Repository: fxamacker/cbor (go)

Issue:
Optimize internal calls to UnmarshalCBOR() for ByteString, RawTag, and SimpleValue
## Problem

Currently, the unreleased changes in PR #636 and #645 cause the input data to be checked twice when `UnmarshalCBOR()` is called internally.

`UnmarshalCBOR()` checks input data because it can be called by user apps providing bad data.  However, the codec already checks input data before internally calling `UnmarshalCBOR()` so the 2nd check is redundant.

## Solution

Optimize by making the codec no longer directly call `UnmarshalCBOR()` if the private `unmarshalCBOR()` is implemented by the type:
- Internally, the codec will call the private `unmarshalCBOR()` to avoid validating data twice.
- `MarshalCBOR()` will be a simple wrapper that checks data for validity before calling the private `unmarshalCBOR()`.

## Status

This optimization is being tested locally.  It will be merged and fuzzed for longer durations before release v2.7.1 and v2.8.0.

Pull request description:
Optimize internal calls to UnmarshalCBOR() for ByteString, RawTag, SimpleValue
Closes #646

This PR resolves a performance regression introduced and mentioned in PR #636 and #645:
> the same data is checked twice for the intended case of the codec calling UnmarshalCBOR() internally. This can be revisited and maybe optimized in the future.

This optimization avoids the redundant 2nd check of input data.

These functions are deprecated (they were created for internal use):
- `ByteString.UnmarshalCBOR()`
- `RawTag.UnmarshalCBOR()`
- `SimpleValue.UnmarshalCBOR()`

`Unmarshal()` should be used instead of the deprecated functions.

## Details

PR #636 and #645 cause the input data to be checked twice when these functions are called internally by `Unmarshal()`:
- `ByteString.UnmarshalCBOR()`
- `RawTag.UnmarshalCBOR()`
- `SimpleValue.UnmarshalCBOR()`

These functions check input data because they can be called by user apps providing bad data. However, the codec already checks input data before internally calling `UnmarshalCBOR()` so the 2nd check is redundant.

This PR avoids redundant check on the input data by having `Unmarshal()` call the private `unmarshalCBOR()` implemented by `ByteString`, `RawTag`, `SimpleValue`:
- Internally, the codec calls the private `unmarshalCBOR()` to avoid the redundant check on input data.
- Externally, `UnmarshalCBOR()` is available as a wrapper that checks input data before calling the private `unmarshalCBOR()`.

To avoid breaking user apps, the 3 `UnmarshalCBOR()` functions are deprecated rather than removed. 


Patch:
diff --git a/cache.go b/cache.go
index 8562fd8..0d96b98 100644
--- a/cache.go
+++ b/cache.go
@@ -32,6 +32,7 @@ type specialType int
 const (
 	specialTypeNone specialType = iota
 	specialTypeUnmarshalerIface
+	specialTypeUnexportedUnmarshalerIface
 	specialTypeEmptyIface
 	specialTypeIface
 	specialTypeTag
@@ -70,6 +71,8 @@ func newTypeInfo(t reflect.Type) *typeInfo {
 		tInfo.spclType = specialTypeTag
 	} else if t == typeTime {
 		tInfo.spclType = specialTypeTime
+	} else if reflect.PointerTo(t).Implements(typeUnexportedUnmarshaler) {
+		tInfo.spclType = specialTypeUnexportedUnmarshalerIface
 	} else if reflect.PointerTo(t).Implements(typeUnmarshaler) {
 		tInfo.spclType = specialTypeUnmarshalerIface
 	}
diff --git a/decode.go b/decode.go
index 52285b1..3c1c168 100644
--- a/decode.go
+++ b/decode.go
@@ -151,6 +151,10 @@ type Unmarshaler interface {
 	UnmarshalCBOR([]byte) error
 }
 
+type unmarshaler interface {
+	unmarshalCBOR([]byte) error
+}
+
 // InvalidUnmarshalError describes an invalid argument passed to Unmarshal.
 type InvalidUnmarshalError struct {
 	s string
@@ -1460,6 +1464,9 @@ func (d *decoder) parseToValue(v reflect.Value, tInfo *typeInfo) error { //nolin
 
 		case specialTypeUnmarshalerIface:
 			return d.parseToUnmarshaler(v)
+
+		case specialTypeUnexportedUnmarshalerIface:
+			return d.parseToUnexportedUnmarshaler(v)
 		}
 	}
 
@@ -1805,6 +1812,26 @@ func (d *decoder) parseToUnmarshaler(v reflect.Value) error {
 	return errors.New("cbor: failed to assert " + v.Type().String() + " as cbor.Unmarshaler")
 }
 
