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Source file src/encoding/xml/typeinfo.go

Documentation: encoding/xml

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package xml
     6  
     7  import (
     8  	"fmt"
     9  	"reflect"
    10  	"strings"
    11  	"sync"
    12  )
    13  
    14  // typeInfo holds details for the xml representation of a type.
    15  type typeInfo struct {
    16  	xmlname *fieldInfo
    17  	fields  []fieldInfo
    18  }
    19  
    20  // fieldInfo holds details for the xml representation of a single field.
    21  type fieldInfo struct {
    22  	idx     []int
    23  	name    string
    24  	xmlns   string
    25  	flags   fieldFlags
    26  	parents []string
    27  }
    28  
    29  type fieldFlags int
    30  
    31  const (
    32  	fElement fieldFlags = 1 << iota
    33  	fAttr
    34  	fCDATA
    35  	fCharData
    36  	fInnerXml
    37  	fComment
    38  	fAny
    39  
    40  	fOmitEmpty
    41  
    42  	fMode = fElement | fAttr | fCDATA | fCharData | fInnerXml | fComment | fAny
    43  
    44  	xmlName = "XMLName"
    45  )
    46  
    47  var tinfoMap sync.Map // map[reflect.Type]*typeInfo
    48  
    49  var nameType = reflect.TypeOf(Name{})
    50  
    51  // getTypeInfo returns the typeInfo structure with details necessary
    52  // for marshaling and unmarshaling typ.
    53  func getTypeInfo(typ reflect.Type) (*typeInfo, error) {
    54  	if ti, ok := tinfoMap.Load(typ); ok {
    55  		return ti.(*typeInfo), nil
    56  	}
    57  
    58  	tinfo := &typeInfo{}
    59  	if typ.Kind() == reflect.Struct && typ != nameType {
    60  		n := typ.NumField()
    61  		for i := 0; i < n; i++ {
    62  			f := typ.Field(i)
    63  			if (f.PkgPath != "" && !f.Anonymous) || f.Tag.Get("xml") == "-" {
    64  				continue // Private field
    65  			}
    66  
    67  			// For embedded structs, embed its fields.
    68  			if f.Anonymous {
    69  				t := f.Type
    70  				if t.Kind() == reflect.Ptr {
    71  					t = t.Elem()
    72  				}
    73  				if t.Kind() == reflect.Struct {
    74  					inner, err := getTypeInfo(t)
    75  					if err != nil {
    76  						return nil, err
    77  					}
    78  					if tinfo.xmlname == nil {
    79  						tinfo.xmlname = inner.xmlname
    80  					}
    81  					for _, finfo := range inner.fields {
    82  						finfo.idx = append([]int{i}, finfo.idx...)
    83  						if err := addFieldInfo(typ, tinfo, &finfo); err != nil {
    84  							return nil, err
    85  						}
    86  					}
    87  					continue
    88  				}
    89  			}
    90  
    91  			finfo, err := structFieldInfo(typ, &f)
    92  			if err != nil {
    93  				return nil, err
    94  			}
    95  
    96  			if f.Name == xmlName {
    97  				tinfo.xmlname = finfo
    98  				continue
    99  			}
   100  
   101  			// Add the field if it doesn't conflict with other fields.
   102  			if err := addFieldInfo(typ, tinfo, finfo); err != nil {
   103  				return nil, err
   104  			}
   105  		}
   106  	}
   107  
   108  	ti, _ := tinfoMap.LoadOrStore(typ, tinfo)
   109  	return ti.(*typeInfo), nil
   110  }
   111  
   112  // structFieldInfo builds and returns a fieldInfo for f.
   113  func structFieldInfo(typ reflect.Type, f *reflect.StructField) (*fieldInfo, error) {
   114  	finfo := &fieldInfo{idx: f.Index}
   115  
   116  	// Split the tag from the xml namespace if necessary.
   117  	tag := f.Tag.Get("xml")
   118  	if i := strings.Index(tag, " "); i >= 0 {
   119  		finfo.xmlns, tag = tag[:i], tag[i+1:]
   120  	}
   121  
   122  	// Parse flags.
   123  	tokens := strings.Split(tag, ",")
   124  	if len(tokens) == 1 {
   125  		finfo.flags = fElement
   126  	} else {
   127  		tag = tokens[0]
   128  		for _, flag := range tokens[1:] {
   129  			switch flag {
   130  			case "attr":
   131  				finfo.flags |= fAttr
   132  			case "cdata":
   133  				finfo.flags |= fCDATA
   134  			case "chardata":
   135  				finfo.flags |= fCharData
   136  			case "innerxml":
   137  				finfo.flags |= fInnerXml
   138  			case "comment":
   139  				finfo.flags |= fComment
   140  			case "any":
   141  				finfo.flags |= fAny
   142  			case "omitempty":
   143  				finfo.flags |= fOmitEmpty
   144  			}
   145  		}
   146  
   147  		// Validate the flags used.
