exec.go

     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 template
     6  
     7  import (
     8  	"errors"
     9  	"fmt"
    10  	"internal/fmtsort"
    11  	"io"
    12  	"reflect"
    13  	"runtime"
    14  	"strings"
    15  	"text/template/parse"
    16  )
    17  
    18  // maxExecDepth specifies the maximum stack depth of templates within
    19  // templates. This limit is only practically reached by accidentally
    20  // recursive template invocations. This limit allows us to return
    21  // an error instead of triggering a stack overflow.
    22  var maxExecDepth = initMaxExecDepth()
    23  
    24  func initMaxExecDepth() int {
    25  	if runtime.GOARCH == "wasm" {
    26  		return 1000
    27  	}
    28  	return 100000
    29  }
    30  
    31  // state represents the state of an execution. It's not part of the
    32  // template so that multiple executions of the same template
    33  // can execute in parallel.
    34  type state struct {
    35  	tmpl  *Template
    36  	wr    io.Writer
    37  	node  parse.Node // current node, for errors
    38  	vars  []variable // push-down stack of variable values.
    39  	depth int        // the height of the stack of executing templates.
    40  }
    41  
    42  // variable holds the dynamic value of a variable such as $, $x etc.
    43  type variable struct {
    44  	name  string
    45  	value reflect.Value
    46  }
    47  
    48  // push pushes a new variable on the stack.
    49  func (s *state) push(name string, value reflect.Value) {
    50  	s.vars = append(s.vars, variable{name, value})
    51  }
    52  
    53  // mark returns the length of the variable stack.
    54  func (s *state) mark() int {
    55  	return len(s.vars)
    56  }
    57  
    58  // pop pops the variable stack up to the mark.
    59  func (s *state) pop(mark int) {
    60  	s.vars = s.vars[0:mark]
    61  }
    62  
    63  // setVar overwrites the last declared variable with the given name.
    64  // Used by variable assignments.
    65  func (s *state) setVar(name string, value reflect.Value) {
    66  	for i := s.mark() - 1; i >= 0; i-- {
    67  		if s.vars[i].name == name {
    68  			s.vars[i].value = value
    69  			return
    70  		}
    71  	}
    72  	s.errorf("undefined variable: %s", name)
    73  }
    74  
    75  // setTopVar overwrites the top-nth variable on the stack. Used by range iterations.
    76  func (s *state) setTopVar(n int, value reflect.Value) {
    77  	s.vars[len(s.vars)-n].value = value
    78  }
    79  
    80  // varValue returns the value of the named variable.
    81  func (s *state) varValue(name string) reflect.Value {
    82  	for i := s.mark() - 1; i >= 0; i-- {
    83  		if s.vars[i].name == name {
    84  			return s.vars[i].value
    85  		}
    86  	}
    87  	s.errorf("undefined variable: %s", name)
    88  	return zero
    89  }
    90  
    91  var zero reflect.Value
    92  
    93  type missingValType struct{}
    94  
    95  var missingVal = reflect.ValueOf(missingValType{})
    96  
    97  var missingValReflectType = reflect.TypeFor[missingValType]()
    98  
    99  func isMissing(v reflect.Value) bool {
   100  	return v.IsValid() && v.Type() == missingValReflectType
   101  }
   102  
   103  // at marks the state to be on node n, for error reporting.
   104  func (s *state) at(node parse.Node) {
   105  	s.node = node
   106  }
   107  
   108  // doublePercent returns the string with %'s replaced by %%, if necessary,
   109  // so it can be used safely inside a Printf format string.
   110  func doublePercent(str string) string {
   111  	return strings.ReplaceAll(str, "%", "%%")
   112  }
   113  
   114  // TODO: It would be nice if ExecError was more broken down, but
   115  // the way ErrorContext embeds the template name makes the
   116  // processing too clumsy.
   117  
   118  // ExecError is the custom error type returned when Execute has an
   119  // error evaluating its template. (If a write error occurs, the actual
   120  // error is returned; it will not be of type ExecError.)
   121  type ExecError struct {
   122  	Name string // Name of template.
   123  	Err  error  // Pre-formatted error.
   124  }
   125  
   126  func (e ExecError) Error() string {
   127  	return e.Err.Error()
   128  }
   129  
   130  func (e ExecError) Unwrap() error {
   131  	return e.Err
   132  }
   133  
   134  // errorf records an ExecError and terminates processing.
   135  func (s *state) errorf(format string, args ...any) {
   136  	name := doublePercent(s.tmpl.Name())
   137  	if s.node == nil {
   138  		format = fmt.Sprintf("template: %s: %s", name, format)
   139  	} else {
   140  		location, context := s.tmpl.ErrorContext(s.node)
   141  		format = fmt.Sprintf("template: %s: executing %q at <%s>: %s", location, name, doublePercent(context), format)
   142  	}
   143  	panic(ExecError{
   144  		Name: s.tmpl.Name(),
   145  		Err:  fmt.Errorf(format, args...),
   146  	})
   147  }
   148  
   149  // writeError is the wrapper type used internally when Execute has an
   150  // error writing to its output. We strip the wrapper in errRecover.
   151  // Note that this is not an implementation of error, so it cannot escape
   152  // from the package as an error value.
   153  type writeError struct {
   154  	Err error // Original error.
   155  }
   156  
   157  func (s *state) writeError(err error) {
   158  	panic(writeError{
   159  		Err: err,
   160  	})
   161  }
   162  
   163  // errRecover is the handler that turns panics into returns from the top
   164  // level of Parse.