+// parseToUnexportedUnmarshaler parses CBOR data to value implementing unmarshaler interface.
+// It assumes data is well-formed, and does not perform bounds checking.
+func (d *decoder) parseToUnexportedUnmarshaler(v reflect.Value) error {
+	if d.nextCBORNil() && v.Kind() == reflect.Pointer && v.IsNil() {
+		d.skip()
+		return nil
+	}
+
+	if v.Kind() != reflect.Pointer && v.CanAddr() {
+		v = v.Addr()
+	}
+	if u, ok := v.Interface().(unmarshaler); ok {
+		start := d.off
+		d.skip()
+		return u.unmarshalCBOR(d.data[start:d.off])
+	}
+	d.skip()
+	return errors.New("cbor: failed to assert " + v.Type().String() + " as cbor.unmarshaler")
+}
+
 // parse parses CBOR data and returns value in default Go type.
 // It assumes data is well-formed, and does not perform bounds checking.
 func (d *decoder) parse(skipSelfDescribedTag bool) (any, error) { //nolint:gocyclo
@@ -2969,13 +2996,14 @@ func (d *decoder) nextCBORNil() bool {
 }
 
 var (
-	typeIntf              = reflect.TypeOf([]any(nil)).Elem()
-	typeTime              = reflect.TypeOf(time.Time{})
-	typeBigInt            = reflect.TypeOf(big.Int{})
-	typeUnmarshaler       = reflect.TypeOf((*Unmarshaler)(nil)).Elem()
-	typeBinaryUnmarshaler = reflect.TypeOf((*encoding.BinaryUnmarshaler)(nil)).Elem()
-	typeString            = reflect.TypeOf("")
-	typeByteSlice         = reflect.TypeOf([]byte(nil))
+	typeIntf                  = reflect.TypeOf([]any(nil)).Elem()
+	typeTime                  = reflect.TypeOf(time.Time{})
+	typeBigInt                = reflect.TypeOf(big.Int{})
+	typeUnmarshaler           = reflect.TypeOf((*Unmarshaler)(nil)).Elem()
+	typeUnexportedUnmarshaler = reflect.TypeOf((*unmarshaler)(nil)).Elem()
+	typeBinaryUnmarshaler     = reflect.TypeOf((*encoding.BinaryUnmarshaler)(nil)).Elem()
+	typeString                = reflect.TypeOf("")
+	typeByteSlice             = reflect.TypeOf([]byte(nil))
 )
 
 func fillNil(_ cborType, v reflect.Value) error {
diff --git a/simplevalue.go b/simplevalue.go
index 5c6c668..30f7281 100644
--- a/simplevalue.go
+++ b/simplevalue.go
@@ -48,6 +48,9 @@ func (sv SimpleValue) MarshalCBOR() ([]byte, error) {
 }
 
 // UnmarshalCBOR decodes CBOR simple value (major type 7) to SimpleValue.
+//
+// Deprecated: No longer used by this codec; kept for compatibility
+// with user apps that directly call this function.
 func (sv *SimpleValue) UnmarshalCBOR(data []byte) error {
 	if sv == nil {
 		return errors.New("cbor.SimpleValue: UnmarshalCBOR on nil pointer")
@@ -56,9 +59,7 @@ func (sv *SimpleValue) UnmarshalCBOR(data []byte) error {
 	d := decoder{data: data, dm: defaultDecMode}
 
 	// Check well-formedness of CBOR data item.
-	// NOTE: well-formedness check here is redundant when
-	// Unmarshal() invokes SimpleValue.UnmarshalCBOR().
-	// However, SimpleValue.UnmarshalCBOR() is exported, so
+	// SimpleValue.UnmarshalCBOR() is exported, so
 	// the codec needs to support same behavior for:
 	// - Unmarshal(data, *SimpleValue)
 	// - SimpleValue.UnmarshalCBOR(data)
@@ -67,8 +68,18 @@ func (sv *SimpleValue) UnmarshalCBOR(data []byte) error {
 		return err
 	}
 
-	// Restore decoder offset after well-formedness check.
-	d.off = 0
+	return sv.unmarshalCBOR(data)
+}
+
+// unmarshalCBOR decodes CBOR simple value (major type 7) to SimpleValue.
+// This function assumes data is well-formed, and does not perform bounds checking.
+// This function is called by Unmarshal().
+func (sv *SimpleValue) unmarshalCBOR(data []byte) error {
+	if sv == nil {
+		return errors.New("cbor.SimpleValue: UnmarshalCBOR on nil pointer")
+	}
+
+	d := decoder{data: data, dm: defaultDecMode}
 
 	typ, ai, val := d.getHead()
 
diff --git a/tag.go b/tag.go
index def4c3f..47bcca8 100644
--- a/tag.go
+++ b/tag.go
@@ -26,22 +26,18 @@ type RawTag struct {
 }
 