   148  		valid := true
   149  		switch mode := finfo.flags & fMode; mode {
   150  		case 0:
   151  			finfo.flags |= fElement
   152  		case fAttr, fCDATA, fCharData, fInnerXml, fComment, fAny, fAny | fAttr:
   153  			if f.Name == xmlName || tag != "" && mode != fAttr {
   154  				valid = false
   155  			}
   156  		default:
   157  			// This will also catch multiple modes in a single field.
   158  			valid = false
   159  		}
   160  		if finfo.flags&fMode == fAny {
   161  			finfo.flags |= fElement
   162  		}
   163  		if finfo.flags&fOmitEmpty != 0 && finfo.flags&(fElement|fAttr) == 0 {
   164  			valid = false
   165  		}
   166  		if !valid {
   167  			return nil, fmt.Errorf("xml: invalid tag in field %s of type %s: %q",
   168  				f.Name, typ, f.Tag.Get("xml"))
   169  		}
   170  	}
   171  
   172  	// Use of xmlns without a name is not allowed.
   173  	if finfo.xmlns != "" && tag == "" {
   174  		return nil, fmt.Errorf("xml: namespace without name in field %s of type %s: %q",
   175  			f.Name, typ, f.Tag.Get("xml"))
   176  	}
   177  
   178  	if f.Name == xmlName {
   179  		// The XMLName field records the XML element name. Don't
   180  		// process it as usual because its name should default to
   181  		// empty rather than to the field name.
   182  		finfo.name = tag
   183  		return finfo, nil
   184  	}
   185  
   186  	if tag == "" {
   187  		// If the name part of the tag is completely empty, get
   188  		// default from XMLName of underlying struct if feasible,
   189  		// or field name otherwise.
   190  		if xmlname := lookupXMLName(f.Type); xmlname != nil {
   191  			finfo.xmlns, finfo.name = xmlname.xmlns, xmlname.name
   192  		} else {
   193  			finfo.name = f.Name
   194  		}
   195  		return finfo, nil
   196  	}
   197  
   198  	// Prepare field name and parents.
   199  	parents := strings.Split(tag, ">")
   200  	if parents[0] == "" {
   201  		parents[0] = f.Name
   202  	}
   203  	if parents[len(parents)-1] == "" {
   204  		return nil, fmt.Errorf("xml: trailing '>' in field %s of type %s", f.Name, typ)
   205  	}
   206  	finfo.name = parents[len(parents)-1]
   207  	if len(parents) > 1 {
   208  		if (finfo.flags & fElement) == 0 {
   209  			return nil, fmt.Errorf("xml: %s chain not valid with %s flag", tag, strings.Join(tokens[1:], ","))
   210  		}
   211  		finfo.parents = parents[:len(parents)-1]
   212  	}
   213  
   214  	// If the field type has an XMLName field, the names must match
   215  	// so that the behavior of both marshaling and unmarshaling
   216  	// is straightforward and unambiguous.
   217  	if finfo.flags&fElement != 0 {
   218  		ftyp := f.Type
   219  		xmlname := lookupXMLName(ftyp)
   220  		if xmlname != nil && xmlname.name != finfo.name {
   221  			return nil, fmt.Errorf("xml: name %q in tag of %s.%s conflicts with name %q in %s.XMLName",
   222  				finfo.name, typ, f.Name, xmlname.name, ftyp)
   223  		}
   224  	}
   225  	return finfo, nil
   226  }
   227  
   228  // lookupXMLName returns the fieldInfo for typ's XMLName field
   229  // in case it exists and has a valid xml field tag, otherwise
   230  // it returns nil.
   231  func lookupXMLName(typ reflect.Type) (xmlname *fieldInfo) {
   232  	for typ.Kind() == reflect.Ptr {
   233  		typ = typ.Elem()
   234  	}
   235  	if typ.Kind() != reflect.Struct {
   236  		return nil
   237  	}
   238  	for i, n := 0, typ.NumField(); i < n; i++ {
   239  		f := typ.Field(i)
   240  		if f.Name != xmlName {
   241  			continue
   242  		}
   243  		finfo, err := structFieldInfo(typ, &f)
   244  		if err == nil && finfo.name != "" {
   245  			return finfo
   246  		}
   247  		// Also consider errors as a non-existent field tag
   248  		// and let getTypeInfo itself report the error.
