What causes "close temporary dir" in thanos-io/thanos?

Check open file descriptor limits (ulimit -n) and close leaked handles elsewhere in the app.

close temporary dir

Error message

close temporary dir

What it means

Flush fails when df.Close() cannot close the temporary block directory handle after a successful sync. The close is required before renaming the tmp dir into place (particularly on Windows, where open handles prevent renames).

Solutions

Check open file descriptor limits (ulimit -n) and close leaked handles elsewhere in the app.
Retry the Flush; ensure no double-close of the same dir handle.
On Windows, confirm no other process holds an open handle inside the tmp dir.
Discard the tmp dir and re-create the block if the close keeps failing.

Example fix

// before
$ ulimit -n 1024
// after
$ ulimit -n 65536

Defensive patterns

Strategy: retry

Validate before calling

// ensure fd headroom
var lim unix.Rlimit
if unix.Getrlimit(unix.RLIMIT_NOFILE, &lim) == nil && lim.Cur < 4096 {
    lim.Cur = 4096
    unix.Setrlimit(unix.RLIMIT_NOFILE, &lim)
}

Try / catch

if _, err := d.Flush(ctx); err != nil {
    if strings.Contains(err.Error(), "close temporary dir") {
        // fd/handle issue: GC and retry once, else re-create block
        runtime.GC()
        return fmt.Errorf("closing tmp dir failed: %w", err)
    }
    return err
}

Prevention

Raise ulimit -n on hosts doing heavy block IO.
Avoid holding duplicate handles to the block tmp dir.
On Windows, ensure no other process opens files inside the tmp dir during flush.

When it happens

Trigger: df.Close() returning an error — typically fd exhaustion-related state issues, double-close conditions, or OS-level errors on the directory handle.

Common situations: Running out of file descriptors leading to handle corruption; platform quirks on Windows where the dir must be closed before rename; rare kernel-level errors.

Understand the failure class

Background: "open() failed", "failed to open file", "cannot create file" — what a file open error means and how to fix it — this error's family across 42 libraries.

Related errors

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/a96902f03c8908ff. Report an issue: GitHub.

Appendix: source

Thrown at pkg/block/writer.go:128

		}
	}()
	df, err := fileutil.OpenDir(d.bTmp)
	if err != nil {
		return tsdb.BlockStats{}, errors.Wrap(err, "open temporary block dir")
	}
	defer func() {
		if df != nil {
			err = tsdb_errors.NewMulti(err, df.Close()).Err()
		}
	}()

	if err := df.Sync(); err != nil {
		return tsdb.BlockStats{}, errors.Wrap(err, "sync temporary dir file")
	}

	// Close temp dir before rename block dir (for windows platform).
	if err = df.Close(); err != nil {
		return tsdb.BlockStats{}, errors.Wrap(err, "close temporary dir")
	}
	df = nil

	if err := tsdb_errors.CloseAll(d.closers); err != nil {
		d.closers = nil
		return tsdb.BlockStats{}, err
	}
	d.closers = nil

	// Block files successfully written, make them visible by moving files from tmp dir.
	if err := fileutil.Replace(filepath.Join(d.bTmp, IndexFilename), filepath.Join(d.bDir, IndexFilename)); err != nil {
		return tsdb.BlockStats{}, errors.Wrap(err, "replace index file")
	}
	if err := fileutil.Replace(filepath.Join(d.bTmp, ChunksDirname), filepath.Join(d.bDir, ChunksDirname)); err != nil {
		return tsdb.BlockStats{}, errors.Wrap(err, "replace chunks dir")
	}
	return d.stats, nil
}

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