Add Dart Sass support

But note that the Dart Sass Embedded Protocol is still in beta (beta 5), a main release scheduled for Q1 2021.

Fixes #7380
Fixes #8102

3 files changed, 343 insertions, 0 deletions

diff --git a/resources/resource_transformers/tocss/dartsass/client.go b/resources/resource_transformers/tocss/dartsass/client.go
new file mode 100644
index 000000000..1d8250dc5
--- /dev/null
+++ b/resources/resource_transformers/tocss/dartsass/client.go
@@ -0,0 +1,115 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package godartsass integrates with the Dass Sass Embedded protocol to transpile
+// SCSS/SASS.
+package dartsass
+
+import (
+	"io"
+
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/hugolib/filesystems"
+	"github.com/gohugoio/hugo/resources"
+	"github.com/gohugoio/hugo/resources/resource"
+	"github.com/spf13/afero"
+
+	"github.com/bep/godartsass"
+	"github.com/mitchellh/mapstructure"
+)
+
+// used as part of the cache key.
+const transformationName = "tocss-dart"
+
+func New(fs *filesystems.SourceFilesystem, rs *resources.Spec) (*Client, error) {
+	if !Supports() {
+		return &Client{dartSassNoAvailable: true}, nil
+	}
+	transpiler, err := godartsass.Start(godartsass.Options{})
+	if err != nil {
+		return nil, err
+	}
+	return &Client{sfs: fs, workFs: rs.BaseFs.Work, rs: rs, transpiler: transpiler}, nil
+}
+
+type Client struct {
+	dartSassNoAvailable bool
+	rs                  *resources.Spec
+	sfs                 *filesystems.SourceFilesystem
+	workFs              afero.Fs
+	transpiler          *godartsass.Transpiler
+}
+
+func (c *Client) ToCSS(res resources.ResourceTransformer, args map[string]interface{}) (resource.Resource, error) {
+	if c.dartSassNoAvailable {
+		return res.Transform(resources.NewFeatureNotAvailableTransformer(transformationName, args))
+	}
+	return res.Transform(&transform{c: c, optsm: args})
+}
+
+func (c *Client) Close() error {
+	if c.transpiler == nil {
+		return nil
+	}
+	return c.transpiler.Close()
+}
+
+func (c *Client) toCSS(args godartsass.Args, src io.Reader) (godartsass.Result, error) {
+	var res godartsass.Result
+
+	in := helpers.ReaderToString(src)
+	args.Source = in
+
+	res, err := c.transpiler.Execute(args)
+	if err != nil {
+		return res, err
+	}
+
+	return res, err
+}
+
+type Options struct {
+
+	// Hugo, will by default, just replace the extension of the source
+	// to .css, e.g. "scss/main.scss" becomes "scss/main.css". You can
+	// control this by setting this, e.g. "styles/main.css" will create
+	// a Resource with that as a base for RelPermalink etc.
+	TargetPath string
+
+	// Hugo automatically adds the entry directories (where the main.scss lives)
+	// for project and themes to the list of include paths sent to LibSASS.
+	// Any paths set in this setting will be appended. Note that these will be
+	// treated as relative to the working dir, i.e. no include paths outside the
+	// project/themes.
+	IncludePaths []string
+
+	// Default is nested.
+	// One of nested, expanded, compact, compressed.
+	OutputStyle string
+
+	// When enabled, Hugo will generate a source map.
