given socket path.
     #[cfg(feature = "virtiofs")]
@@ -183,6 +187,10 @@ enum Commands {
         /// (exhaustive) listing.
         #[clap(long, short, action)]
         list_root: bool,
+
+        #[arg(long, default_value_t = true)]
+        /// Whether to expose blob and directory digests as extended attributes.
+        show_xattr: bool,
     },
 }
 
@@ -459,6 +467,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
             list_root,
             threads,
             allow_other,
+            show_xattr,
         } => {
             let (blob_service, directory_service, path_info_service) =
                 tvix_store::utils::construct_services(
@@ -474,6 +483,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                     directory_service,
                     Arc::from(path_info_service),
                     list_root,
+                    show_xattr,
                 );
                 info!(mount_path=?dest, "mounting");
 
@@ -499,6 +509,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
             directory_service_addr,
             path_info_service_addr,
             list_root,
+            show_xattr,
         } => {
             let (blob_service, directory_service, path_info_service) =
                 tvix_store::utils::construct_services(
@@ -514,6 +525,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                     directory_service,
                     Arc::from(path_info_service),
                     list_root,
+                    show_xattr,
                 );
                 info!(socket_path=?socket, "starting virtiofs-daemon");
 
diff --git a/tvix/store/src/pathinfoservice/fs/mod.rs b/tvix/store/src/pathinfoservice/fs/mod.rs
index 45d59fd0bc..aa64b1c01f 100644
--- a/tvix/store/src/pathinfoservice/fs/mod.rs
+++ b/tvix/store/src/pathinfoservice/fs/mod.rs
@@ -17,6 +17,7 @@ pub fn make_fs<BS, DS, PS>(
     directory_service: DS,
     path_info_service: PS,
     list_root: bool,
+    show_xattr: bool,
 ) -> TvixStoreFs<BS, DS, RootNodesWrapper<PS>>
 where
     BS: AsRef<dyn BlobService> + Send + Clone + 'static,
@@ -28,6 +29,7 @@ where
         directory_service,
         RootNodesWrapper(path_info_service),
         list_root,
+        show_xattr,
     )
 }