 // UnmarshalCBOR sets *t with tag number and raw tag content copied from data.
+//
+// Deprecated: No longer used by this codec; kept for compatibility
+// with user apps that directly call this function.
 func (t *RawTag) UnmarshalCBOR(data []byte) error {
 	if t == nil {
 		return errors.New("cbor.RawTag: UnmarshalCBOR on nil pointer")
 	}
 
-	// Decoding CBOR null and undefined to cbor.RawTag is no-op.
-	if len(data) == 1 && (data[0] == 0xf6 || data[0] == 0xf7) {
-		return nil
-	}
-
 	d := decoder{data: data, dm: defaultDecMode}
 
 	// Check if data is a well-formed CBOR data item.
-	// NOTE: well-formedness check here is redundant when
-	// Unmarshal() invokes RawTag.UnmarshalCBOR().
-	// However, RawTag.UnmarshalCBOR() is exported, so
+	// RawTag.UnmarshalCBOR() is exported, so
 	// the codec needs to support same behavior for:
 	// - Unmarshal(data, *RawTag)
 	// - RawTag.UnmarshalCBOR(data)
@@ -50,8 +46,23 @@ func (t *RawTag) UnmarshalCBOR(data []byte) error {
 		return err
 	}
 
-	// Restore decoder offset after well-formedness check.
-	d.off = 0
+	return t.unmarshalCBOR(data)
+}
+
+// unmarshalCBOR sets *t with tag number and raw tag content copied from data.
+// This function assumes data is well-formed, and does not perform bounds checking.
+// This function is called by Unmarshal().
+func (t *RawTag) unmarshalCBOR(data []byte) error {
+	if t == nil {
+		return errors.New("cbor.RawTag: UnmarshalCBOR on nil pointer")
+	}
+
+	// Decoding CBOR null and undefined to cbor.RawTag is no-op.
+	if len(data) == 1 && (data[0] == 0xf6 || data[0] == 0xf7) {
+		return nil
+	}
+
+	d := decoder{data: data, dm: defaultDecMode}
 
 	// Unmarshal tag number.
 	typ, _, num := d.getHead()


Test patch:
diff --git a/bytestring.go b/bytestring.go
index 6bc081f..23c5724 100644
--- a/bytestring.go
+++ b/bytestring.go
@@ -38,24 +38,18 @@ func (bs ByteString) MarshalCBOR() ([]byte, error) {
 
 // UnmarshalCBOR decodes CBOR byte string (major type 2) to ByteString.
 // Decoding CBOR null and CBOR undefined sets ByteString to be empty.
+//
+// Deprecated: No longer used by this codec; kept for compatibility
+// with user apps that directly call this function.
 func (bs *ByteString) UnmarshalCBOR(data []byte) error {
 	if bs == nil {
 		return errors.New("cbor.ByteString: UnmarshalCBOR on nil pointer")
 	}
 
-	// Decoding CBOR null and CBOR undefined to ByteString resets data.
-	// This behavior is similar to decoding CBOR null and CBOR undefined to []byte.
-	if len(data) == 1 && (data[0] == 0xf6 || data[0] == 0xf7) {
-		*bs = ""
-		return nil
-	}
-
 	d := decoder{data: data, dm: defaultDecMode}
 
 	// Check well-formedness of CBOR data item.
-	// NOTE: well-formedness check here is redundant when
-	// Unmarshal() invokes ByteString.UnmarshalCBOR().
-	// However, ByteString.UnmarshalCBOR() is exported, so
+	// ByteString.UnmarshalCBOR() is exported, so
 	// the codec needs to support same behavior for:
 	// - Unmarshal(data, *ByteString)
 	// - ByteString.UnmarshalCBOR(data)
@@ -64,8 +58,26 @@ func (bs *ByteString) UnmarshalCBOR(data []byte) error {
 		return err
 	}
 