   165  func errRecover(errp *error) {
   166  	e := recover()
   167  	if e != nil {
   168  		switch err := e.(type) {
   169  		case runtime.Error:
   170  			panic(e)
   171  		case writeError:
   172  			*errp = err.Err // Strip the wrapper.
   173  		case ExecError:
   174  			*errp = err // Keep the wrapper.
   175  		default:
   176  			panic(e)
   177  		}
   178  	}
   179  }
   180  
   181  // ExecuteTemplate applies the template associated with t that has the given name
   182  // to the specified data object and writes the output to wr.
   183  // If an error occurs executing the template or writing its output,
   184  // execution stops, but partial results may already have been written to
   185  // the output writer.
   186  // A template may be executed safely in parallel, although if parallel
   187  // executions share a Writer the output may be interleaved.
   188  func (t *Template) ExecuteTemplate(wr io.Writer, name string, data any) error {
   189  	tmpl := t.Lookup(name)
   190  	if tmpl == nil {
   191  		return fmt.Errorf("template: no template %q associated with template %q", name, t.name)
   192  	}
   193  	return tmpl.Execute(wr, data)
   194  }
   195  
   196  // Execute applies a parsed template to the specified data object,
   197  // and writes the output to wr.
   198  // If an error occurs executing the template or writing its output,
   199  // execution stops, but partial results may already have been written to
   200  // the output writer.
   201  // A template may be executed safely in parallel, although if parallel
   202  // executions share a Writer the output may be interleaved.
   203  //
   204  // If data is a [reflect.Value], the template applies to the concrete
   205  // value that the reflect.Value holds, as in [fmt.Print].
   206  func (t *Template) Execute(wr io.Writer, data any) error {
   207  	return t.execute(wr, data)
   208  }
   209  
   210  func (t *Template) execute(wr io.Writer, data any) (err error) {
   211  	defer errRecover(&err)
   212  	value, ok := data.(reflect.Value)
   213  	if !ok {
   214  		value = reflect.ValueOf(data)
   215  	}
   216  	state := &state{
   217  		tmpl: t,
   218  		wr:   wr,
   219  		vars: []variable{{"$", value}},
   220  	}
   221  	if t.Tree == nil || t.Root == nil {
   222  		state.errorf("%q is an incomplete or empty template", t.Name())
   223  	}
   224  	state.walk(value, t.Root)
   225  	return
   226  }
   227  
   228  // DefinedTemplates returns a string listing the defined templates,
   229  // prefixed by the string "; defined templates are: ". If there are none,
   230  // it returns the empty string. For generating an error message here
   231  // and in [html/template].
   232  func (t *Template) DefinedTemplates() string {
   233  	if t.common == nil {
   234  		return ""
   235  	}
   236  	var b strings.Builder
   237  	t.muTmpl.RLock()
   238  	defer t.muTmpl.RUnlock()
   239  	for name, tmpl := range t.tmpl {
   240  		if tmpl.Tree == nil || tmpl.Root == nil {
   241  			continue
   242  		}
   243  		if b.Len() == 0 {
   244  			b.WriteString("; defined templates are: ")
   245  		} else {
   246  			b.WriteString(", ")
   247  		}
   248  		fmt.Fprintf(&b, "%q", name)
   249  	}
   250  	return b.String()
   251  }
   252  
   253  // Sentinel errors for use with panic to signal early exits from range loops.
   254  var (
   255  	walkBreak    = errors.New("break")
   256  	walkContinue = errors.New("continue")
   257  )
   258  
   259  // Walk functions step through the major pieces of the template structure,
   260  // generating output as they go.
   261  func (s *state) walk(dot reflect.Value, node parse.Node) {
   262  	s.at(node)
   263  	switch node := node.(type) {
   264  	case *parse.ActionNode:
   265  		// Do not pop variables so they persist until next end.
   266  		// Also, if the action declares variables, don't print the result.
   267  		val := s.evalPipeline(dot, node.Pipe)
   268  		if len(node.Pipe.Decl) == 0 {
   269  			s.printValue(node, val)
   270  		}
   271  	case *parse.BreakNode:
   272  		panic(walkBreak)
   273  	case *parse.CommentNode:
   274  	case *parse.ContinueNode:
   275  		panic(walkContinue)
   276  	case *parse.IfNode:
   277  		s.walkIfOrWith(parse.NodeIf, dot, node.Pipe, node.List, node.ElseList)
   278  	case *parse.ListNode:
   279  		for _, node := range node.Nodes {
   280  			s.walk(dot, node)
   281  		}
   282  	case *parse.RangeNode:
   283  		s.walkRange(dot, node)
   284  	case *parse.TemplateNode:
   285  		s.walkTemplate(dot, node)
   286  	case *parse.TextNode:
   287  		if _, err := s.wr.Write(node.Text); err != nil {
   288  			s.writeError(err)
   289  		}
   290  	case *parse.WithNode:
   291  		s.walkIfOrWith(parse.NodeWith, dot, node.Pipe, node.List, node.ElseList)
   292  	default:
   293  		s.errorf("unknown node: %s", node)
   294  	}
   295  }
   296  
   297  // walkIfOrWith walks an 'if' or 'with' node. The two control structures
   298  // are identical in behavior except that 'with' sets dot.
