Reject a metadata database truncated to the age header and nonce (closes #152)

An object holding exactly the age header plus its 16-byte nonce decrypts
without error: age.Decrypt succeeds, and on the first read the age stream's
io.ErrUnexpectedEOF is mapped by the zstd decoder to a clean io.EOF at frame
start. blobgen.Reader.Read then reported zero bytes and no error, so a
truncated stream was indistinguishable from a valid empty one. Blobs are
caught by the Close-time hash check, but the metadata database export was not:
restore built a fresh schema on the empty file and exited reporting success.

blobgen.Reader.Read now, on EOF, reads once more from the age reader and
surfaces io.ErrUnexpectedEOF unless that read is (0, io.EOF) — the state a
genuine end leaves behind, so an empty payload still round-trips to empty with
no error. downloadSnapshotDB additionally rejects a zero-length decrypted
database before OpenReadOnly applies a schema.

On next the decrypted-DB identity check already rejects an empty database, so
the restore-level test guards the required end-to-end property; the blobgen
tests are the ones that fail without the reader change.

Model: opus-4-8

internal/blobgen/truncation_test.go

@@ -0,0 +1,70 @@
+package blobgen_test
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	"filippo.io/age"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/vaultik/internal/blobgen"
+)
+
+// TestReaderRejectsHeaderNonceTruncation guards against a stream cut right
+// after the age header plus its 16-byte nonce. age.Decrypt still succeeds on
+// such an object, and the zstd decoder maps the age reader's
+// io.ErrUnexpectedEOF to a clean io.EOF at frame start, so without the extra
+// check the truncated stream would read as a valid empty one. Reading it must
+// now fail.
+func TestReaderRejectsHeaderNonceTruncation(t *testing.T) {
+	t.Parallel()
+
+	identity, err := age.GenerateX25519Identity()
+	require.NoError(t, err)
+
+	// Encrypting empty plaintext yields header + nonce(16) + a single
+	// 16-byte final chunk tag. Dropping the trailing tag leaves exactly the
+	// age header plus its nonce — the truncation point that triggers the bug.
+	var full bytes.Buffer
+
+	w, err := age.Encrypt(&full, identity.Recipient())
+	require.NoError(t, err)
+	require.NoError(t, w.Close())
+
+	truncated := full.Bytes()[:full.Len()-16]
+
+	reader, err := blobgen.NewReader(bytes.NewReader(truncated), identity)
+	require.NoError(t, err)
+
+	defer func() { _ = reader.Close() }()
+
+	_, err = io.ReadAll(reader)
+	require.Error(t, err)
+	require.ErrorIs(t, err, io.ErrUnexpectedEOF)
+}
+
+// TestReaderReadsGenuinelyEmptyBlob confirms the truncation check does not
+// reject a legitimately empty payload: a blob written with no data must round
+// trip back to zero bytes with no error.
+func TestReaderReadsGenuinelyEmptyBlob(t *testing.T) {
+	t.Parallel()
+
+	identity, err := age.GenerateX25519Identity()
+	require.NoError(t, err)
+
+	var encrypted bytes.Buffer
+
+	writer, err := blobgen.NewWriter(
+		&encrypted, 3, []string{identity.Recipient().String()})
+	require.NoError(t, err)
+	require.NoError(t, writer.Close())
+
+	reader, err := blobgen.NewReader(bytes.NewReader(encrypted.Bytes()), identity)
+	require.NoError(t, err)
+
+	defer func() { _ = reader.Close() }()
+
+	data, err := io.ReadAll(reader)
+	require.NoError(t, err)
+	require.Empty(t, data)
+}