Rework template handling for function and map lookups

This is a big commit, but it deletes lots of code and simplifies a lot.

* Resolving the template funcs at execution time means we don't have to create template clones per site
* Having a custom map resolver means that we can remove the AST lower case transformation for the special lower case Params map

Not only is the above easier to reason about, it's also faster, especially if you have more than one language, as in the benchmark below:

```
name                          old time/op    new time/op    delta
SiteNew/Deep_content_tree-16    53.7ms ± 0%    48.1ms ± 2%  -10.38%  (p=0.029 n=4+4)

name                          old alloc/op   new alloc/op   delta
SiteNew/Deep_content_tree-16    41.0MB ± 0%    36.8MB ± 0%  -10.26%  (p=0.029 n=4+4)

name                          old allocs/op  new allocs/op  delta
SiteNew/Deep_content_tree-16      481k ± 0%      410k ± 0%  -14.66%  (p=0.029 n=4+4)
```

This should be even better if you also have lots of templates.

Closes #6594

tpl/template.go

@@ -14,39 +14,22 @@
 package tpl
 
 import (
-	"fmt"
-	"github.com/gohugoio/hugo/tpl/internal/go_templates/htmltemplate"
+	"reflect"
 
 	"io"
-	"path/filepath"
 	"regexp"
-	"strings"
-	"time"
 
 	"github.com/gohugoio/hugo/output"
 
-	"github.com/gohugoio/hugo/common/herrors"
-
-	"github.com/gohugoio/hugo/hugofs"
-
-	"github.com/spf13/afero"
-
 	texttemplate "github.com/gohugoio/hugo/tpl/internal/go_templates/texttemplate"
-	"github.com/gohugoio/hugo/tpl/internal/go_templates/texttemplate/parse"
-
-	bp "github.com/gohugoio/hugo/bufferpool"
-	"github.com/gohugoio/hugo/metrics"
-	"github.com/pkg/errors"
 )
 
-var (
-	_ TemplateExecutor     = (*TemplateAdapter)(nil)
-	_ TemplateInfoProvider = (*TemplateAdapter)(nil)
-)
+var _ TemplateInfoProvider = (*TemplateInfo)(nil)
 
-// TemplateHandler manages the collection of templates.
-type TemplateHandler interface {
-	TemplateFinder
+// TemplateManager manages the collection of templates.
+type TemplateManager interface {
+	TemplateHandler
+	TemplateFuncGetter
 	AddTemplate(name, tpl string) error
 	AddLateTemplate(name, tpl string) error
 	LoadTemplates(prefix string) error
@@ -68,6 +51,12 @@ type TemplateFinder interface {
 	TemplateLookupVariant
 }
 
+// TemplateHandler finds and executes templates.
+type TemplateHandler interface {
+	TemplateFinder
+	Execute(t Template, wr io.Writer, data interface{}) error
+}
+
 type TemplateLookup interface {
 	Lookup(name string) (Template, bool)
 }
@@ -87,8 +76,8 @@ type TemplateLookupVariant interface {
 
 // Template is the common interface between text/template and html/template.
 type Template interface {
-	Execute(wr io.Writer, data interface{}) error
 	Name() string
+	Prepare() (*texttemplate.Template, error)
 }
 
 // TemplateInfoProvider provides some contextual information about a template.
@@ -107,30 +96,15 @@ type TemplateParseFinder interface {
 	TemplateFinder
 }
 
-// TemplateExecutor adds some extras to Template.
-type TemplateExecutor interface {
-	Template
-	ExecuteToString(data interface{}) (string, error)
-	Tree() string
-}
-
 // TemplateDebugger prints some debug info to stdoud.
 type TemplateDebugger interface {
 	Debug()
 }
 
-// TemplateAdapter implements the TemplateExecutor interface.
-type TemplateAdapter struct {
+// TemplateInfo wraps a Template with some additional information.
+type TemplateInfo struct {
 	Template
-	Metrics metrics.Provider
-
 	Info Info
-
-	// The filesystem where the templates are stored.
-	Fs afero.Fs
-
-	// Maps to base template if relevant.
-	NameBaseTemplateName map[string]string
 }
 
 var baseOfRe = regexp.MustCompile("template: (.*?):")
@@ -143,165 +117,11 @@ func extractBaseOf(err string) string {
 	return ""
 }
 