   299  func (s *state) walkIfOrWith(typ parse.NodeType, dot reflect.Value, pipe *parse.PipeNode, list, elseList *parse.ListNode) {
   300  	defer s.pop(s.mark())
   301  	val := s.evalPipeline(dot, pipe)
   302  	truth, ok := isTrue(indirectInterface(val))
   303  	if !ok {
   304  		s.errorf("if/with can't use %v", val)
   305  	}
   306  	if truth {
   307  		if typ == parse.NodeWith {
   308  			s.walk(val, list)
   309  		} else {
   310  			s.walk(dot, list)
   311  		}
   312  	} else if elseList != nil {
   313  		s.walk(dot, elseList)
   314  	}
   315  }
   316  
   317  // IsTrue reports whether the value is 'true', in the sense of not the zero of its type,
   318  // and whether the value has a meaningful truth value. This is the definition of
   319  // truth used by if and other such actions.
   320  func IsTrue(val any) (truth, ok bool) {
   321  	return isTrue(reflect.ValueOf(val))
   322  }
   323  
   324  func isTrue(val reflect.Value) (truth, ok bool) {
   325  	if !val.IsValid() {
   326  		// Something like var x interface{}, never set. It's a form of nil.
   327  		return false, true
   328  	}
   329  	switch val.Kind() {
   330  	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
   331  		truth = val.Len() > 0
   332  	case reflect.Bool:
   333  		truth = val.Bool()
   334  	case reflect.Complex64, reflect.Complex128:
   335  		truth = val.Complex() != 0
   336  	case reflect.Chan, reflect.Func, reflect.Pointer, reflect.Interface:
   337  		truth = !val.IsNil()
   338  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   339  		truth = val.Int() != 0
   340  	case reflect.Float32, reflect.Float64:
   341  		truth = val.Float() != 0
   342  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   343  		truth = val.Uint() != 0
   344  	case reflect.Struct:
   345  		truth = true // Struct values are always true.
   346  	default:
   347  		return
   348  	}
   349  	return truth, true
   350  }
   351  
   352  func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
   353  	s.at(r)
   354  	defer func() {
   355  		if r := recover(); r != nil && r != walkBreak {
   356  			panic(r)
   357  		}
   358  	}()
   359  	defer s.pop(s.mark())
   360  	val, _ := indirect(s.evalPipeline(dot, r.Pipe))
   361  	// mark top of stack before any variables in the body are pushed.
   362  	mark := s.mark()
   363  	oneIteration := func(index, elem reflect.Value) {
   364  		if len(r.Pipe.Decl) > 0 {
   365  			if r.Pipe.IsAssign {
   366  				// With two variables, index comes first.
   367  				// With one, we use the element.
   368  				if len(r.Pipe.Decl) > 1 {
   369  					s.setVar(r.Pipe.Decl[0].Ident[0], index)
   370  				} else {
   371  					s.setVar(r.Pipe.Decl[0].Ident[0], elem)
   372  				}
   373  			} else {
   374  				// Set top var (lexically the second if there
   375  				// are two) to the element.
   376  				s.setTopVar(1, elem)
   377  			}
   378  		}
   379  		if len(r.Pipe.Decl) > 1 {
   380  			if r.Pipe.IsAssign {
   381  				s.setVar(r.Pipe.Decl[1].Ident[0], elem)
   382  			} else {
   383  				// Set next var (lexically the first if there
   384  				// are two) to the index.
   385  				s.setTopVar(2, index)
   386  			}
   387  		}
   388  		defer s.pop(mark)
   389  		defer func() {
   390  			// Consume panic(walkContinue)
   391  			if r := recover(); r != nil && r != walkContinue {
   392  				panic(r)
   393  			}
   394  		}()
   395  		s.walk(elem, r.List)
   396  	}
   397  	switch val.Kind() {
   398  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
   399  		reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   400  		if len(r.Pipe.Decl) > 1 {
   401  			s.errorf("can't use %v to iterate over more than one variable", val)
   402  			break
   403  		}
   404  		run := false
   405  		for v := range val.Seq() {
   406  			run = true
   407  			// Pass element as second value, as we do for channels.
   408  			oneIteration(reflect.Value{}, v)
   409  		}
   410  		if !run {
   411  			break
   412  		}
   413  		return
   414  	case reflect.Array, reflect.Slice:
   415  		if val.Len() == 0 {
   416  			break
   417  		}
   418  		for i := 0; i < val.Len(); i++ {
   419  			oneIteration(reflect.ValueOf(i), val.Index(i))
   420  		}
   421  		return
   422  	case reflect.Map:
   423  		if val.Len() == 0 {
   424  			break
   425  		}
   426  		om := fmtsort.Sort(val)
   427  		for _, m := range om {
   428  			oneIteration(m.Key, m.Value)
   429  		}
   430  		return
   431  	case reflect.Chan:
   432  		if val.IsNil() {
   433  			break
   434  		}
   435  		if val.Type().ChanDir() == reflect.SendDir {
   436  			s.errorf("range over send-only channel %v", val)
   437  			break
   438  		}
   439  		i := 0
   440  		for ; ; i++ {
   441  			elem, ok := val.Recv()
   442  			if !ok {
   443  				break
   444  			}
   445  			oneIteration(reflect.ValueOf(i), elem)
   446  		}
   447  		if i == 0 {
   448  			break
   449  		}
   450  		return
   451  	case reflect.Invalid:
   452  		break // An invalid value is likely a nil map, etc. and acts like an empty map.
   453  	case reflect.Func:
   454  		if val.Type().CanSeq() {
   455  			if len(r.Pipe.Decl) > 1 {
   456  				s.errorf("can't use %v iterate over more than one variable", val)
   457  				break
   458  			}
   459  			run := false
   460  			for v := range val.Seq() {
   461  				run = true
   462  				// Pass element as second value,
   463  				// as we do for channels.
