pola-rs/polars · error

failed to update file last accessed time

Error message

failed to update file last accessed time: {e}

What it means

This panic fires when the OS-level call to update a cached file's timestamps (via file.set_times) fails while Polars updates the last-accessed time of a file in its cache. The library treats cache metadata upkeep as a hard invariant, so failure is a panic rather than a recoverable error. It typically indicates an OS/filesystem-level problem with the already-open file handle.

Solutions

1. Verify the file resides on a filesystem that supports access-time updates (avoid unusual network/FUSE mounts for the cache path).
2. Check file permissions and that no other process holds the file open in a mode blocking timestamp updates.
3. Point POLARS_TEMP_DIR / cache directory at a local POSIX filesystem.
4. If it persists, report to polars with OS and filesystem details, as set_times failing on a valid handle is environmental.

Example fix

// before
let cache_dir = "/mnt/nfs-share/cache"; // exotic mount, may not support set_times
// after
let cache_dir = "/tmp/polars-cache"; // local filesystem with timestamp support

let meta = std::fs::metadata(&path)?;
let modified = meta.modified()?; // panics inside polars if None; check first
assert!(modified.elapsed().is_ok(), "filesystem lacks timestamps");

fn fs_supports_times(p: &std::path::Path) -> bool {
    std::fs::metadata(p).ok()
        .and_then(|m| m.modified().ok())
        .is_some()
}

// polars panics here; guard the environment before running
if !fs_supports_times(&file_path) {
    // copy the file to a local FS or skip caching
}

When it happens

Trigger: Calling try_open_assume_latest, try_open_check_latest, or finish_open in the file cache when the underlying std::fs::File rejects set_times(); also triggered if file_metadata.modified() returns None and unwrap panics on filesystems without modification times.

Common situations: Files on filesystems that don't support access/modified timestamps (some network mounts, tmpfs variants, FUSE filesystems); permission-restricted files; Windows files locked by another process.

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

• could not read
• failed to create spill file
• failed to open spill file
• Expecting to be able to downcast into bytes from read…
• failed to create file cache data directory: path =

AI-assisted analysis of pola-rs/polars@fe841f959e (2026-09-18). Data as JSON: /api/errors/59089ef69a153a62. Report an issue: GitHub.

Appendix: source

Thrown at crates/polars-io/src/file_cache/utils.rs:46

        metadata
            .modified()
            .unwrap()
            .duration_since(UNIX_EPOCH)
            .unwrap()
            .as_millis(),
    )
    .unwrap()
}

pub(super) fn update_last_accessed(file: &std::fs::File) {
    let file_metadata = file.metadata().unwrap();

    if let Err(e) = file.set_times(
        std::fs::FileTimes::new()
            .set_modified(file_metadata.modified().unwrap())
            .set_accessed(std::time::SystemTime::now()),
    ) {
        panic!("failed to update file last accessed time: {e}");
    }
}

pub async fn init_entries_from_uri_list(
    uri_list: impl ExactSizeIterator<Item = PlRefPath> + Send + 'static,
    cloud_options: Option<&CloudOptions>,
) -> PolarsResult<Vec<Arc<FileCacheEntry>>> {
    init_entries_from_uri_list_impl(Box::new(uri_list), cloud_options).await
}

async fn init_entries_from_uri_list_impl(
    uri_list: Box<dyn ExactSizeIterator<Item = PlRefPath> + Send + 'static>,
    cloud_options: Option<&CloudOptions>,
) -> PolarsResult<Vec<Arc<FileCacheEntry>>> {
    #[allow(clippy::len_zero)]
    if uri_list.len() == 0 {
        return Ok(Default::default());
    }

View on GitHub (pinned to fe841f959e)