text: // Copyright 2019 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // Package messageset encodes and decodes the obsolete MessageSet wire format.
  package messageset
  
  import (
  	"math"
  
  	"google.golang.org/protobuf/encoding/protowire"
  	"google.golang.org/protobuf/internal/errors"
  	pref "google.golang.org/protobuf/reflect/protoreflect"
  	preg "google.golang.org/protobuf/reflect/protoregistry"
  )
  
  // The MessageSet wire format is equivalent to a message defiend as follows,
  // where each Item defines an extension field with a field number of 'type_id'
  // and content of 'message'. MessageSet extensions must be non-repeated message
  // fields.
  //
  //	message MessageSet {
  //		repeated group Item = 1 {
  //			required int32 type_id = 2;
  //			required string message = 3;
  //		}
  //	}
  const (
  	FieldItem    = protowire.Number(1)
  	FieldTypeID  = protowire.Number(2)
  	FieldMessage = protowire.Number(3)
  )
  
  // ExtensionName is the field name for extensions of MessageSet.
  //
  // A valid MessageSet extension must be of the form:
  //	message MyMessage {
  //		extend proto2.bridge.MessageSet {
  //			optional MyMessage message_set_extension = 1234;
  //		}
  //		...
  //	}
  const ExtensionName = "message_set_extension"
  
  // IsMessageSet returns whether the message uses the MessageSet wire format.
  func IsMessageSet(md pref.MessageDescriptor) bool {
  	xmd, ok := md.(interface{ IsMessageSet() bool })
  	return ok && xmd.IsMessageSet()
  }
  
  // IsMessageSetExtension reports this field extends a MessageSet.
  func IsMessageSetExtension(fd pref.FieldDescriptor) bool {
  	if fd.Name() != ExtensionName {
  		return false
  	}
  	if fd.FullName().Parent() != fd.Message().FullName() {
  		return false
  	}
  	return IsMessageSet(fd.ContainingMessage())
  }
  
  // FindMessageSetExtension locates a MessageSet extension field by name.
  // In text and JSON formats, the extension name used is the message itself.
  // The extension field name is derived by appending ExtensionName.
  func FindMessageSetExtension(r preg.ExtensionTypeResolver, s pref.FullName) (pref.ExtensionType, error) {
  	name := s.Append(ExtensionName)
  	xt, err := r.FindExtensionByName(name)
  	if err != nil {
  		if err == preg.NotFound {
  			return nil, err
  		}
  		return nil, errors.Wrap(err, "%q", name)
  	}
  	if !IsMessageSetExtension(xt.TypeDescriptor()) {
  		return nil, preg.NotFound
  	}
  	return xt, nil
  }
  
  // SizeField returns the size of a MessageSet item field containing an extension
  // with the given field number, not counting the contents of the message subfield.
  func SizeField(num protowire.Number) int {
  	return 2*protowire.SizeTag(FieldItem) + protowire.SizeTag(FieldTypeID) + protowire.SizeVarint(uint64(num))
  }
  
  // Unmarshal parses a MessageSet.
  //
  // It calls fn with the type ID and value of each item in the MessageSet.
  // Unknown fields are discarded.
  //
  // If wantLen is true, the item values include the varint length prefix.
  // This is ugly, but simplifies the fast-path decoder in internal/impl.
  func Unmarshal(b []byte, wantLen bool, fn func(typeID protowire.Number, value []byte) error) error {
  	for len(b) > 0 {
  		num, wtyp, n := protowire.ConsumeTag(b)
  		if n < 0 {
  			return protowire.ParseError(n)
  		}
  		b = b[n:]
  		if num != FieldItem || wtyp != protowire.StartGroupType {
  			n := protowire.ConsumeFieldValue(num, wtyp, b)
  			if n < 0 {
  				return protowire.ParseError(n)
  			}
  			b = b[n:]
  			continue
  		}
  		typeID, value, n, err := ConsumeFieldValue(b, wantLen)
  		if err != nil {
  			return err
  		}
  		b = b[n:]
  		if typeID == 0 {
  			continue
  		}
  		if err := fn(typeID, value); err != nil {
  			return err
  		}
  	}
  	return nil
  }
  