   464  				oneIteration(reflect.Value{}, v)
   465  			}
   466  			if !run {
   467  				break
   468  			}
   469  			return
   470  		}
   471  		if val.Type().CanSeq2() {
   472  			run := false
   473  			for i, v := range val.Seq2() {
   474  				run = true
   475  				if len(r.Pipe.Decl) > 1 {
   476  					oneIteration(i, v)
   477  				} else {
   478  					// If there is only one range variable,
   479  					// oneIteration will use the
   480  					// second value.
   481  					oneIteration(reflect.Value{}, i)
   482  				}
   483  			}
   484  			if !run {
   485  				break
   486  			}
   487  			return
   488  		}
   489  		fallthrough
   490  	default:
   491  		s.errorf("range can't iterate over %v", val)
   492  	}
   493  	if r.ElseList != nil {
   494  		s.walk(dot, r.ElseList)
   495  	}
   496  }
   497  
   498  func (s *state) walkTemplate(dot reflect.Value, t *parse.TemplateNode) {
   499  	s.at(t)
   500  	tmpl := s.tmpl.Lookup(t.Name)
   501  	if tmpl == nil {
   502  		s.errorf("template %q not defined", t.Name)
   503  	}
   504  	if s.depth == maxExecDepth {
   505  		s.errorf("exceeded maximum template depth (%v)", maxExecDepth)
   506  	}
   507  	// Variables declared by the pipeline persist.
   508  	dot = s.evalPipeline(dot, t.Pipe)
   509  	newState := *s
   510  	newState.depth++
   511  	newState.tmpl = tmpl
   512  	// No dynamic scoping: template invocations inherit no variables.
   513  	newState.vars = []variable{{"$", dot}}
   514  	newState.walk(dot, tmpl.Root)
   515  }
   516  
   517  // Eval functions evaluate pipelines, commands, and their elements and extract
   518  // values from the data structure by examining fields, calling methods, and so on.
   519  // The printing of those values happens only through walk functions.
   520  
   521  // evalPipeline returns the value acquired by evaluating a pipeline. If the
   522  // pipeline has a variable declaration, the variable will be pushed on the
   523  // stack. Callers should therefore pop the stack after they are finished
   524  // executing commands depending on the pipeline value.
   525  func (s *state) evalPipeline(dot reflect.Value, pipe *parse.PipeNode) (value reflect.Value) {
   526  	if pipe == nil {
   527  		return
   528  	}
   529  	s.at(pipe)
   530  	value = missingVal
   531  	for _, cmd := range pipe.Cmds {
   532  		value = s.evalCommand(dot, cmd, value) // previous value is this one's final arg.
   533  		// If the object has type interface{}, dig down one level to the thing inside.
   534  		if value.Kind() == reflect.Interface && value.Type().NumMethod() == 0 {
   535  			value = value.Elem()
   536  		}
   537  	}
   538  	for _, variable := range pipe.Decl {
   539  		if pipe.IsAssign {
   540  			s.setVar(variable.Ident[0], value)
   541  		} else {
   542  			s.push(variable.Ident[0], value)
   543  		}
   544  	}
   545  	return value
   546  }
   547  
   548  func (s *state) notAFunction(args []parse.Node, final reflect.Value) {
   549  	if len(args) > 1 || !isMissing(final) {
   550  		s.errorf("can't give argument to non-function %s", args[0])
   551  	}
   552  }
   553  
   554  func (s *state) evalCommand(dot reflect.Value, cmd *parse.CommandNode, final reflect.Value) reflect.Value {
   555  	firstWord := cmd.Args[0]
   556  	switch n := firstWord.(type) {
   557  	case *parse.FieldNode:
   558  		return s.evalFieldNode(dot, n, cmd.Args, final)
   559  	case *parse.ChainNode:
   560  		return s.evalChainNode(dot, n, cmd.Args, final)
   561  	case *parse.IdentifierNode:
   562  		// Must be a function.
   563  		return s.evalFunction(dot, n, cmd, cmd.Args, final)
   564  	case *parse.PipeNode:
   565  		// Parenthesized pipeline. The arguments are all inside the pipeline; final must be absent.
   566  		s.notAFunction(cmd.Args, final)
   567  		return s.evalPipeline(dot, n)
   568  	case *parse.VariableNode:
   569  		return s.evalVariableNode(dot, n, cmd.Args, final)
   570  	}
   571  	s.at(firstWord)
   572  	s.notAFunction(cmd.Args, final)
   573  	switch word := firstWord.(type) {
   574  	case *parse.BoolNode:
   575  		return reflect.ValueOf(word.True)
   576  	case *parse.DotNode:
   577  		return dot
   578  	case *parse.NilNode:
   579  		s.errorf("nil is not a command")
   580  	case *parse.NumberNode:
   581  		return s.idealConstant(word)
   582  	case *parse.StringNode:
   583  		return reflect.ValueOf(word.Text)
   584  	}
   585  	s.errorf("can't evaluate command %q", firstWord)
   586  	panic("not reached")
   587  }
   588  
   589  // idealConstant is called to return the value of a number in a context where
   590  // we don't know the type. In that case, the syntax of the number tells us
   591  // its type, and we use Go rules to resolve. Note there is no such thing as
   592  // a uint ideal constant in this situation - the value must be of int type.
   593  func (s *state) idealConstant(constant *parse.NumberNode) reflect.Value {
   594  	// These are ideal constants but we don't know the type
   595  	// and we have no context.  (If it was a method argument,
   596  	// we'd know what we need.) The syntax guides us to some extent.
   597  	s.at(constant)
   598  	switch {
   599  	case constant.IsComplex:
   600  		return reflect.ValueOf(constant.Complex128) // incontrovertible.
