Reject a blob_size_limit below the largest possible chunk (closes #167)

Validate only rejected blob_size_limit below chunk_size, but the chunker can emit chunks up to chunk_size times the FastCDC size spread (four times), and the packer puts a single chunk of any size into an otherwise empty blob. A limit between one and four times chunk_size therefore let a blob reach four times the configured maximum, with most blobs holding a single chunk and so exposing individual chunk lengths to anyone who can list the destination. Validate now rejects blob_size_limit below chunk_size times the spread, reusing the chunker's one constant (now exported as ChunkSizeSpread) instead of a second literal. The rule is stated in the error text, the Validate comment, the README config table, config.example.yml, and the generated config template. Model: opus-4-8
2026-09-22 11:45:41 +02:00
parent 86361c8b50
commit 4c83e82543
6 changed files with 109 additions and 16 deletions
@@ -518,7 +518,7 @@ complete annotated example also lives in
 | `s3.*` | | Legacy S3 configuration (endpoint, bucket, credentials) |
 | `exclude` | | Global exclude patterns (applied to all snapshots) |
 | `chunk_size` | `10MB` | Average chunk size for content-defined chunking |
-| `blob_size_limit` | `10GB` | Maximum blob size before splitting |
+| `blob_size_limit` | `10GB` | Maximum blob size before splitting. Must be at least four times `chunk_size` (the largest chunk the chunker can emit), otherwise a single-chunk blob could exceed the limit |
 | `compression_level` | `3` | zstd compression level (1-19) |
 | `hostname` | system hostname | Hostname used in snapshot IDs |
 | `index_path` | platform data dir | Local SQLite index path |