Source: template.go in package html/template


package template

import (
	"fmt"
	"io"
	"io/fs"
	"os"
	"path"
	"path/filepath"
	"sync"
	"text/template"
	"text/template/parse"
)

Template is a specialized Template from "text/template" that produces a safe HTML document fragment.

Sticky error if escaping fails, or escapeOK if succeeded.

We could embed the text/template field, but it's safer not to because we need to keep our version of the name space and the underlying template's in sync.

The underlying template's parse tree, updated to be HTML-safe.

	Tree       *parse.Tree
	*nameSpace // common to all associated templates
}

escapeOK is a sentinel value used to indicate valid escaping.

var escapeOK = fmt.Errorf("template escaped correctly")

nameSpace is the data structure shared by all templates in an association.

type nameSpace struct {
	mu      sync.Mutex
	set     map[string]*Template
	escaped bool
	esc     escaper
}

Templates returns a slice of the templates associated with t, including t itself.

func (t *Template) Templates() []*Template {
	ns := t.nameSpace
	ns.mu.Lock()

Return a slice so we don't expose the map.

	m := make([]*Template, 0, len(ns.set))
	for _, v := range ns.set {
		m = append(m, v)
	}
	return m
}

Option sets options for the template. Options are described by strings, either a simple string or "key=value". There can be at most one equals sign in an option string. If the option string is unrecognized or otherwise invalid, Option panics. Known options: missingkey: Control the behavior during execution if a map is indexed with a key that is not present in the map. "missingkey=default" or "missingkey=invalid" The default behavior: Do nothing and continue execution. If printed, the result of the index operation is the string "<no value>". "missingkey=zero" The operation returns the zero value for the map type's element. "missingkey=error" Execution stops immediately with an error.

func (t *Template) Option(opt ...string) *Template {
	t.text.Option(opt...)
	return t
}

checkCanParse checks whether it is OK to parse templates. If not, it returns an error.

func (t *Template) checkCanParse() error {
	if t == nil {
		return nil
	}
	t.nameSpace.mu.Lock()
	defer t.nameSpace.mu.Unlock()
	if t.nameSpace.escaped {
		return fmt.Errorf("html/template: cannot Parse after Execute")
	}
	return nil
}

escape escapes all associated templates.

func (t *Template) escape() error {
	t.nameSpace.mu.Lock()
	defer t.nameSpace.mu.Unlock()
	t.nameSpace.escaped = true
	if t.escapeErr == nil {
		if t.Tree == nil {
			return fmt.Errorf("template: %q is an incomplete or empty template", t.Name())
		}
		if err := escapeTemplate(t, t.text.Root, t.Name()); err != nil {
			return err
		}
	} else if t.escapeErr != escapeOK {
		return t.escapeErr
	}
	return nil
}

Execute applies a parsed template to the specified data object, writing the output to wr. If an error occurs executing the template or writing its output, execution stops, but partial results may already have been written to the output writer. A template may be executed safely in parallel, although if parallel executions share a Writer the output may be interleaved.

func (t *Template) Execute(wr io.Writer, data interface{}) error {
	if err := t.escape(); err != nil {
		return err
	}
	return t.text.Execute(wr, data)
}

ExecuteTemplate applies the template associated with t that has the given name to the specified data object and writes the output to wr. If an error occurs executing the template or writing its output, execution stops, but partial results may already have been written to the output writer. A template may be executed safely in parallel, although if parallel executions share a Writer the output may be interleaved.

func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
	tmpl, err := t.lookupAndEscapeTemplate(name)
	if err != nil {
		return err
	}
	return tmpl.text.Execute(wr, data)
}

lookupAndEscapeTemplate guarantees that the template with the given name is escaped, or returns an error if it cannot be. It returns the named template.