  // ConsumeFieldValue parses b as a MessageSet item field value until and including
  // the trailing end group marker. It assumes the start group tag has already been parsed.
  // It returns the contents of the type_id and message subfields and the total
  // item length.
  //
  // If wantLen is true, the returned message value includes the length prefix.
  func ConsumeFieldValue(b []byte, wantLen bool) (typeid protowire.Number, message []byte, n int, err error) {
  	ilen := len(b)
  	for {
  		num, wtyp, n := protowire.ConsumeTag(b)
  		if n < 0 {
  			return 0, nil, 0, protowire.ParseError(n)
  		}
  		b = b[n:]
  		switch {
  		case num == FieldItem && wtyp == protowire.EndGroupType:
  			if wantLen && len(message) == 0 {
  				// The message field was missing, which should never happen.
  				// Be prepared for this case anyway.
  				message = protowire.AppendVarint(message, 0)
  			}
  			return typeid, message, ilen - len(b), nil
  		case num == FieldTypeID && wtyp == protowire.VarintType:
  			v, n := protowire.ConsumeVarint(b)
  			if n < 0 {
  				return 0, nil, 0, protowire.ParseError(n)
  			}
  			b = b[n:]
  			if v < 1 || v > math.MaxInt32 {
  				return 0, nil, 0, errors.New("invalid type_id in message set")
  			}
  			typeid = protowire.Number(v)
  		case num == FieldMessage && wtyp == protowire.BytesType:
  			m, n := protowire.ConsumeBytes(b)
  			if n < 0 {
  				return 0, nil, 0, protowire.ParseError(n)
  			}
  			if message == nil {
  				if wantLen {
  					message = b[:n:n]
  				} else {
  					message = m[:len(m):len(m)]
  				}
  			} else {
  				// This case should never happen in practice, but handle it for
  				// correctness: The MessageSet item contains multiple message
  				// fields, which need to be merged.
  				//
  				// In the case where we're returning the length, this becomes
  				// quite inefficient since we need to strip the length off
  				// the existing data and reconstruct it with the combined length.
  				if wantLen {
  					_, nn := protowire.ConsumeVarint(message)
  					m0 := message[nn:]
  					message = nil
  					message = protowire.AppendVarint(message, uint64(len(m0)+len(m)))
  					message = append(message, m0...)
  					message = append(message, m...)
  				} else {
  					message = append(message, m...)
  				}
  			}
  			b = b[n:]
  		default:
  			// We have no place to put it, so we just ignore unknown fields.
  			n := protowire.ConsumeFieldValue(num, wtyp, b)
  			if n < 0 {
  				return 0, nil, 0, protowire.ParseError(n)
  			}
  			b = b[n:]
  		}
  	}
  }
  
  // AppendFieldStart appends the start of a MessageSet item field containing
  // an extension with the given number. The caller must add the message
  // subfield (including the tag).
  func AppendFieldStart(b []byte, num protowire.Number) []byte {
  	b = protowire.AppendTag(b, FieldItem, protowire.StartGroupType)
  	b = protowire.AppendTag(b, FieldTypeID, protowire.VarintType)
  	b = protowire.AppendVarint(b, uint64(num))
  	return b
  }
  
  // AppendFieldEnd appends the trailing end group marker for a MessageSet item field.
  func AppendFieldEnd(b []byte) []byte {
  	return protowire.AppendTag(b, FieldItem, protowire.EndGroupType)
  }
  
  // SizeUnknown returns the size of an unknown fields section in MessageSet format.
  //
  // See AppendUnknown.
  func SizeUnknown(unknown []byte) (size int) {
  	for len(unknown) > 0 {
  		num, typ, n := protowire.ConsumeTag(unknown)
  		if n < 0 || typ != protowire.BytesType {
  			return 0
  		}
  		unknown = unknown[n:]
  		_, n = protowire.ConsumeBytes(unknown)
  		if n < 0 {
  			return 0
  		}
  		unknown = unknown[n:]
  		size += SizeField(num) + protowire.SizeTag(FieldMessage) + n
  	}
  	return size
  }
  
  // AppendUnknown appends unknown fields to b in MessageSet format.
  //
  // For historic reasons, unresolved items in a MessageSet are stored in a
  // message's unknown fields section in non-MessageSet format. That is, an
  // unknown item with typeID T and value V appears in the unknown fields as
  // a field with number T and value V.
  //
  // This function converts the unknown fields back into MessageSet form.
  func AppendUnknown(b, unknown []byte) ([]byte, error) {
  	for len(unknown) > 0 {
  		num, typ, n := protowire.ConsumeTag(unknown)
  		if n < 0 || typ != protowire.BytesType {
  			return nil, errors.New("invalid data in message set unknown fields")
  		}
  		unknown = unknown[n:]
  		_, n = protowire.ConsumeBytes(unknown)
  		if n < 0 {
  			return nil, errors.New("invalid data in message set unknown fields")
  		}
  		b = AppendFieldStart(b, num)
  		b = protowire.AppendTag(b, FieldMessage, protowire.BytesType)
  		b = append(b, unknown[:n]...)
  		b = AppendFieldEnd(b)
  		unknown = unknown[n:]
  	}
  	return b, nil
  }
  