   601  
   602  	case constant.IsFloat &&
   603  		!isHexInt(constant.Text) && !isRuneInt(constant.Text) &&
   604  		strings.ContainsAny(constant.Text, ".eEpP"):
   605  		return reflect.ValueOf(constant.Float64)
   606  
   607  	case constant.IsInt:
   608  		n := int(constant.Int64)
   609  		if int64(n) != constant.Int64 {
   610  			s.errorf("%s overflows int", constant.Text)
   611  		}
   612  		return reflect.ValueOf(n)
   613  
   614  	case constant.IsUint:
   615  		s.errorf("%s overflows int", constant.Text)
   616  	}
   617  	return zero
   618  }
   619  
   620  func isRuneInt(s string) bool {
   621  	return len(s) > 0 && s[0] == '\''
   622  }
   623  
   624  func isHexInt(s string) bool {
   625  	return len(s) > 2 && s[0] == '0' && (s[1] == 'x' || s[1] == 'X') && !strings.ContainsAny(s, "pP")
   626  }
   627  
   628  func (s *state) evalFieldNode(dot reflect.Value, field *parse.FieldNode, args []parse.Node, final reflect.Value) reflect.Value {
   629  	s.at(field)
   630  	return s.evalFieldChain(dot, dot, field, field.Ident, args, final)
   631  }
   632  
   633  func (s *state) evalChainNode(dot reflect.Value, chain *parse.ChainNode, args []parse.Node, final reflect.Value) reflect.Value {
   634  	s.at(chain)
   635  	if len(chain.Field) == 0 {
   636  		s.errorf("internal error: no fields in evalChainNode")
   637  	}
   638  	if chain.Node.Type() == parse.NodeNil {
   639  		s.errorf("indirection through explicit nil in %s", chain)
   640  	}
   641  	// (pipe).Field1.Field2 has pipe as .Node, fields as .Field. Eval the pipeline, then the fields.
   642  	pipe := s.evalArg(dot, nil, chain.Node)
   643  	return s.evalFieldChain(dot, pipe, chain, chain.Field, args, final)
   644  }
   645  
   646  func (s *state) evalVariableNode(dot reflect.Value, variable *parse.VariableNode, args []parse.Node, final reflect.Value) reflect.Value {
   647  	// $x.Field has $x as the first ident, Field as the second. Eval the var, then the fields.
   648  	s.at(variable)
   649  	value := s.varValue(variable.Ident[0])
   650  	if len(variable.Ident) == 1 {
   651  		s.notAFunction(args, final)
   652  		return value
   653  	}
   654  	return s.evalFieldChain(dot, value, variable, variable.Ident[1:], args, final)
   655  }
   656  
   657  // evalFieldChain evaluates .X.Y.Z possibly followed by arguments.
   658  // dot is the environment in which to evaluate arguments, while
   659  // receiver is the value being walked along the chain.
   660  func (s *state) evalFieldChain(dot, receiver reflect.Value, node parse.Node, ident []string, args []parse.Node, final reflect.Value) reflect.Value {
   661  	n := len(ident)
   662  	for i := 0; i < n-1; i++ {
   663  		receiver = s.evalField(dot, ident[i], node, nil, missingVal, receiver)
   664  	}
   665  	// Now if it's a method, it gets the arguments.
   666  	return s.evalField(dot, ident[n-1], node, args, final, receiver)
   667  }
   668  
   669  func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd parse.Node, args []parse.Node, final reflect.Value) reflect.Value {
   670  	s.at(node)
   671  	name := node.Ident
   672  	function, isBuiltin, ok := findFunction(name, s.tmpl)
   673  	if !ok {
   674  		s.errorf("%q is not a defined function", name)
   675  	}
   676  	return s.evalCall(dot, function, isBuiltin, cmd, name, args, final)
   677  }
   678  
   679  // evalField evaluates an expression like (.Field) or (.Field arg1 arg2).
   680  // The 'final' argument represents the return value from the preceding
   681  // value of the pipeline, if any.
   682  func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node, args []parse.Node, final, receiver reflect.Value) reflect.Value {
   683  	if !receiver.IsValid() {
   684  		if s.tmpl.option.missingKey == mapError { // Treat invalid value as missing map key.
   685  			s.errorf("nil data; no entry for key %q", fieldName)
   686  		}
   687  		return zero
   688  	}
   689  	typ := receiver.Type()
   690  	receiver, isNil := indirect(receiver)
   691  	if receiver.Kind() == reflect.Interface && isNil {
   692  		// Calling a method on a nil interface can't work. The
   693  		// MethodByName method call below would panic.
   694  		s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
   695  		return zero
   696  	}
   697  
   698  	// Unless it's an interface, need to get to a value of type *T to guarantee
   699  	// we see all methods of T and *T.
   700  	ptr := receiver
   701  	if ptr.Kind() != reflect.Interface && ptr.Kind() != reflect.Pointer && ptr.CanAddr() {
   702  		ptr = ptr.Addr()
   703  	}
   704  	if method := ptr.MethodByName(fieldName); method.IsValid() {
   705  		return s.evalCall(dot, method, false, node, fieldName, args, final)
   706  	}
   707  	hasArgs := len(args) > 1 || !isMissing(final)
   708  	// It's not a method; must be a field of a struct or an element of a map.
   709  	switch receiver.Kind() {
   710  	case reflect.Struct:
   711  		tField, ok := receiver.Type().FieldByName(fieldName)
   712  		if ok {
   713  			field, err := receiver.FieldByIndexErr(tField.Index)
   714  			if !tField.IsExported() {
   715  				s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
   716  			}
   717  			if err != nil {
   718  				s.errorf("%v", err)
   719  			}
   720  			// If it's a function, we must call it.
