tpl/tplimpl: Rework template management to get rid of concurrency issues

This more or less completes the simplification of the template handling code in Hugo started in v0.62.

The main motivation was to fix a long lasting issue about a crash in HTML content files  without front matter.

But this commit also comes with a big functional improvement.

As we now have moved the base template evaluation to the build stage we now use the same lookup rules for `baseof` as for `list` etc. type of templates.

This means that in this simple example you can have a `baseof` template for the `blog` section without having to duplicate the others:

```
layouts
├── _default
│   ├── baseof.html
│   ├── list.html
│   └── single.html
└── blog
    └── baseof.html
```

Also, when simplifying code, you often get rid of some double work, as shown in the "site building" benchmarks below.

These benchmarks looks suspiciously good, but I have repeated the below with ca. the same result. Compared to master:

```
name                              old time/op    new time/op    delta
SiteNew/Bundle_with_image-16        13.1ms ± 1%    10.5ms ± 1%  -19.34%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16    13.0ms ± 0%    10.7ms ± 1%  -18.05%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16      46.4ms ± 2%    43.1ms ± 1%   -7.15%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16            52.2ms ± 2%    47.8ms ± 1%   -8.30%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16        77.9ms ± 1%    70.9ms ± 1%   -9.01%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16      43.0ms ± 0%    37.2ms ± 1%  -13.54%  (p=0.029 n=4+4)
SiteNew/Page_collections-16         58.2ms ± 1%    52.4ms ± 1%   -9.95%  (p=0.029 n=4+4)

name                              old alloc/op   new alloc/op   delta
SiteNew/Bundle_with_image-16        3.81MB ± 0%    2.22MB ± 0%  -41.70%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16    3.60MB ± 0%    2.01MB ± 0%  -44.20%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16      19.3MB ± 1%    14.1MB ± 0%  -26.91%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16            70.7MB ± 0%    69.0MB ± 0%   -2.40%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16        37.1MB ± 0%    31.2MB ± 0%  -15.94%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16      17.6MB ± 0%    10.6MB ± 0%  -39.92%  (p=0.029 n=4+4)
SiteNew/Page_collections-16         25.9MB ± 0%    21.2MB ± 0%  -17.99%  (p=0.029 n=4+4)

name                              old allocs/op  new allocs/op  delta
SiteNew/Bundle_with_image-16         52.3k ± 0%     26.1k ± 0%  -50.18%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16     52.3k ± 0%     26.1k ± 0%  -50.16%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16        336k ± 1%      269k ± 0%  -19.90%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16              422k ± 0%      395k ± 0%   -6.43%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16          401k ± 0%      313k ± 0%  -21.79%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16        247k ± 0%      143k ± 0%  -42.17%  (p=0.029 n=4+4)
SiteNew/Page_collections-16           282k ± 0%      207k ± 0%  -26.55%  (p=0.029 n=4+4)
```

Fixes #6716
Fixes #6760
Fixes #6768
Fixes #6778

1 files changed, 26 insertions, 51 deletions

diff --git a/hugolib/site.go b/hugolib/site.go
index eb232c629..bbcbcd27a 100644
--- a/hugolib/site.go
+++ b/hugolib/site.go
@@ -105,8 +105,6 @@ type Site struct {
 	Sections Taxonomy
 	Info     SiteInfo
 
-	layoutHandler *output.LayoutHandler
-
 	language *langs.Language
 
 	siteCfg siteConfigHolder
@@ -324,7 +322,6 @@ func (s *Site) isEnabled(kind string) bool {
 // reset returns a new Site prepared for rebuild.
 func (s *Site) reset() *Site {
 	return &Site{Deps: s.Deps,
-		layoutHandler:          output.NewLayoutHandler(),
 		disabledKinds:          s.disabledKinds,
 		titleFunc:              s.titleFunc,
 		relatedDocsHandler:     s.relatedDocsHandler.Clone(),
@@ -439,7 +436,6 @@ func newSite(cfg deps.DepsCfg) (*Site, error) {
 
 	s := &Site{
 		PageCollections:        c,
-		layoutHandler:          output.NewLayoutHandler(),
 		language:               cfg.Language,
 		disabledKinds:          disabledKinds,
 		titleFunc:              titleFunc,
@@ -936,7 +932,7 @@ func (s *Site) processPartial(config *BuildCfg, init func(config *BuildCfg) erro
 				sourceChanged = append(sourceChanged, ev)
 			case files.ComponentFolderLayouts:
 				tmplChanged = true
-				if _, found := s.Tmpl.Lookup(id.Path); !found {
+				if !s.Tmpl().HasTemplate(id.Path) {
 					tmplAdded = true
 				}
 				if tmplAdded {
@@ -1030,7 +1026,7 @@ func (s *Site) processPartial(config *BuildCfg, init func(config *BuildCfg) erro
 		sourceFilesChanged[ev.Name] = true
 	}
 