+	EnableSourceMap bool
+}
+
+func decodeOptions(m map[string]interface{}) (opts Options, err error) {
+	if m == nil {
+		return
+	}
+	err = mapstructure.WeakDecode(m, &opts)
+
+	if opts.TargetPath != "" {
+		opts.TargetPath = helpers.ToSlashTrimLeading(opts.TargetPath)
+	}
+
+	return
+}
diff --git a/resources/resource_transformers/tocss/dartsass/transform.go b/resources/resource_transformers/tocss/dartsass/transform.go
new file mode 100644
index 000000000..4cbb35fb9
--- /dev/null
+++ b/resources/resource_transformers/tocss/dartsass/transform.go
@@ -0,0 +1,222 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package dartsass
+
+import (
+	"fmt"
+	"io"
+	"net/url"
+	"path"
+	"path/filepath"
+	"strings"
+
+	"github.com/cli/safeexec"
+
+	"github.com/gohugoio/hugo/common/herrors"
+	"github.com/gohugoio/hugo/htesting"
+	"github.com/gohugoio/hugo/media"
+
+	"github.com/gohugoio/hugo/resources"
+
+	"github.com/gohugoio/hugo/resources/internal"
+
+	"github.com/spf13/afero"
+
+	"github.com/gohugoio/hugo/hugofs"
+
+	"github.com/bep/godartsass"
+)
+
+// See https://github.com/sass/dart-sass-embedded/issues/24
+const stdinPlaceholder = "HUGOSTDIN"
+
+// Supports returns whether dart-sass-embedded is found in $PATH.
+func Supports() bool {
+	if htesting.SupportsAll() {
+		return true
+	}
+	p, err := safeexec.LookPath("dart-sass-embedded")
+	return err == nil && p != ""
+}
+
+type transform struct {
+	optsm map[string]interface{}
+	c     *Client
+}
+
+func (t *transform) Key() internal.ResourceTransformationKey {
+	return internal.NewResourceTransformationKey(transformationName, t.optsm)
+}
+
+func (t *transform) Transform(ctx *resources.ResourceTransformationCtx) error {
+	ctx.OutMediaType = media.CSSType
+
+	opts, err := decodeOptions(t.optsm)
+	if err != nil {
+		return err
+	}
+
+	if opts.TargetPath != "" {
+		ctx.OutPath = opts.TargetPath
+	} else {
+		ctx.ReplaceOutPathExtension(".css")
+	}
+
+	baseDir := path.Dir(ctx.SourcePath)
+
+	args := godartsass.Args{
+		URL:          stdinPlaceholder,
+		IncludePaths: t.c.sfs.RealDirs(baseDir),
+		ImportResolver: importResolver{
+			baseDir: baseDir,
+			c:       t.c,
+		},
+		EnableSourceMap: opts.EnableSourceMap,
+	}
+
+	// Append any workDir relative include paths
+	for _, ip := range opts.IncludePaths {
+		info, err := t.c.workFs.Stat(filepath.Clean(ip))
+		if err == nil {
+			filename := info.(hugofs.FileMetaInfo).Meta().Filename()
+			args.IncludePaths = append(args.IncludePaths, filename)
+		}
+	}
+
+	if ctx.InMediaType.SubType == media.SASSType.SubType {
+		args.SourceSyntax = godartsass.SourceSyntaxSASS
+	}
+
+	res, err := t.c.toCSS(args, ctx.From)
+	if err != nil {
+		if sassErr, ok := err.(godartsass.SassError); ok {
+			start := sassErr.Span.Start
+			context := strings.TrimSpace(sassErr.Span.Context)
+			filename, _ := urlToFilename(sassErr.Span.Url)
+			if filename == stdinPlaceholder {
+				if ctx.SourcePath == "" {
+					return sassErr
+				}
+				filename = t.c.sfs.RealFilename(ctx.SourcePath)
+			}
+
+			offsetMatcher := func(m herrors.LineMatcher) bool {
+				return m.Offset+len(m.Line) >= start.Offset && strings.Contains(m.Line, context)
+			}
+
+			ferr, ok := herrors.WithFileContextForFile(
+				herrors.NewFileError("scss", -1, -1, start.Column, sassErr),
+				filename,
+				filename,
+				hugofs.Os,
+				offsetMatcher)
+
+			if !ok {
+				return sassErr
+			}
+
+			return ferr
+		}
+		return err
+	}
+
+	out := res.CSS
+
+	_, err = io.WriteString(ctx.To, out)
+	if err != nil {
+		return err
+	}
+
+	if opts.EnableSourceMap && res.SourceMap != "" {
+		if err := ctx.PublishSourceMap(res.SourceMap); err != nil {
+			return err
+		}
+		_, err = fmt.Fprintf(ctx.To, "\n\n/*# sourceMappingURL=%s */", path.Base(ctx.OutPath)+".map")
+	}
+
+	return err
+}
+
+type importResolver struct {
+	baseDir string
+	c       *Client
+}
+
+func (t importResolver) CanonicalizeURL(url string) (string, error) {
+	filePath, isURL := urlToFilename(url)
+	var prevDir string
+	var pathDir string
+	if isURL {
+		var found bool
+		prevDir, found = t.c.sfs.MakePathRelative(filepath.Dir(filePath))
+
+		if !found {
+			// Not a member of this filesystem, let Dart Sass handle it.