-	// Restore decoder offset after well-formedness check.
-	d.off = 0
+	return bs.unmarshalCBOR(data)
+}
+
+// unmarshalCBOR decodes CBOR byte string (major type 2) to ByteString.
+// Decoding CBOR null and CBOR undefined sets ByteString to be empty.
+// This function assumes data is well-formed, and does not perform bounds checking.
+// This function is called by Unmarshal().
+func (bs *ByteString) unmarshalCBOR(data []byte) error {
+	if bs == nil {
+		return errors.New("cbor.ByteString: UnmarshalCBOR on nil pointer")
+	}
+
+	// Decoding CBOR null and CBOR undefined to ByteString resets data.
+	// This behavior is similar to decoding CBOR null and CBOR undefined to []byte.
+	if len(data) == 1 && (data[0] == 0xf6 || data[0] == 0xf7) {
+		*bs = ""
+		return nil
+	}
+
+	d := decoder{data: data, dm: defaultDecMode}
 
 	// Check if CBOR data type is byte string
 	if typ := d.nextCBORType(); typ != cborTypeByteString {
diff --git a/bytestring_test.go b/bytestring_test.go
index ae208ae..2001638 100644
--- a/bytestring_test.go
+++ b/bytestring_test.go
@@ -195,16 +195,16 @@ func TestUnmarshalByteStringOnBadData(t *testing.T) {
 					t.Errorf("UnmarshalCBOR(%x) returned error %q, want %q", tc.data, err.Error(), tc.errMsg)
 				}
 			}
-			// Test Unmarshal(data, *ByteString), which calls ByteString.UnmarshalCBOR() under the hood
+			// Test Unmarshal(data, *ByteString), which calls ByteString.unmarshalCBOR() under the hood
 			{
 				var v ByteString
 
 				err := Unmarshal(tc.data, &v)
 				if err == nil {
-					t.Errorf("UnmarshalCBOR(%x) didn't return error", tc.data)
+					t.Errorf("Unmarshal(%x) didn't return error", tc.data)
 				}
 				if !strings.HasPrefix(err.Error(), tc.errMsg) {
-					t.Errorf("UnmarshalCBOR(%x) returned error %q, want %q", tc.data, err.Error(), tc.errMsg)
+					t.Errorf("Unmarshal(%x) returned error %q, want %q", tc.data, err.Error(), tc.errMsg)
 				}
 			}
 		})
diff --git a/simplevalue_test.go b/simplevalue_test.go
index 1022c59..e877e63 100644
--- a/simplevalue_test.go
+++ b/simplevalue_test.go
@@ -156,16 +156,16 @@ func TestUnmarshalSimpleValueOnBadData(t *testing.T) {
 					t.Errorf("UnmarshalCBOR(%x) returned error %q, want %q", tc.data, err.Error(), tc.errMsg)
 				}
 			}
-			// Test Unmarshal(data, *SimpleValue), which calls SimpleValue.UnmarshalCBOR() under the hood
+			// Test Unmarshal(data, *SimpleValue), which calls SimpleValue.unmarshalCBOR() under the hood
 			{
 				var v SimpleValue
 
 				err := Unmarshal(tc.data, &v)
 				if err == nil {
-					t.Errorf("UnmarshalCBOR(%x) didn't return error", tc.data)
+					t.Errorf("Unmarshal(%x) didn't return error", tc.data)
 				}
 				if !strings.HasPrefix(err.Error(), tc.errMsg) {
-					t.Errorf("UnmarshalCBOR(%x) returned error %q, want %q", tc.data, err.Error(), tc.errMsg)
+					t.Errorf("Unmarshal(%x) returned error %q, want %q", tc.data, err.Error(), tc.errMsg)
 				}
 			}
 		})
diff --git a/tag_test.go b/tag_test.go
index 8d4e9c5..5fccfdf 100644
--- a/tag_test.go
+++ b/tag_test.go
@@ -1646,16 +1646,16 @@ func TestUnmarshalRawTagOnBadData(t *testing.T) {
 					t.Errorf("UnmarshalCBOR(%x) returned error %q, want %q", tc.data, err.Error(), tc.errMsg)
 				}
 			}
-			// Test Unmarshal(data, *RawTag), which calls RawTag.UnmarshalCBOR() under the hood
+			// Test Unmarshal(data, *RawTag), which calls RawTag.unmarshalCBOR() under the hood
 			{
 				var v RawTag
 
 				err := Unmarshal(tc.data, &v)
 				if err == nil {
-					t.Errorf("UnmarshalCBOR(%x) didn't return error", tc.data)
+					t.Errorf("Unmarshal(%x) didn't return error", tc.data)
 				}
 				if !strings.HasPrefix(err.Error(), tc.errMsg) {
-					t.Errorf("UnmarshalCBOR(%x) returned error %q, want %q", tc.data, err.Error(), tc.errMsg)
+					t.Errorf("Unmarshal(%x) returned error %q, want %q", tc.data, err.Error(), tc.errMsg)
 				}
 			}
 		})