func (t *Template) lookupAndEscapeTemplate(name string) (tmpl *Template, err error) {
	t.nameSpace.mu.Lock()
	defer t.nameSpace.mu.Unlock()
	t.nameSpace.escaped = true
	tmpl = t.set[name]
	if tmpl == nil {
		return nil, fmt.Errorf("html/template: %q is undefined", name)
	}
	if tmpl.escapeErr != nil && tmpl.escapeErr != escapeOK {
		return nil, tmpl.escapeErr
	}
	if tmpl.text.Tree == nil || tmpl.text.Root == nil {
		return nil, fmt.Errorf("html/template: %q is an incomplete template", name)
	}
	if t.text.Lookup(name) == nil {
		panic("html/template internal error: template escaping out of sync")
	}
	if tmpl.escapeErr == nil {
		err = escapeTemplate(tmpl, tmpl.text.Root, name)
	}
	return tmpl, err
}

DefinedTemplates returns a string listing the defined templates, prefixed by the string "; defined templates are: ". If there are none, it returns the empty string. Used to generate an error message.

func (t *Template) DefinedTemplates() string {
	return t.text.DefinedTemplates()
}

Parse parses text as a template body for t. Named template definitions ({{define ...}} or {{block ...}} statements) in text define additional templates associated with t and are removed from the definition of t itself. Templates can be redefined in successive calls to Parse, before the first use of Execute on t or any associated template. A template definition with a body containing only white space and comments is considered empty and will not replace an existing template's body. This allows using Parse to add new named template definitions without overwriting the main template body.

func (t *Template) Parse(text string) (*Template, error) {
	if err := t.checkCanParse(); err != nil {
		return nil, err
	}

	ret, err := t.text.Parse(text)
	if err != nil {
		return nil, err
	}

In general, all the named templates might have changed underfoot. Regardless, some new ones may have been defined. The template.Template set has been updated; update ours.

	t.nameSpace.mu.Lock()
	defer t.nameSpace.mu.Unlock()
	for _, v := range ret.Templates() {
		name := v.Name()
		tmpl := t.set[name]
		if tmpl == nil {
			tmpl = t.new(name)
		}
		tmpl.text = v
		tmpl.Tree = v.Tree
	}
	return t, nil
}

AddParseTree creates a new template with the name and parse tree and associates it with t. It returns an error if t or any associated template has already been executed.

func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
	if err := t.checkCanParse(); err != nil {
		return nil, err
	}

	t.nameSpace.mu.Lock()
	defer t.nameSpace.mu.Unlock()
	text, err := t.text.AddParseTree(name, tree)
	if err != nil {
		return nil, err
	}
	ret := &Template{
		nil,
		text,
		text.Tree,
		t.nameSpace,
	}
	t.set[name] = ret
	return ret, nil
}

Clone returns a duplicate of the template, including all associated templates. The actual representation is not copied, but the name space of associated templates is, so further calls to Parse in the copy will add templates to the copy but not to the original. Clone can be used to prepare common templates and use them with variant definitions for other templates by adding the variants after the clone is made. It returns an error if t has already been executed.

func (t *Template) Clone() (*Template, error) {
	t.nameSpace.mu.Lock()
	defer t.nameSpace.mu.Unlock()
	if t.escapeErr != nil {
		return nil, fmt.Errorf("html/template: cannot Clone %q after it has executed", t.Name())
	}
	textClone, err := t.text.Clone()
	if err != nil {
		return nil, err
	}
	ns := &nameSpace{set: make(map[string]*Template)}
	ns.esc = makeEscaper(ns)
	ret := &Template{
		nil,
		textClone,
		textClone.Tree,
		ns,
	}
	ret.set[ret.Name()] = ret
	for _, x := range textClone.Templates() {
		name := x.Name()
		src := t.set[name]
		if src == nil || src.escapeErr != nil {
			return nil, fmt.Errorf("html/template: cannot Clone %q after it has executed", t.Name())
		}
		x.Tree = x.Tree.Copy()
		ret.set[name] = &Template{
			nil,
			x,
			x.Tree,
			ret.nameSpace,
		}

Return the template associated with the name of this template.

