Add Hugo Piper with SCSS support and much more

Before this commit, you would have to use page bundles to do image processing etc. in Hugo.

This commit adds

* A new `/assets` top-level project or theme dir (configurable via `assetDir`)
* A new template func, `resources.Get` which can be used to "get a resource" that can be further processed.

This means that you can now do this in your templates (or shortcodes):

```bash
{{ $sunset := (resources.Get "images/sunset.jpg").Fill "300x200" }}
```

This also adds a new `extended` build tag that enables powerful SCSS/SASS support with source maps. To compile this from source, you will also need a C compiler installed:

```
HUGO_BUILD_TAGS=extended mage install
```

Note that you can use output of the SCSS processing later in a non-SCSSS-enabled Hugo.

The `SCSS` processor is a _Resource transformation step_ and it can be chained with the many others in a pipeline:

```bash
{{ $css := resources.Get "styles.scss" | resources.ToCSS | resources.PostCSS | resources.Minify | resources.Fingerprint }}
<link rel="stylesheet" href="{{ $styles.RelPermalink }}" integrity="{{ $styles.Data.Digest }}" media="screen">
```

The transformation funcs above have aliases, so it can be shortened to:

```bash
{{ $css := resources.Get "styles.scss" | toCSS | postCSS | minify | fingerprint }}
<link rel="stylesheet" href="{{ $styles.RelPermalink }}" integrity="{{ $styles.Data.Digest }}" media="screen">
```

A quick tip would be to avoid the fingerprinting part, and possibly also the not-superfast `postCSS` when you're doing development, as it allows Hugo to be smarter about the rebuilding.

Documentation will follow, but have a look at the demo repo in https://github.com/bep/hugo-sass-test

New functions to create `Resource` objects:

* `resources.Get` (see above)
* `resources.FromString`: Create a Resource from a string.

New `Resource` transformation funcs:

* `resources.ToCSS`: Compile `SCSS` or `SASS` into `CSS`.
* `resources.PostCSS`: Process your CSS with PostCSS. Config file support (project or theme or passed as an option).
* `resources.Minify`: Currently supports `css`, `js`, `json`, `html`, `svg`, `xml`.
* `resources.Fingerprint`: Creates a fingerprinted version of the given Resource with Subresource Integrity..
* `resources.Concat`: Concatenates a list of Resource objects. Think of this as a poor man's bundler.
* `resources.ExecuteAsTemplate`: Parses and executes the given Resource and data context (e.g. .Site) as a Go template.

Fixes #4381
Fixes #4903
Fixes #4858

hugolib/site.go

@@ -27,12 +27,12 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/resource"
+
 	"github.com/gohugoio/hugo/langs"
 
 	src "github.com/gohugoio/hugo/source"
 
-	"github.com/gohugoio/hugo/resource"
-
 	"golang.org/x/sync/errgroup"
 
 	"github.com/gohugoio/hugo/config"
@@ -140,8 +140,7 @@ type Site struct {
 	renderFormats output.Formats
 
 	// Logger etc.
-	*deps.Deps   `json:"-"`
-	resourceSpec *resource.Spec
+	*deps.Deps `json:"-"`
 
 	// The func used to title case titles.
 	titleFunc func(s string) string
@@ -188,7 +187,6 @@ func (s *Site) reset() *Site {
 		outputFormatsConfig: s.outputFormatsConfig,
 		frontmatterHandler:  s.frontmatterHandler,
 		mediaTypesConfig:    s.mediaTypesConfig,
-		resourceSpec:        s.resourceSpec,
 		Language:            s.Language,
 		owner:               s.owner,
 		PageCollections:     newPageCollections()}
@@ -691,7 +689,11 @@ func (s *Site) processPartial(events []fsnotify.Event) (whatChanged, error) {
 		logger = helpers.NewDistinctFeedbackLogger()
 	)
 
