Source file src/text/template/exec.go

Documentation: text/template

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

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