+			return "", nil
+		}
+	} else {
+		prevDir = t.baseDir
+		pathDir = path.Dir(url)
+	}
+
+	basePath := filepath.Join(prevDir, pathDir)
+	name := filepath.Base(filePath)
+
+	// Pick the first match.
+	var namePatterns []string
+	if strings.Contains(name, ".") {
+		namePatterns = []string{"_%s", "%s"}
+	} else if strings.HasPrefix(name, "_") {
+		namePatterns = []string{"_%s.scss", "_%s.sass"}
+	} else {
+		namePatterns = []string{"_%s.scss", "%s.scss", "_%s.sass", "%s.sass"}
+	}
+
+	name = strings.TrimPrefix(name, "_")
+
+	for _, namePattern := range namePatterns {
+		filenameToCheck := filepath.Join(basePath, fmt.Sprintf(namePattern, name))
+		fi, err := t.c.sfs.Fs.Stat(filenameToCheck)
+		if err == nil {
+			if fim, ok := fi.(hugofs.FileMetaInfo); ok {
+				return "file://" + filepath.ToSlash(fim.Meta().Filename()), nil
+			}
+		}
+	}
+
+	// Not found, let Dart Dass handle it
+	return "", nil
+}
+
+func (t importResolver) Load(url string) (string, error) {
+	filename, _ := urlToFilename(url)
+	b, err := afero.ReadFile(hugofs.Os, filename)
+	return string(b), err
+}
+
+// TODO(bep) add tests
+func urlToFilename(urls string) (string, bool) {
+	u, err := url.ParseRequestURI(urls)
+	if err != nil {
+		return filepath.FromSlash(urls), false
+	}
+	p := filepath.FromSlash(u.Path)
+
+	if u.Host != "" {
+		// C:\data\file.txt
+		p = strings.ToUpper(u.Host) + ":" + p
+	}
+
+	return p, true
+}
diff --git a/resources/transform.go b/resources/transform.go
index 9007ead18..f276f00c3 100644
--- a/resources/transform.go
+++ b/resources/transform.go
@@ -411,6 +411,9 @@ func (r *resourceAdapter) transform(publish, setContent bool) error {
 					errMsg = ". Check your PostCSS installation; install with \"npm install postcss-cli\". See https://gohugo.io/hugo-pipes/postcss/"
 				} else if tr.Key().Name == "tocss" {
 					errMsg = ". Check your Hugo installation; you need the extended version to build SCSS/SASS."
+				} else if tr.Key().Name == "tocss-dart" {
+					errMsg = ". You need dart-sass-embedded in your system $PATH."
+
 				} else if tr.Key().Name == "babel" {
 					errMsg = ". You need to install Babel, see https://gohugo.io/hugo-pipes/babel/"
 				}
@@ -442,6 +445,9 @@ func (r *resourceAdapter) transform(publish, setContent bool) error {
 		if tryFileCache {
 			f := r.target.tryTransformedFileCache(key, updates)
 			if f == nil {
+				if err != nil {
+					return newErr(err)
+				}
 				return newErr(errors.Errorf("resource %q not found in file cache", key))
 			}
 			transformedContentr = f