-	for _, ev := range events {
+	cachePartitions := make([]string, len(events))
+
+	for i, ev := range events {
+		cachePartitions[i] = resource.ResourceKeyPartition(ev.Name)
+
 		if s.isContentDirEvent(ev) {
 			logger.Println("Source changed", ev)
 			sourceChanged = append(sourceChanged, ev)
@@ -717,6 +719,11 @@ func (s *Site) processPartial(events []fsnotify.Event) (whatChanged, error) {
 		}
 	}
 
+	// These in memory resource caches will be rebuilt on demand.
+	for _, s := range s.owner.Sites {
+		s.ResourceSpec.ResourceCache.DeletePartitions(cachePartitions...)
+	}
+
 	if len(tmplChanged) > 0 || len(i18nChanged) > 0 {
 		sites := s.owner.Sites
 		first := sites[0]
@@ -731,7 +738,11 @@ func (s *Site) processPartial(events []fsnotify.Event) (whatChanged, error) {
 		for i := 1; i < len(sites); i++ {
 			site := sites[i]
 			var err error
-			site.Deps, err = first.Deps.ForLanguage(site.Language)
+			depsCfg := deps.DepsCfg{
+				Language:   site.Language,
+				MediaTypes: site.mediaTypesConfig,
+			}
+			site.Deps, err = first.Deps.ForLanguage(depsCfg)
 			if err != nil {
 				return whatChanged{}, err
 			}
@@ -797,6 +808,7 @@ func (s *Site) processPartial(events []fsnotify.Event) (whatChanged, error) {
 		if err := s.readAndProcessContent(filenamesChanged...); err != nil {
 			return whatChanged{}, err
 		}
+
 	}
 
 	changed := whatChanged{
@@ -1240,7 +1252,7 @@ func (s *Site) readAndProcessContent(filenames ...string) error {
 
 	mainHandler := &contentCaptureResultHandler{contentProcessors: contentProcessors, defaultContentProcessor: defaultContentProcessor}
 
-	sourceSpec := source.NewSourceSpec(s.PathSpec, s.BaseFs.ContentFs)
+	sourceSpec := source.NewSourceSpec(s.PathSpec, s.BaseFs.Content.Fs)
 
 	if s.running() {
 		// Need to track changes.
@@ -1717,6 +1729,8 @@ func (s *Site) renderForLayouts(name string, d interface{}, w io.Writer, layouts
 				templName = templ.Name()
 			}
 			s.DistinctErrorLog.Printf("Failed to render %q: %s", templName, r)
+			s.DistinctErrorLog.Printf("Stack Trace:\n%s", stackTrace(1200))
+
 			// TOD(bep) we really need to fix this. Also see below.
 			if !s.running() && !testMode {
 				os.Exit(-1)
@@ -1753,7 +1767,7 @@ func (s *Site) renderForLayouts(name string, d interface{}, w io.Writer, layouts
 
 func (s *Site) findFirstTemplate(layouts ...string) tpl.Template {
 	for _, layout := range layouts {
-		if templ := s.Tmpl.Lookup(layout); templ != nil {
+		if templ, found := s.Tmpl.Lookup(layout); found {
 			return templ
 		}
 	}
@@ -1782,7 +1796,7 @@ func (s *Site) newNodePage(typ string, sections ...string) *Page {
 		pageContentInit: &pageContentInit{},
 		Kind:            typ,
 		Source:          Source{File: &source.FileInfo{}},
-		Data:            make(map[string]interface{}),
+		data:            make(map[string]interface{}),
 		Site:            &s.Info,
 		sections:        sections,
 		s:               s}
@@ -1797,7 +1811,7 @@ func (s *Site) newHomePage() *Page {
 	p := s.newNodePage(KindHome)
 	p.title = s.Info.Title
 	pages := Pages{}
-	p.Data["Pages"] = pages
+	p.data["Pages"] = pages
 	p.Pages = pages
 	return p
 }