	return ret.set[ret.Name()], nil
}

New allocates a new HTML template with the given name.

func New(name string) *Template {
	ns := &nameSpace{set: make(map[string]*Template)}
	ns.esc = makeEscaper(ns)
	tmpl := &Template{
		nil,
		template.New(name),
		nil,
		ns,
	}
	tmpl.set[name] = tmpl
	return tmpl
}

New allocates a new HTML template associated with the given one and with the same delimiters. The association, which is transitive, allows one template to invoke another with a {{template}} action. If a template with the given name already exists, the new HTML template will replace it. The existing template will be reset and disassociated with t.

func (t *Template) New(name string) *Template {
	t.nameSpace.mu.Lock()
	defer t.nameSpace.mu.Unlock()
	return t.new(name)
}

new is the implementation of New, without the lock.

func (t *Template) new(name string) *Template {
	tmpl := &Template{
		nil,
		t.text.New(name),
		nil,
		t.nameSpace,
	}
	if existing, ok := tmpl.set[name]; ok {
		emptyTmpl := New(existing.Name())
		*existing = *emptyTmpl
	}
	tmpl.set[name] = tmpl
	return tmpl
}

Name returns the name of the template.

func (t *Template) Name() string {
	return t.text.Name()
}

FuncMap is the type of the map defining the mapping from names to functions. Each function must have either a single return value, or two return values of which the second has type error. In that case, if the second (error) argument evaluates to non-nil during execution, execution terminates and Execute returns that error. FuncMap has the same base type as FuncMap in "text/template", copied here so clients need not import "text/template".

type FuncMap map[string]interface{}

Funcs adds the elements of the argument map to the template's function map. It must be called before the template is parsed. It panics if a value in the map is not a function with appropriate return type. However, it is legal to overwrite elements of the map. The return value is the template, so calls can be chained.

func (t *Template) Funcs(funcMap FuncMap) *Template {
	t.text.Funcs(template.FuncMap(funcMap))
	return t
}

Delims sets the action delimiters to the specified strings, to be used in subsequent calls to Parse, ParseFiles, or ParseGlob. Nested template definitions will inherit the settings. An empty delimiter stands for the corresponding default: {{ or }}. The return value is the template, so calls can be chained.

func (t *Template) Delims(left, right string) *Template {
	t.text.Delims(left, right)
	return t
}

Lookup returns the template with the given name that is associated with t, or nil if there is no such template.

func (t *Template) Lookup(name string) *Template {
	t.nameSpace.mu.Lock()
	defer t.nameSpace.mu.Unlock()
	return t.set[name]
}

Must is a helper that wraps a call to a function returning (*Template, error) and panics if the error is non-nil. It is intended for use in variable initializations such as var t = template.Must(template.New("name").Parse("html"))

func Must(t *Template, err error) *Template {
	if err != nil {
		panic(err)
	}
	return t
}

ParseFiles creates a new Template and parses the template definitions from the named files. The returned template's name will have the (base) name and (parsed) contents of the first file. There must be at least one file. If an error occurs, parsing stops and the returned *Template is nil. When parsing multiple files with the same name in different directories, the last one mentioned will be the one that results. For instance, ParseFiles("a/foo", "b/foo") stores "b/foo" as the template named "foo", while "a/foo" is unavailable.

func ParseFiles(filenames ...string) (*Template, error) {
	return parseFiles(nil, readFileOS, filenames...)
}

ParseFiles parses the named files and associates the resulting templates with t. If an error occurs, parsing stops and the returned template is nil; otherwise it is t. There must be at least one file. When parsing multiple files with the same name in different directories, the last one mentioned will be the one that results. ParseFiles returns an error if t or any associated template has already been executed.

func (t *Template) ParseFiles(filenames ...string) (*Template, error) {
	return parseFiles(t, readFileOS, filenames...)
}

parseFiles is the helper for the method and function. If the argument template is nil, it is created from the first file.

