error determining whether index blob

Error message

error determining whether index blob %v has been downloaded

What it means

Raised by missingIndexBlobs when querying the local cache (cache.hasIndexBlobID) to decide which index blobs still need downloading fails. It is not about the blob being missing (that is handled), but about the cache lookup itself erroring. The wrapped cause indicates the local cache backend (typically the disk cache) could not be inspected.

Solutions

Check the wrapped cause for the cache-backend error and fix filesystem permissions or I/O issues
Remove and rebuild the local cache directory so lookups succeed again
Verify the cache path configuration points to an existing writable location
Retry the repository open/maintenance after repairing the cache

Defensive patterns

Strategy: fallback

Validate before calling

st, err := os.Stat(cacheDir)
if err != nil || !st.IsDir() {
    // recreate cacheDir with 0700 before using the repository
}

Try / catch

ids, err := idx.MissingIndexBlobs(ctx, blobs)
if err != nil {
    // on cache lookup failure: wipe cache dir and retry once
    os.RemoveAll(cacheDir)
    ids, err = idx.MissingIndexBlobs(ctx, blobs)
}

Prevention

Ensure the cache directory exists, is a directory, and is writable before opening the repo
Don't run multiple Kopia processes against the same cache concurrently with conflicting versions
Recreate the cache after OS crashes or disk errors
Check file permissions when running as different users (root vs service account)

When it happens

Trigger: c.cache.hasIndexBlobID(ctx, id) returns non-nil while fetchIndexBlobs computes the list of blobs to download via missingIndexBlobs.

Common situations: Local cache directory inaccessible or corrupted; permission errors stat-ing cache files; underlying cache openIndex probing failed (e.g. unreadable cached index file).

Related errors

AI-assisted analysis of kopia/kopia@82495e54b5 (2026-09-07). Data as JSON: /api/errors/fb23f2f564248a8b. Report an issue: GitHub.

Appendix: source

Thrown at repo/content/committed_content_index.go:366

	if err := eg.Wait(); err != nil {
		return errors.Wrap(err, "error downloading indexes")
	}

	contentlog.Log(ctx, c.log, "Index blobs downloaded")

	return nil
}

// missingIndexBlobs returns a closed channel filled with blob IDs that are not in committedContents cache.
func (c *committedContentIndex) missingIndexBlobs(ctx context.Context, blobs []blob.ID) (<-chan blob.ID, error) {
	ch := make(chan blob.ID, len(blobs))
	defer close(ch)

	for _, id := range blobs {
		has, err := c.cache.hasIndexBlobID(ctx, id)
		if err != nil {
			return nil, errors.Wrapf(err, "error determining whether index blob %v has been downloaded", id)
		}

		if !has {
			ch <- id
		}
	}

	return ch, nil
}

func newCommittedContentIndex(caching *CachingOptions,
	v1PerContentOverhead func() int,
	formatProvider format.Provider,
	permissiveCacheLoading bool,
	fetchIndexBlob func(ctx context.Context, blobID blob.ID, output *gather.WriteBuffer) error,
	log *contentlog.Logger,
	minSweepAge time.Duration,
) *committedContentIndex {

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