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_test.go

@@ -1,9 +1,12 @@
 package config //nolint:testpackage // exercises unexported extractAgeSecretKey
 
 import (
+	"errors"
 	"os"
 	"path/filepath"
 	"testing"
+
+	"sneak.berlin/go/vaultik/internal/chunker"
 )
 
 const (
@@ -101,6 +104,80 @@ func TestConfigFromEnv(t *testing.T) {
 	}
 }
 
+// TestValidateBlobSizeLimit checks the blob_size_limit boundary: it must be at
+// least the largest chunk the chunker can emit (chunk_size times
+// chunker.ChunkSizeSpread), because the packer places a single such chunk into
+// an otherwise empty blob. A limit between chunk_size and that bound is rejected.
+func TestValidateBlobSizeLimit(t *testing.T) {
+	t.Parallel()
+
+	const chunkSize = Size(10 * 1024 * 1024) // 10MB
+
+	largestChunk := chunkSize.Int64() * chunker.ChunkSizeSpread
+
+	newConfig := func(blobLimit Size) *Config {
+		return &Config{
+			AgeRecipients:    []string{testSneakAgePublicKey},
+			Snapshots:        map[string]SnapshotConfig{"test": {Paths: []string{"/tmp/src"}}},
+			StorageURL:       "file:///tmp/vaultik-test-store",
+			ChunkSize:        chunkSize,
+			BlobSizeLimit:    blobLimit,
+			CompressionLevel: 3,
+		}
+	}
+
+	tests := []struct {
+		name      string
+		blobLimit Size
+		wantErr   bool
+	}{
+		{
+			name:      "at chunk_size but below largest chunk is rejected",
+			blobLimit: chunkSize,
+			wantErr:   true,
+		},
+		{
+			name:      "between chunk_size and largest chunk is rejected",
+			blobLimit: Size(chunkSize.Int64() * 2),
+			wantErr:   true,
+		},
+		{
+			name:      "one byte below largest chunk is rejected",
+			blobLimit: Size(largestChunk - 1),
+			wantErr:   true,
+		},
+		{
+			name:      "exactly at largest chunk is accepted",
+			blobLimit: Size(largestChunk),
+			wantErr:   false,
+		},
+		{
+			name:      "above largest chunk is accepted",
+			blobLimit: Size(largestChunk * 100),
+			wantErr:   false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := newConfig(tt.blobLimit).Validate()
+			if tt.wantErr {
+				if !errors.Is(err, errBlobSizeTooSmall) {
+					t.Fatalf("Validate() error = %v, want errBlobSizeTooSmall", err)
+				}
+
+				return
+			}
+
+			if err != nil {
+				t.Fatalf("Validate() unexpected error: %v", err)
+			}
+		})
+	}
+}
+
 // TestExtractAgeSecretKey tests extraction of AGE-SECRET-KEY from various inputs
 func TestExtractAgeSecretKey(t *testing.T) {
 	t.Parallel()