   721  			if hasArgs {
   722  				s.errorf("%s has arguments but cannot be invoked as function", fieldName)
   723  			}
   724  			return field
   725  		}
   726  	case reflect.Map:
   727  		// If it's a map, attempt to use the field name as a key.
   728  		nameVal := reflect.ValueOf(fieldName)
   729  		if nameVal.Type().AssignableTo(receiver.Type().Key()) {
   730  			if hasArgs {
   731  				s.errorf("%s is not a method but has arguments", fieldName)
   732  			}
   733  			result := receiver.MapIndex(nameVal)
   734  			if !result.IsValid() {
   735  				switch s.tmpl.option.missingKey {
   736  				case mapInvalid:
   737  					// Just use the invalid value.
   738  				case mapZeroValue:
   739  					result = reflect.Zero(receiver.Type().Elem())
   740  				case mapError:
   741  					s.errorf("map has no entry for key %q", fieldName)
   742  				}
   743  			}
   744  			return result
   745  		}
   746  	case reflect.Pointer:
   747  		etyp := receiver.Type().Elem()
   748  		if etyp.Kind() == reflect.Struct {
   749  			if _, ok := etyp.FieldByName(fieldName); !ok {
   750  				// If there's no such field, say "can't evaluate"
   751  				// instead of "nil pointer evaluating".
   752  				break
   753  			}
   754  		}
   755  		if isNil {
   756  			s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
   757  		}
   758  	}
   759  	s.errorf("can't evaluate field %s in type %s", fieldName, typ)
   760  	panic("not reached")
   761  }
   762  
   763  var (
   764  	errorType        = reflect.TypeFor[error]()
   765  	fmtStringerType  = reflect.TypeFor[fmt.Stringer]()
   766  	reflectValueType = reflect.TypeFor[reflect.Value]()
   767  )
   768  
   769  // evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
   770  // it looks just like a function call. The arg list, if non-nil, includes (in the manner of the shell), arg[0]
   771  // as the function itself.
   772  func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node, name string, args []parse.Node, final reflect.Value) reflect.Value {
   773  	if args != nil {
   774  		args = args[1:] // Zeroth arg is function name/node; not passed to function.
   775  	}
   776  	typ := fun.Type()
   777  	numIn := len(args)
   778  	if !isMissing(final) {
   779  		numIn++
   780  	}
   781  	numFixed := len(args)
   782  	if typ.IsVariadic() {
   783  		numFixed = typ.NumIn() - 1 // last arg is the variadic one.
   784  		if numIn < numFixed {
   785  			s.errorf("wrong number of args for %s: want at least %d got %d", name, typ.NumIn()-1, len(args))
   786  		}
   787  	} else if numIn != typ.NumIn() {
   788  		s.errorf("wrong number of args for %s: want %d got %d", name, typ.NumIn(), numIn)
   789  	}
   790  	if err := goodFunc(name, typ); err != nil {
   791  		s.errorf("%v", err)
   792  	}
   793  
   794  	unwrap := func(v reflect.Value) reflect.Value {
   795  		if v.Type() == reflectValueType {
   796  			v = v.Interface().(reflect.Value)
   797  		}
   798  		return v
   799  	}
   800  
   801  	// Special case for builtin and/or, which short-circuit.
   802  	if isBuiltin && (name == "and" || name == "or") {
   803  		argType := typ.In(0)
   804  		var v reflect.Value
   805  		for _, arg := range args {
   806  			v = s.evalArg(dot, argType, arg).Interface().(reflect.Value)
   807  			if truth(v) == (name == "or") {
   808  				// This value was already unwrapped
   809  				// by the .Interface().(reflect.Value).
   810  				return v
   811  			}
   812  		}
   813  		if !final.Equal(missingVal) {
   814  			// The last argument to and/or is coming from
   815  			// the pipeline. We didn't short circuit on an earlier
   816  			// argument, so we are going to return this one.
   817  			// We don't have to evaluate final, but we do
   818  			// have to check its type. Then, since we are
   819  			// going to return it, we have to unwrap it.
   820  			v = unwrap(s.validateType(final, argType))
   821  		}
   822  		return v
   823  	}
   824  
   825  	// Build the arg list.
   826  	argv := make([]reflect.Value, numIn)
   827  	// Args must be evaluated. Fixed args first.
   828  	i := 0
   829  	for ; i < numFixed && i < len(args); i++ {
   830  		argv[i] = s.evalArg(dot, typ.In(i), args[i])
   831  	}
   832  	// Now the ... args.
   833  	if typ.IsVariadic() {
   834  		argType := typ.In(typ.NumIn() - 1).Elem() // Argument is a slice.
   835  		for ; i < len(args); i++ {
   836  			argv[i] = s.evalArg(dot, argType, args[i])
   837  		}
   838  	}
   839  	// Add final value if necessary.
   840  	if !isMissing(final) {
   841  		t := typ.In(typ.NumIn() - 1)
   842  		if typ.IsVariadic() {
   843  			if numIn-1 < numFixed {
   844  				// The added final argument corresponds to a fixed parameter of the function.
   845  				// Validate against the type of the actual parameter.
   846  				t = typ.In(numIn - 1)
   847  			} else {
   848  				// The added final argument corresponds to the variadic part.
   849  				// Validate against the type of the elements of the variadic slice.
   850  				t = t.Elem()
   851  			}
   852  		}
   853  		argv[i] = s.validateType(final, t)
   854  	}
   855  
   856  	// Special case for the "call" builtin.
   857  	// Insert the name of the callee function as the first argument.
