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

internal/config/config.go

@@ -16,6 +16,7 @@ import (
 	"github.com/adrg/xdg"
 	"go.uber.org/fx"
 	"gopkg.in/yaml.v3"
+	"sneak.berlin/go/vaultik/internal/chunker"
 	"sneak.berlin/go/vaultik/internal/log"
 )
 
@@ -41,9 +42,11 @@ var (
 		"at least one snapshot must be configured (see config.example.yml)")
 	errSnapshotNoPaths   = errors.New("snapshot must have at least one path")
 	errChunkSizeTooSmall = errors.New("chunk_size must be at least 1MB")
-	errBlobSizeTooSmall  = errors.New("blob_size_limit must be at least chunk_size")
-	errBadCompression    = errors.New("compression_level must be between 1 and 19")
-	errBadStorageScheme  = errors.New(
+	errBlobSizeTooSmall  = errors.New(
+		"blob_size_limit must be at least the largest chunk the chunker can " +
+			"emit (chunk_size times the FastCDC size spread)")
+	errBadCompression   = errors.New("compression_level must be between 1 and 19")
+	errBadStorageScheme = errors.New(
 		"storage_url must start with s3://, file://, or rclone://")
 	errStorageNotConfigured = errors.New(
 		"storage not configured; set storage_url or provide s3.endpoint + " +
@@ -287,12 +290,15 @@ func Load(path string) (*Config, error) {
 
 // Validate checks if the configuration is valid and complete.
 // It ensures all required fields are present and have valid values:
-// - At least one age recipient must be specified
-// - At least one snapshot must be configured with at least one path
-// - Storage must be configured (either storage_url or s3.* fields)
-// - Chunk size must be at least 1MB
-// - Blob size limit must be at least the chunk size
-// - Compression level must be between 1 and 19
+//   - At least one age recipient must be specified
+//   - At least one snapshot must be configured with at least one path
+//   - Storage must be configured (either storage_url or s3.* fields)
+//   - Chunk size must be at least 1MB
+//   - Blob size limit must be at least the largest chunk the chunker can emit
+//     (chunk_size times chunker.ChunkSizeSpread), so a single-chunk blob never
+//     exceeds the configured limit
+//   - Compression level must be between 1 and 19
+//
 // Returns an error describing the first validation failure encountered.
 func (c *Config) Validate() error {
 	if len(c.AgeRecipients) == 0 {
@@ -319,8 +325,13 @@ func (c *Config) Validate() error {
 		return errChunkSizeTooSmall
 	}
 
-	if c.BlobSizeLimit.Int64() < c.ChunkSize.Int64() {
-		return errBlobSizeTooSmall
+	// The chunker can emit chunks up to chunk_size * ChunkSizeSpread, and the
+	// packer places a single such chunk into an otherwise empty blob. A limit
+	// below that bound would let a blob exceed it, so reject it.
+	largestChunk := c.ChunkSize.Int64() * chunker.ChunkSizeSpread
+	if c.BlobSizeLimit.Int64() < largestChunk {
+		return fmt.Errorf("%w: need at least %d bytes",
+			errBlobSizeTooSmall, largestChunk)
 	}
 
 	if c.CompressionLevel < minCompressionLevel ||