   249  		break
   250  	}
   251  	return nil
   252  }
   253  
   254  func min(a, b int) int {
   255  	if a <= b {
   256  		return a
   257  	}
   258  	return b
   259  }
   260  
   261  // addFieldInfo adds finfo to tinfo.fields if there are no
   262  // conflicts, or if conflicts arise from previous fields that were
   263  // obtained from deeper embedded structures than finfo. In the latter
   264  // case, the conflicting entries are dropped.
   265  // A conflict occurs when the path (parent + name) to a field is
   266  // itself a prefix of another path, or when two paths match exactly.
   267  // It is okay for field paths to share a common, shorter prefix.
   268  func addFieldInfo(typ reflect.Type, tinfo *typeInfo, newf *fieldInfo) error {
   269  	var conflicts []int
   270  Loop:
   271  	// First, figure all conflicts. Most working code will have none.
   272  	for i := range tinfo.fields {
   273  		oldf := &tinfo.fields[i]
   274  		if oldf.flags&fMode != newf.flags&fMode {
   275  			continue
   276  		}
   277  		if oldf.xmlns != "" && newf.xmlns != "" && oldf.xmlns != newf.xmlns {
   278  			continue
   279  		}
   280  		minl := min(len(newf.parents), len(oldf.parents))
   281  		for p := 0; p < minl; p++ {
   282  			if oldf.parents[p] != newf.parents[p] {
   283  				continue Loop
   284  			}
   285  		}
   286  		if len(oldf.parents) > len(newf.parents) {
   287  			if oldf.parents[len(newf.parents)] == newf.name {
   288  				conflicts = append(conflicts, i)
   289  			}
   290  		} else if len(oldf.parents) < len(newf.parents) {
   291  			if newf.parents[len(oldf.parents)] == oldf.name {
   292  				conflicts = append(conflicts, i)
   293  			}
   294  		} else {
   295  			if newf.name == oldf.name {
   296  				conflicts = append(conflicts, i)
   297  			}
   298  		}
   299  	}
   300  	// Without conflicts, add the new field and return.
   301  	if conflicts == nil {
   302  		tinfo.fields = append(tinfo.fields, *newf)
   303  		return nil
   304  	}
   305  
   306  	// If any conflict is shallower, ignore the new field.
   307  	// This matches the Go field resolution on embedding.
   308  	for _, i := range conflicts {
   309  		if len(tinfo.fields[i].idx) < len(newf.idx) {
   310  			return nil
   311  		}
   312  	}
   313  
   314  	// Otherwise, if any of them is at the same depth level, it's an error.
   315  	for _, i := range conflicts {
   316  		oldf := &tinfo.fields[i]
   317  		if len(oldf.idx) == len(newf.idx) {
   318  			f1 := typ.FieldByIndex(oldf.idx)
   319  			f2 := typ.FieldByIndex(newf.idx)
   320  			return &TagPathError{typ, f1.Name, f1.Tag.Get("xml"), f2.Name, f2.Tag.Get("xml")}
   321  		}
   322  	}
   323  
   324  	// Otherwise, the new field is shallower, and thus takes precedence,
   325  	// so drop the conflicting fields from tinfo and append the new one.
   326  	for c := len(conflicts) - 1; c >= 0; c-- {
   327  		i := conflicts[c]
   328  		copy(tinfo.fields[i:], tinfo.fields[i+1:])
   329  		tinfo.fields = tinfo.fields[:len(tinfo.fields)-1]
   330  	}
   331  	tinfo.fields = append(tinfo.fields, *newf)
   332  	return nil
   333  }
   334  
   335  // A TagPathError represents an error in the unmarshaling process
   336  // caused by the use of field tags with conflicting paths.
   337  type TagPathError struct {
   338  	Struct       reflect.Type
   339  	Field1, Tag1 string
   340  	Field2, Tag2 string
   341  }
   342  
   343  func (e *TagPathError) Error() string {
   344  	return fmt.Sprintf("%s field %q with tag %q conflicts with field %q with tag %q", e.Struct, e.Field1, e.Tag1, e.Field2, e.Tag2)
   345  }
   346  
   347  // value returns v's field value corresponding to finfo.
   348  // It's equivalent to v.FieldByIndex(finfo.idx), but initializes
   349  // and dereferences pointers as necessary.
   350  func (finfo *fieldInfo) value(v reflect.Value) reflect.Value {
   351  	for i, x := range finfo.idx {
   352  		if i > 0 {
   353  			t := v.Type()
   354  			if t.Kind() == reflect.Ptr && t.Elem().Kind() == reflect.Struct {
   355  				if v.IsNil() {
   356  					v.Set(reflect.New(v.Type().Elem()))
   357  				}
   358  				v = v.Elem()
   359  			}
   360  		}
   361  		v = v.Field(x)
   362  	}
   363  	return v
   364  }
   365  

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