   858  	if isBuiltin && name == "call" {
   859  		calleeName := args[0].String()
   860  		argv = append([]reflect.Value{reflect.ValueOf(calleeName)}, argv...)
   861  		fun = reflect.ValueOf(call)
   862  	}
   863  
   864  	v, err := safeCall(fun, argv)
   865  	// If we have an error that is not nil, stop execution and return that
   866  	// error to the caller.
   867  	if err != nil {
   868  		s.at(node)
   869  		s.errorf("error calling %s: %w", name, err)
   870  	}
   871  	return unwrap(v)
   872  }
   873  
   874  // canBeNil reports whether an untyped nil can be assigned to the type. See reflect.Zero.
   875  func canBeNil(typ reflect.Type) bool {
   876  	switch typ.Kind() {
   877  	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
   878  		return true
   879  	case reflect.Struct:
   880  		return typ == reflectValueType
   881  	}
   882  	return false
   883  }
   884  
   885  // validateType guarantees that the value is valid and assignable to the type.
   886  func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
   887  	if !value.IsValid() {
   888  		if typ == nil {
   889  			// An untyped nil interface{}. Accept as a proper nil value.
   890  			return reflect.ValueOf(nil)
   891  		}
   892  		if canBeNil(typ) {
   893  			// Like above, but use the zero value of the non-nil type.
   894  			return reflect.Zero(typ)
   895  		}
   896  		s.errorf("invalid value; expected %s", typ)
   897  	}
   898  	if typ == reflectValueType && value.Type() != typ {
   899  		return reflect.ValueOf(value)
   900  	}
   901  	if typ != nil && !value.Type().AssignableTo(typ) {
   902  		if value.Kind() == reflect.Interface && !value.IsNil() {
   903  			value = value.Elem()
   904  			if value.Type().AssignableTo(typ) {
   905  				return value
   906  			}
   907  			// fallthrough
   908  		}
   909  		// Does one dereference or indirection work? We could do more, as we
   910  		// do with method receivers, but that gets messy and method receivers
   911  		// are much more constrained, so it makes more sense there than here.
   912  		// Besides, one is almost always all you need.
   913  		switch {
   914  		case value.Kind() == reflect.Pointer && value.Type().Elem().AssignableTo(typ):
   915  			value = value.Elem()
   916  			if !value.IsValid() {
   917  				s.errorf("dereference of nil pointer of type %s", typ)
   918  			}
   919  		case reflect.PointerTo(value.Type()).AssignableTo(typ) && value.CanAddr():
   920  			value = value.Addr()
   921  		default:
   922  			s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
   923  		}
   924  	}
   925  	return value
   926  }
   927  
   928  func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) reflect.Value {
   929  	s.at(n)
   930  	switch arg := n.(type) {
   931  	case *parse.DotNode:
   932  		return s.validateType(dot, typ)
   933  	case *parse.NilNode:
   934  		if canBeNil(typ) {
   935  			return reflect.Zero(typ)
   936  		}
   937  		s.errorf("cannot assign nil to %s", typ)
   938  	case *parse.FieldNode:
   939  		return s.validateType(s.evalFieldNode(dot, arg, []parse.Node{n}, missingVal), typ)
   940  	case *parse.VariableNode:
   941  		return s.validateType(s.evalVariableNode(dot, arg, nil, missingVal), typ)
   942  	case *parse.PipeNode:
   943  		return s.validateType(s.evalPipeline(dot, arg), typ)
   944  	case *parse.IdentifierNode:
   945  		return s.validateType(s.evalFunction(dot, arg, arg, nil, missingVal), typ)
   946  	case *parse.ChainNode:
   947  		return s.validateType(s.evalChainNode(dot, arg, nil, missingVal), typ)
   948  	}
   949  	switch typ.Kind() {
   950  	case reflect.Bool:
   951  		return s.evalBool(typ, n)
   952  	case reflect.Complex64, reflect.Complex128:
   953  		return s.evalComplex(typ, n)
   954  	case reflect.Float32, reflect.Float64:
   955  		return s.evalFloat(typ, n)
   956  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   957  		return s.evalInteger(typ, n)
   958  	case reflect.Interface:
   959  		if typ.NumMethod() == 0 {
   960  			return s.evalEmptyInterface(dot, n)
   961  		}
   962  	case reflect.Struct:
   963  		if typ == reflectValueType {
   964  			return reflect.ValueOf(s.evalEmptyInterface(dot, n))
   965  		}
   966  	case reflect.String:
   967  		return s.evalString(typ, n)
   968  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   969  		return s.evalUnsignedInteger(typ, n)
   970  	}
   971  	s.errorf("can't handle %s for arg of type %s", n, typ)
   972  	panic("not reached")
   973  }
   974  
   975  func (s *state) evalBool(typ reflect.Type, n parse.Node) reflect.Value {
   976  	s.at(n)
   977  	if n, ok := n.(*parse.BoolNode); ok {
   978  		value := reflect.New(typ).Elem()
   979  		value.SetBool(n.True)
   980  		return value
   981  	}
   982  	s.errorf("expected bool; found %s", n)
   983  	panic("not reached")
   984  }
   985  
   986  func (s *state) evalString(typ reflect.Type, n parse.Node) reflect.Value {
   987  	s.at(n)
   988  	if n, ok := n.(*parse.StringNode); ok {
   989  		value := reflect.New(typ).Elem()
