tempodb: tolerate blocks removed during local listing

[arkoc]

What this fixes

The local backend blocklist poll and retention run on the same interval. When retention removes a compacted block directory while ListBlocks is walking the tenant directory, fs.WalkDir reports fs.ErrNotExist for that child and the entire tenant poll fails.

A concurrently removed child is no longer part of the blocklist, so this change skips only fs.ErrNotExist below the tenant root. Errors opening the tenant root and all other filesystem errors still fail the poll.

This was observed on v2.9.0, and the affected ListBlocks implementation is also present on release-v2.9. A backport would make the fix available to 2.9 users.

Testing

• Added a regression test that continuously removes and recreates a block while ListBlocks runs. It reliably fails before this change with open UUID: no such file or directory.
• go test -race ./tempodb/backend/local -run TestListBlocks -count=20
• go test ./tempodb/backend/...

[github-actions]

Signed commits report

1 of 1 commit between main and fix/local-backend-list-retention-race could not be fully verified:

Commit	Author	Reason	Message
ce4e6c2b	Aram Kocharyan	unsigned	tempodb: tolerate blocks removed during local listing

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