fix: return partition table not exist when trying to read an empty dev

Fixes: #51

Signed-off-by: Artem Chernyshev <artem.chernyshev@talos-systems.com>

blockdevice/blockdevice_linux.go

@@ -5,7 +5,7 @@
 package blockdevice
 
 import (
-	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -88,22 +88,16 @@ func Open(devname string, setters ...Option) (bd *BlockDevice, err error) {
 
 		bd.g = g
 	} else {
-		buf := make([]byte, 1)
-		// PMBR protective entry starts at 446. The partition type is at offset
-		// 4 from the start of the PMBR protective entry.
-		_, err = f.ReadAt(buf, 450)
-		if err != nil {
+		var g *gpt.GPT
+		if g, err = gpt.Open(f); err != nil {
+			if errors.Is(err, gpt.ErrPartitionTableDoesNotExist) {
+				return bd, nil
+			}
+
 			return nil, err
 		}
 
-		// For GPT, the partition type should be 0xee (EFI GPT).
-		if bytes.Equal(buf, []byte{0xee}) {
-			var g *gpt.GPT
-			if g, err = gpt.Open(f); err != nil {
-				return nil, err
-			}
-			bd.g = g
-		}
+		bd.g = g
 	}
 
 	return bd, nil

blockdevice/partition/gpt/gpt.go

@@ -26,11 +26,11 @@ const (
 
 var (
 	// ErrPartitionTableDoesNotExist indicates that the partition table does not exist.
-	ErrPartitionTableDoesNotExist = errors.New("does not exist")
+	ErrPartitionTableDoesNotExist = errors.New("block device partition table does not exist")
 	// ErrHeaderCRCMismatch indicates that the header CRC does not match what is on disk.
-	ErrHeaderCRCMismatch = errors.New("header CRC mismatch")
+	ErrHeaderCRCMismatch = errors.New("block device partition table header CRC mismatch")
 	// ErrEntriesCRCMismatch indicates that the partitions array CRC does not match what is on disk.
-	ErrEntriesCRCMismatch = errors.New("entries CRC mismatch")
+	ErrEntriesCRCMismatch = errors.New("block device partition table entries CRC mismatch")
 )
 
 type outOfSpaceError struct {
@@ -84,6 +84,10 @@ func Open(f *os.File) (g *GPT, err error) {
 			markMBRBootable: buf[0] == 0x80,
 		}
 
+		if err = h.DeserializeSignature(); err != nil {
+			return nil, ErrPartitionTableDoesNotExist
+		}
+
 		return g, nil
 	}
 

blockdevice/partition/gpt/gpt_test.go

@@ -35,7 +35,7 @@ func (suite *GPTSuite) SetupTest() {
 
 func (suite *GPTSuite) TestEmpty() {
 	_, err := gpt.Open(suite.Dev)
-	suite.Require().Error(err)
+	suite.Require().EqualError(err, gpt.ErrPartitionTableDoesNotExist.Error())
 }
 
 func (suite *GPTSuite) TestReset() {