func parseFiles(t *Template, readFile func(string) (string, []byte, error), filenames ...string) (*Template, error) {
	if err := t.checkCanParse(); err != nil {
		return nil, err
	}

Not really a problem, but be consistent.

		return nil, fmt.Errorf("html/template: no files named in call to ParseFiles")
	}
	for _, filename := range filenames {
		name, b, err := readFile(filename)
		if err != nil {
			return nil, err
		}

First template becomes return value if not already defined, and we use that one for subsequent New calls to associate all the templates together. Also, if this file has the same name as t, this file becomes the contents of t, so t, err := New(name).Funcs(xxx).ParseFiles(name) works. Otherwise we create a new template associated with t.

		var tmpl *Template
		if t == nil {
			t = New(name)
		}
		if name == t.Name() {
			tmpl = t
		} else {
			tmpl = t.New(name)
		}
		_, err = tmpl.Parse(s)
		if err != nil {
			return nil, err
		}
	}
	return t, nil
}

ParseGlob creates a new Template and parses the template definitions from the files identified by the pattern. The files are matched according to the semantics of filepath.Match, and the pattern must match at least one file. The returned template will have the (base) name and (parsed) contents of the first file matched by the pattern. ParseGlob is equivalent to calling ParseFiles with the list of files matched by the pattern. When parsing multiple files with the same name in different directories, the last one mentioned will be the one that results.

func ParseGlob(pattern string) (*Template, error) {
	return parseGlob(nil, pattern)
}

ParseGlob parses the template definitions in the files identified by the pattern and associates the resulting templates with t. The files are matched according to the semantics of filepath.Match, and the pattern must match at least one file. ParseGlob is equivalent to calling t.ParseFiles with the list of files matched by the pattern. When parsing multiple files with the same name in different directories, the last one mentioned will be the one that results. ParseGlob returns an error if t or any associated template has already been executed.

func (t *Template) ParseGlob(pattern string) (*Template, error) {
	return parseGlob(t, pattern)
}

parseGlob is the implementation of the function and method ParseGlob.

func parseGlob(t *Template, pattern string) (*Template, error) {
	if err := t.checkCanParse(); err != nil {
		return nil, err
	}
	filenames, err := filepath.Glob(pattern)
	if err != nil {
		return nil, err
	}
	if len(filenames) == 0 {
		return nil, fmt.Errorf("html/template: pattern matches no files: %#q", pattern)
	}
	return parseFiles(t, readFileOS, filenames...)
}

IsTrue reports whether the value is 'true', in the sense of not the zero of its type, and whether the value has a meaningful truth value. This is the definition of truth used by if and other such actions.

func IsTrue(val interface{}) (truth, ok bool) {
	return template.IsTrue(val)
}

ParseFS is like ParseFiles or ParseGlob but reads from the file system fs instead of the host operating system's file system. It accepts a list of glob patterns. (Note that most file names serve as glob patterns matching only themselves.)

func ParseFS(fs fs.FS, patterns ...string) (*Template, error) {
	return parseFS(nil, fs, patterns)
}

func (t *Template) ParseFS(fs fs.FS, patterns ...string) (*Template, error) {
	return parseFS(t, fs, patterns)
}

func parseFS(t *Template, fsys fs.FS, patterns []string) (*Template, error) {
	var filenames []string
	for _, pattern := range patterns {
		list, err := fs.Glob(fsys, pattern)
		if err != nil {
			return nil, err
		}
		if len(list) == 0 {
			return nil, fmt.Errorf("template: pattern matches no files: %#q", pattern)
		}
		filenames = append(filenames, list...)
	}
	return parseFiles(t, readFileFS(fsys), filenames...)
}

func readFileOS(file string) (name string, b []byte, err error) {
	name = filepath.Base(file)
	b, err = os.ReadFile(file)
	return
}

func readFileFS(fsys fs.FS) func(string) (string, []byte, error) {
	return func(file string) (name string, b []byte, err error) {
		name = path.Base(file)
		b, err = fs.ReadFile(fsys, file)
		return
	}