-	if config.ErrRecovery || tmplAdded {
+	if config.ErrRecovery || tmplAdded || dataChanged {
 		h.resetPageState()
 	} else {
 		h.resetPageStateFromEvents(changeIdentities)
@@ -1226,10 +1222,9 @@ func (s *Site) initializeSiteInfo() error {
 func (s *Site) eventToIdentity(e fsnotify.Event) (identity.PathIdentity, bool) {
 	for _, fs := range s.BaseFs.SourceFilesystems.FileSystems() {
 		if p := fs.Path(e.Name); p != "" {
-			return identity.NewPathIdentity(fs.Name, p), true
+			return identity.NewPathIdentity(fs.Name, filepath.ToSlash(p)), true
 		}
 	}
-
 	return identity.PathIdentity{}, false
 }
 
@@ -1464,12 +1459,22 @@ func (s *Site) permalink(link string) string {
 
 }
 
-func (s *Site) renderAndWriteXML(statCounter *uint64, name string, targetPath string, d interface{}, layouts ...string) error {
+func (s *Site) lookupLayouts(layouts ...string) tpl.Template {
+	for _, l := range layouts {
+		if templ, found := s.Tmpl().Lookup(l); found {
+			return templ
+		}
+	}
+
+	return nil
+}
+
+func (s *Site) renderAndWriteXML(statCounter *uint64, name string, targetPath string, d interface{}, templ tpl.Template) error {
 	s.Log.DEBUG.Printf("Render XML for %q to %q", name, targetPath)
 	renderBuffer := bp.GetBuffer()
 	defer bp.PutBuffer(renderBuffer)
 
-	if err := s.renderForLayouts(name, "", d, renderBuffer, layouts...); err != nil {
+	if err := s.renderForTemplate(name, "", d, renderBuffer, templ); err != nil {
 		return err
 	}
 
@@ -1498,13 +1503,13 @@ func (s *Site) renderAndWriteXML(statCounter *uint64, name string, targetPath st
 
 }
 
-func (s *Site) renderAndWritePage(statCounter *uint64, name string, targetPath string, p *pageState, layouts ...string) error {
+func (s *Site) renderAndWritePage(statCounter *uint64, name string, targetPath string, p *pageState, templ tpl.Template) error {
 	renderBuffer := bp.GetBuffer()
 	defer bp.PutBuffer(renderBuffer)
 
 	of := p.outputFormat()
 
-	if err := s.renderForLayouts(p.Kind(), of.Name, p, renderBuffer, layouts...); err != nil {
+	if err := s.renderForTemplate(p.Kind(), of.Name, p, renderBuffer, templ); err != nil {
 		return err
 	}
 
@@ -1571,56 +1576,26 @@ func (r contentLinkRenderer) Render(w io.Writer, ctx hooks.LinkContext) error {
 	return r.templateHandler.Execute(r.templ, w, ctx)
 }
 
-func (s *Site) lookupTemplate(layouts ...string) (tpl.Template, bool) {
-	for _, l := range layouts {
-		if templ, found := s.Tmpl.Lookup(l); found {
-			return templ, true
-		}
-	}
-
-	return nil, false
-}
-
-func (s *Site) renderForLayouts(name, outputFormat string, d interface{}, w io.Writer, layouts ...string) (err error) {
-	templ := s.findFirstTemplate(layouts...)
+func (s *Site) renderForTemplate(name, outputFormat string, d interface{}, w io.Writer, templ tpl.Template) (err error) {
 	if templ == nil {
-		log := s.Log.WARN
-		if infoOnMissingLayout[name] {
-			log = s.Log.INFO
-		}
-
-		errMsg := "You should create a template file which matches Hugo Layouts Lookup Rules for this combination."
-		var args []interface{}
-		msg := "found no layout file for"
-		if outputFormat != "" {
-			msg += " %q"
-			args = append(args, outputFormat)
-		}
-		if name != "" {
-			msg += " for %q"
-			args = append(args, name)
-		}
-
-		msg += ": " + errMsg
-
-		log.Printf(msg, args...)
-
+		s.logMissingLayout(name, "", outputFormat)
 		return nil
 	}
 
-	if err = s.Tmpl.Execute(templ, w, d); err != nil {
+	if err = s.Tmpl().Execute(templ, w, d); err != nil {
 		return _errors.Wrapf(err, "render of %q failed", name)
 	}
 	return
 }
 
-func (s *Site) findFirstTemplate(layouts ...string) tpl.Template {
-	for _, layout := range layouts {
-		if templ, found := s.Tmpl.Lookup(layout); found {
-			return templ
+func (s *Site) lookupTemplate(layouts ...string) (tpl.Template, bool) {
+	for _, l := range layouts {
+		if templ, found := s.Tmpl().Lookup(l); found {
+			return templ, true
 		}
 	}
-	return nil
+
+	return nil, false
 }
 
 func (s *Site) publish(statCounter *uint64, path string, r io.Reader) (err error) {