crl-release-26.2: manifest: fix BackingValueSize lost during version edit round-trip

When encoding blob references for virtual tables, if BackingValueSize
equaled ValueSize for all references, the encoder used
customTagBlobReferences (which omits BackingValueSize). On decode,
BackingValueSize became 0 — a sentinel meaning "no backing value size
info available." After manifest replay, this caused blob files to be
incorrectly marked ineligible for rewrite compaction (via
virtualReferencesWithoutBackingValueSize), preventing blob garbage
collection.

Fix the encoding condition to write BackingValueSize whenever it is
known (> 0) for virtual tables, regardless of whether it equals
ValueSize. Also use max(BackingValueSize, ValueSize) when computing
ReferencedBlobValueSizeTotal during accumulation, for robustness
against old-format data.

Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>

Author: RaduBerinde

internal/manifest/version_edit.go

@@ -951,7 +951,7 @@ func (v *VersionEdit) Encode(w io.Writer) error {
 				writeBackingValueSize := false
 				if x.Meta.Virtual {
 					for _, ref := range x.Meta.BlobReferences {
-						if ref.BackingValueSize > 0 && ref.BackingValueSize != ref.ValueSize {
+						if ref.BackingValueSize > 0 {
 							writeBackingValueSize = true
 							break
 						}
@@ -1247,7 +1247,7 @@ func (b *BulkVersionEdit) Accumulate(ve *VersionEdit) error {
 			// blob references.
 			if backing.ReferencedBlobValueSizeTotal == 0 {
 				for _, br := range nf.Meta.BlobReferences {
-					backing.ReferencedBlobValueSizeTotal += br.BackingValueSize
+					backing.ReferencedBlobValueSizeTotal += max(br.BackingValueSize, br.ValueSize)
 				}
 			}
 			nf.Meta.AttachVirtualBacking(backing)

internal/manifest/version_edit_test.go

@@ -124,6 +124,77 @@ func TestVERoundTripAndAccumulate(t *testing.T) {
 	}
 }
 
+// TestVERoundTripBackingValueSizeEqualValueSize verifies that BackingValueSize
+// is preserved through encode/decode when it equals ValueSize for a virtual
+// table. This is a regression test for a bug where the encoder omitted
+// BackingValueSize when it equaled ValueSize, causing it to decode as 0 (the
+// sentinel for "unknown").
+func TestVERoundTripBackingValueSizeEqualValueSize(t *testing.T) {
+	cmp := base.DefaultComparer.Compare
+	m1 := (&TableMetadata{
+		TableNum:              810,
+		Size:                  8090,
+		CreationTime:          809060,
+		SeqNums:               base.SeqNumRange{Low: 9, High: 11},
+		LargestSeqNumAbsolute: 11,
+		BlobReferences: []BlobReference{
+			{FileID: 900, ValueSize: 1024, BackingValueSize: 1024},
+			{FileID: 910, ValueSize: 8090, BackingValueSize: 8090},
+		},
+	}).ExtendPointKeyBounds(
+		cmp,
+		base.MakeInternalKey([]byte("a"), 0, base.InternalKeyKindSet),
+		base.MakeInternalKey([]byte("m"), 0, base.InternalKeyKindSet),
+	)
+	m1.InitPhysicalBacking()
+
+	// Virtual table where BackingValueSize == ValueSize for all references.
+	m2 := (&TableMetadata{
+		TableNum:              812,
+		Size:                  8090,
+		CreationTime:          809060,
+		SeqNums:               base.SeqNumRange{Low: 9, High: 11},
+		LargestSeqNumAbsolute: 11,
+		Virtual:               true,
+		BlobReferences: []BlobReference{
+			{FileID: 900, ValueSize: 1024, BackingValueSize: 1024},
+			{FileID: 910, ValueSize: 8090, BackingValueSize: 8090},
+		},
+	}).ExtendPointKeyBounds(
+		cmp,
+		base.MakeInternalKey([]byte("a"), 0, base.InternalKeyKindSet),
+		base.MakeInternalKey([]byte("c"), 0, base.InternalKeyKindSet),
+	)
+	m2.AttachVirtualBacking(m1.TableBacking)
+
+	ve1 := VersionEdit{
+		CreatedBackingTables: []*TableBacking{m1.TableBacking},
+		NewTables: []NewTableEntry{
+			{
+				Level:          4,
+				Meta:           m2,
+				BackingFileNum: m2.TableBacking.DiskFileNum,
+			},
+		},
+	}
+	buf := new(bytes.Buffer)
+	require.NoError(t, ve1.Encode(buf))
+	var ve2 VersionEdit
+	require.NoError(t, ve2.Decode(buf))
+	var bve BulkVersionEdit
+	require.NoError(t, bve.Accumulate(&ve2))
+
+	// Verify BackingValueSize is preserved after round-trip.
+	require.Equal(t, len(ve1.NewTables), len(ve2.NewTables))
+	for i, nt := range ve2.NewTables {
+		for j, ref := range nt.Meta.BlobReferences {
+			orig := ve1.NewTables[i].Meta.BlobReferences[j]
+			require.Equal(t, orig.BackingValueSize, ref.BackingValueSize,
+				"BlobReferences[%d].BackingValueSize: got %d, want %d", j, ref.BackingValueSize, orig.BackingValueSize)
+		}
+	}
+}
+
 func TestVersionEditRoundTrip(t *testing.T) {
 	cmp := base.DefaultComparer.Compare
 	m1 := (&TableMetadata{