   990  		value.SetString(n.Text)
   991  		return value
   992  	}
   993  	s.errorf("expected string; found %s", n)
   994  	panic("not reached")
   995  }
   996  
   997  func (s *state) evalInteger(typ reflect.Type, n parse.Node) reflect.Value {
   998  	s.at(n)
   999  	if n, ok := n.(*parse.NumberNode); ok && n.IsInt {
  1000  		value := reflect.New(typ).Elem()
  1001  		value.SetInt(n.Int64)
  1002  		return value
  1003  	}
  1004  	s.errorf("expected integer; found %s", n)
  1005  	panic("not reached")
  1006  }
  1007  
  1008  func (s *state) evalUnsignedInteger(typ reflect.Type, n parse.Node) reflect.Value {
  1009  	s.at(n)
  1010  	if n, ok := n.(*parse.NumberNode); ok && n.IsUint {
  1011  		value := reflect.New(typ).Elem()
  1012  		value.SetUint(n.Uint64)
  1013  		return value
  1014  	}
  1015  	s.errorf("expected unsigned integer; found %s", n)
  1016  	panic("not reached")
  1017  }
  1018  
  1019  func (s *state) evalFloat(typ reflect.Type, n parse.Node) reflect.Value {
  1020  	s.at(n)
  1021  	if n, ok := n.(*parse.NumberNode); ok && n.IsFloat {
  1022  		value := reflect.New(typ).Elem()
  1023  		value.SetFloat(n.Float64)
  1024  		return value
  1025  	}
  1026  	s.errorf("expected float; found %s", n)
  1027  	panic("not reached")
  1028  }
  1029  
  1030  func (s *state) evalComplex(typ reflect.Type, n parse.Node) reflect.Value {
  1031  	if n, ok := n.(*parse.NumberNode); ok && n.IsComplex {
  1032  		value := reflect.New(typ).Elem()
  1033  		value.SetComplex(n.Complex128)
  1034  		return value
  1035  	}
  1036  	s.errorf("expected complex; found %s", n)
  1037  	panic("not reached")
  1038  }
  1039  
  1040  func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Value {
  1041  	s.at(n)
  1042  	switch n := n.(type) {
  1043  	case *parse.BoolNode:
  1044  		return reflect.ValueOf(n.True)
  1045  	case *parse.DotNode:
  1046  		return dot
  1047  	case *parse.FieldNode:
  1048  		return s.evalFieldNode(dot, n, nil, missingVal)
  1049  	case *parse.IdentifierNode:
  1050  		return s.evalFunction(dot, n, n, nil, missingVal)
  1051  	case *parse.NilNode:
  1052  		// NilNode is handled in evalArg, the only place that calls here.
  1053  		s.errorf("evalEmptyInterface: nil (can't happen)")
  1054  	case *parse.NumberNode:
  1055  		return s.idealConstant(n)
  1056  	case *parse.StringNode:
  1057  		return reflect.ValueOf(n.Text)
  1058  	case *parse.VariableNode:
  1059  		return s.evalVariableNode(dot, n, nil, missingVal)
  1060  	case *parse.PipeNode:
  1061  		return s.evalPipeline(dot, n)
  1062  	}
  1063  	s.errorf("can't handle assignment of %s to empty interface argument", n)
  1064  	panic("not reached")
  1065  }
  1066  
  1067  // indirect returns the item at the end of indirection, and a bool to indicate
  1068  // if it's nil. If the returned bool is true, the returned value's kind will be
  1069  // either a pointer or interface.
  1070  func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
  1071  	for ; v.Kind() == reflect.Pointer || v.Kind() == reflect.Interface; v = v.Elem() {
  1072  		if v.IsNil() {
  1073  			return v, true
  1074  		}
  1075  	}
  1076  	return v, false
  1077  }
  1078  
  1079  // indirectInterface returns the concrete value in an interface value,
  1080  // or else the zero reflect.Value.
  1081  // That is, if v represents the interface value x, the result is the same as reflect.ValueOf(x):
  1082  // the fact that x was an interface value is forgotten.
  1083  func indirectInterface(v reflect.Value) reflect.Value {
  1084  	if v.Kind() != reflect.Interface {
  1085  		return v
  1086  	}
  1087  	if v.IsNil() {
  1088  		return reflect.Value{}
  1089  	}
  1090  	return v.Elem()
  1091  }
  1092  
  1093  // printValue writes the textual representation of the value to the output of
  1094  // the template.
  1095  func (s *state) printValue(n parse.Node, v reflect.Value) {
  1096  	s.at(n)
  1097  	iface, ok := printableValue(v)
  1098  	if !ok {
  1099  		s.errorf("can't print %s of type %s", n, v.Type())
  1100  	}
  1101  	_, err := fmt.Fprint(s.wr, iface)
  1102  	if err != nil {
  1103  		s.writeError(err)
  1104  	}
  1105  }
  1106  
  1107  // printableValue returns the, possibly indirected, interface value inside v that
  1108  // is best for a call to formatted printer.
  1109  func printableValue(v reflect.Value) (any, bool) {
  1110  	if v.Kind() == reflect.Pointer {
  1111  		v, _ = indirect(v) // fmt.Fprint handles nil.
  1112  	}
  1113  	if !v.IsValid() {
  1114  		return "<no value>", true
  1115  	}
  1116  
  1117  	if !v.Type().Implements(errorType) && !v.Type().Implements(fmtStringerType) {
  1118  		if v.CanAddr() && (reflect.PointerTo(v.Type()).Implements(errorType) || reflect.PointerTo(v.Type()).Implements(fmtStringerType)) {
  1119  			v = v.Addr()
  1120  		} else {
  1121  			switch v.Kind() {
  1122  			case reflect.Chan, reflect.Func:
  1123  				return nil, false
  1124  			}
  1125  		}
  1126  	}
  1127  	return v.Interface(), true
  1128  }
  1129
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