-// Execute executes the current template. The actual execution is performed
-// by the embedded text or html template, but we add an implementation here so
-// we can add a timer for some metrics.
-func (t *TemplateAdapter) Execute(w io.Writer, data interface{}) (execErr error) {
-	defer func() {
-		// Panics in templates are a little bit too common (nil pointers etc.)
-		// See https://github.com/gohugoio/hugo/issues/5327
-		if r := recover(); r != nil {
-			execErr = t.addFileContext(t.Name(), fmt.Errorf(`panic in Execute: %s. See "https://github.com/gohugoio/hugo/issues/5327" for the reason why we cannot provide a better error message for this`, r))
-		}
-	}()
-
-	if t.Metrics != nil {
-		defer t.Metrics.MeasureSince(t.Name(), time.Now())
-	}
-
-	execErr = t.Template.Execute(w, data)
-	if execErr != nil {
-		execErr = t.addFileContext(t.Name(), execErr)
-	}
-
-	return
-}
-
-func (t *TemplateAdapter) TemplateInfo() Info {
+func (t *TemplateInfo) TemplateInfo() Info {
 	return t.Info
 }
 
-// The identifiers may be truncated in the log, e.g.
-// "executing "main" at <$scaled.SRelPermalin...>: can't evaluate field SRelPermalink in type *resource.Image"
-var identifiersRe = regexp.MustCompile(`at \<(.*?)(\.{3})?\>:`)
-
-func (t *TemplateAdapter) extractIdentifiers(line string) []string {
-	m := identifiersRe.FindAllStringSubmatch(line, -1)
-	identifiers := make([]string, len(m))
-	for i := 0; i < len(m); i++ {
-		identifiers[i] = m[i][1]
-	}
-	return identifiers
-}
-
-func (t *TemplateAdapter) addFileContext(name string, inerr error) error {
-	if strings.HasPrefix(t.Name(), "_internal") {
-		return inerr
-	}
-
-	f, realFilename, err := t.fileAndFilename(t.Name())
-	if err != nil {
-		return inerr
-
-	}
-	defer f.Close()
-
-	master, hasMaster := t.NameBaseTemplateName[name]
-
-	ferr := errors.Wrap(inerr, "execute of template failed")
-
-	// Since this can be a composite of multiple template files (single.html + baseof.html etc.)
-	// we potentially need to look in both -- and cannot rely on line number alone.
-	lineMatcher := func(m herrors.LineMatcher) bool {
-		if m.Position.LineNumber != m.LineNumber {
-			return false
-		}
-		if !hasMaster {
-			return true
-		}
-
-		identifiers := t.extractIdentifiers(m.Error.Error())
-
-		for _, id := range identifiers {
-			if strings.Contains(m.Line, id) {
-				return true
-			}
-		}
-		return false
-	}
-
-	fe, ok := herrors.WithFileContext(ferr, realFilename, f, lineMatcher)
-	if ok || !hasMaster {
-		return fe
-	}
-
-	// Try the base template if relevant
-	f, realFilename, err = t.fileAndFilename(master)
-	if err != nil {
-		return err
-	}
-	defer f.Close()
-
-	fe, ok = herrors.WithFileContext(ferr, realFilename, f, lineMatcher)
-
-	if !ok {
-		// Return the most specific.
-		return ferr
-
-	}
-	return fe
-
-}
-
-func (t *TemplateAdapter) fileAndFilename(name string) (afero.File, string, error) {
-	fs := t.Fs
-	filename := filepath.FromSlash(name)
-
-	fi, err := fs.Stat(filename)
-	if err != nil {
-		return nil, "", err
-	}
-	fim := fi.(hugofs.FileMetaInfo)
-	meta := fim.Meta()
-
-	f, err := meta.Open()
-	if err != nil {
-		return nil, "", errors.Wrapf(err, "failed to open template file %q:", filename)
-	}
-
-	return f, meta.Filename(), nil
-}
-
-// ExecuteToString executes the current template and returns the result as a
-// string.
-func (t *TemplateAdapter) ExecuteToString(data interface{}) (string, error) {
-	b := bp.GetBuffer()
-	defer bp.PutBuffer(b)
-	if err := t.Execute(b, data); err != nil {
-		return "", err
-	}
-	return b.String(), nil
-}
-
-// Tree returns the template Parse tree as a string.
-// Note: this isn't safe for parallel execution on the same template
-// vs Lookup and Execute.
-func (t *TemplateAdapter) Tree() string {
-	var tree *parse.Tree
-	switch tt := t.Template.(type) {
-	case *template.Template:
-		tree = tt.Tree
-	case *texttemplate.Template:
-		tree = tt.Tree
-	default:
-		panic("Unknown template")
-	}
-
-	if tree == nil || tree.Root == nil {
-		return ""
-	}
-	s := tree.Root.String()
-
-	return s
-}
-
-// TemplateFuncsGetter allows to get a map of functions.
-type TemplateFuncsGetter interface {
-	GetFuncs() map[string]interface{}
-}
-
-// TemplateTestMocker adds a way to override some template funcs during tests.
-// The interface is named so it's not used in regular application code.
-type TemplateTestMocker interface {
-	SetFuncs(funcMap map[string]interface{})
+// TemplateFuncGetter allows to find a template func by name.
+type TemplateFuncGetter interface {
+	GetFunc(name string) (reflect.Value, bool)
 }