sync: allow target partition larger than source

sync/verify.go

@@ -21,26 +21,37 @@ func verifyBlockCopy(d *disk.Disk, from, to int, expectedSize int64) error {
 	if err != nil {
 		return err
 	}
-	// create a sha256sum of both partitions and compare
-	// but limit it to expectedSize
+
+	// Source partition must hold at least expectedSize bytes — the
+	// copy reported writing that many, so reading less than that back
+	// indicates a real inconsistency.
+	if got := origPart.GetSize(); got < expectedSize {
+		return fmt.Errorf("original partition size %d is smaller than expected size %d", got, expectedSize)
+	}
+	// Target partition must hold at least expectedSize bytes; it may
+	// be larger (the common case when growing a partition before a
+	// later swap+delete renames it into place). The verification only
+	// needs to confirm the leading expectedSize bytes match.
+	if got := targetPart.GetSize(); got < expectedSize {
+		return fmt.Errorf("target partition size %d is smaller than expected size %d", got, expectedSize)
+	}
+
+	// Hash the leading expectedSize bytes of both partitions and
+	// compare. NewLimitWriter caps the hasher's input, and
+	// origPart.ReadContents would otherwise feed all of the source
+	// partition's bytes into the hasher — for our purposes, we want
+	// only expectedSize bytes hashed on each side regardless of how
+	// large either partition is.
 	origHasher := sha256.New()
-	size, err := origPart.ReadContents(d.Backend, origHasher)
-	if err != nil {
+	if _, err := origPart.ReadContents(d.Backend, NewLimitWriter(origHasher, expectedSize)); err != nil {
 		return err
 	}
-	if size != expectedSize {
-		return fmt.Errorf("original partition size %d is different than expected size %d", size, expectedSize)
-	}
 	origResult := origHasher.Sum(nil)
 
 	targetHasher := sha256.New()
-	size, err = targetPart.ReadContents(d.Backend, NewLimitWriter(targetHasher, expectedSize))
-	if err != nil {
+	if _, err := targetPart.ReadContents(d.Backend, NewLimitWriter(targetHasher, expectedSize)); err != nil {
 		return err
 	}
-	if size != expectedSize {
-		return fmt.Errorf("target partition size %d is different than expected size %d", size, expectedSize)
-	}
 	targetResult := targetHasher.Sum(nil)
 
 	if !bytes.Equal(origResult, targetResult) {

sync/verify_blockcopy_test.go

@@ -0,0 +1,88 @@
+package sync
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	diskfs "github.com/diskfs/go-diskfs"
+	"github.com/diskfs/go-diskfs/disk"
+	"github.com/diskfs/go-diskfs/partition/gpt"
+)
+
+// TestVerifyBlockCopyTargetLargerThanSource covers the grow case: a smaller
+// source partition copied into a larger target. verifyBlockCopy must compare
+// only the leading expectedSize bytes and accept a target that is larger than
+// the source, while still catching a real mismatch within that region.
+func TestVerifyBlockCopyTargetLargerThanSource(t *testing.T) {
+	const (
+		sectorSize = 512
+		diskSize   = 64 * 1024 * 1024
+		srcStart   = 2048             // sectors => 1 MiB in
+		tgtStart   = srcStart + 32768 // 16 MiB after source start
+		srcSize    = 8 * 1024 * 1024  // source partition
+		tgtSize    = 24 * 1024 * 1024 // target is 3x larger
+		expected   = int64(srcSize)
+	)
+
+	imgPath := filepath.Join(t.TempDir(), "disk.img")
+	d, err := diskfs.Create(imgPath, diskSize, sectorSize)
+	if err != nil {
+		t.Fatalf("create disk: %v", err)
+	}
+	table := &gpt.Table{
+		LogicalSectorSize:  sectorSize,
+		PhysicalSectorSize: sectorSize,
+		ProtectiveMBR:      true,
+		Partitions: []*gpt.Partition{
+			{Index: 1, Start: srcStart, Size: srcSize, Type: gpt.LinuxFilesystem, Name: "source"},
+			{Index: 2, Start: tgtStart, Size: tgtSize, Type: gpt.LinuxFilesystem, Name: "target"},
+		},
+	}
+	if err := d.Partition(table); err != nil {
+		t.Fatalf("write partition table: %v", err)
+	}
+
+	// Write identical leading expectedSize bytes into both partitions; the
+	// target's trailing bytes stay zero and must not affect the result.
+	content := make([]byte, expected)
+	for i := range content {
+		content[i] = byte(i*7 + 1)
+	}
+	writeAt := func(off int64, b []byte) {
+		t.Helper()
+		img, err := os.OpenFile(imgPath, os.O_RDWR, 0)
+		if err != nil {
+			t.Fatalf("open image for write: %v", err)
+		}
+		defer img.Close()
+		if _, err := img.WriteAt(b, off); err != nil {
+			t.Fatalf("write image at %d: %v", off, err)
+		}
+	}
+	writeAt(srcStart*sectorSize, content)
+	writeAt(tgtStart*sectorSize, content)
+
+	openDisk := func() *disk.Disk {
+		t.Helper()
+		dd, err := diskfs.Open(imgPath, diskfs.WithSectorSize(sectorSize))
+		if err != nil {
+			t.Fatalf("open disk: %v", err)
+		}
+		if _, err := dd.GetPartitionTable(); err != nil {
+			t.Fatalf("read partition table: %v", err)
+		}
+		return dd
+	}
+
+	// Grow case: larger target, identical leading bytes — must verify clean.
+	if err := verifyBlockCopy(openDisk(), 1, 2, expected); err != nil {
+		t.Errorf("grow with identical leading bytes: unexpected error: %v", err)
+	}
+
+	// Corrupt a byte inside the compared region of the target; must be caught.
+	writeAt(tgtStart*sectorSize, []byte{^content[0]})
+	if err := verifyBlockCopy(openDisk(), 1, 2, expected); err == nil {
+		t.Error("corrupted target within compared region: expected mismatch error, got nil")
+	}
+}