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

2 files changed, 323 insertions, 0 deletions

diff --git a/tpl/resources/init.go b/tpl/resources/init.go
new file mode 100644
index 000000000..3e750f325
--- /dev/null
+++ b/tpl/resources/init.go
@@ -0,0 +1,68 @@
+// Copyright 2018 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 resources
+
+import (
+	"github.com/gohugoio/hugo/deps"
+	"github.com/gohugoio/hugo/tpl/internal"
+)
+
+const name = "resources"
+
+func init() {
+	f := func(d *deps.Deps) *internal.TemplateFuncsNamespace {
+		ctx, err := New(d)
+		if err != nil {
+			// TODO(bep) no panic.
+			panic(err)
+		}
+
+		ns := &internal.TemplateFuncsNamespace{
+			Name:    name,
+			Context: func(args ...interface{}) interface{} { return ctx },
+		}
+
+		ns.AddMethodMapping(ctx.Get,
+			nil,
+			[][2]string{},
+		)
+
+		// Add aliases for the most common transformations.
+
+		ns.AddMethodMapping(ctx.Fingerprint,
+			[]string{"fingerprint"},
+			[][2]string{},
+		)
+
+		ns.AddMethodMapping(ctx.Minify,
+			[]string{"minify"},
+			[][2]string{},
+		)
+
+		ns.AddMethodMapping(ctx.ToCSS,
+			[]string{"toCSS"},
+			[][2]string{},
+		)
+
+		ns.AddMethodMapping(ctx.PostCSS,
+			[]string{"postCSS"},
+			[][2]string{},
+		)
+
+		return ns
+
+	}
+
+	internal.AddTemplateFuncsNamespace(f)
+}
diff --git a/tpl/resources/resources.go b/tpl/resources/resources.go
new file mode 100644
index 000000000..5d4f6e315
--- /dev/null
+++ b/tpl/resources/resources.go
@@ -0,0 +1,255 @@
+// Copyright 2018 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 resources
+
+import (
+	"errors"
+	"fmt"
+	"path/filepath"
+
+	"github.com/gohugoio/hugo/deps"
+	"github.com/gohugoio/hugo/resource"
+	"github.com/gohugoio/hugo/resource/bundler"
+	"github.com/gohugoio/hugo/resource/create"
+	"github.com/gohugoio/hugo/resource/integrity"
+	"github.com/gohugoio/hugo/resource/minifiers"
+	"github.com/gohugoio/hugo/resource/postcss"
+	"github.com/gohugoio/hugo/resource/templates"
+	"github.com/gohugoio/hugo/resource/tocss/scss"
+	"github.com/spf13/cast"
+)
+
+// New returns a new instance of the resources-namespaced template functions.
+func New(deps *deps.Deps) (*Namespace, error) {
+	scssClient, err := scss.New(deps.BaseFs.Assets, deps.ResourceSpec)
+	if err != nil {
+		return nil, err
+	}
+	return &Namespace{
+		deps:            deps,
+		scssClient:      scssClient,
+		createClient:    create.New(deps.ResourceSpec),
+		bundlerClient:   bundler.New(deps.ResourceSpec),
+		integrityClient: integrity.New(deps.ResourceSpec),
+		minifyClient:    minifiers.New(deps.ResourceSpec),
+		postcssClient:   postcss.New(deps.ResourceSpec),
+		templatesClient: templates.New(deps.ResourceSpec, deps.TextTmpl),
+	}, nil
+}
+
+// Namespace provides template functions for the "resources" namespace.
+type Namespace struct {
+	deps *deps.Deps
+
+	createClient    *create.Client
+	bundlerClient   *bundler.Client
+	scssClient      *scss.Client
+	integrityClient *integrity.Client
+	minifyClient    *minifiers.Client
+	postcssClient   *postcss.Client
+	templatesClient *templates.Client
+}
+
+// Get locates the filename given in Hugo's filesystems: static, assets and content (in that order)
+// and creates a Resource object that can be used for further transformations.
+func (ns *Namespace) Get(filename interface{}) (resource.Resource, error) {
+	filenamestr, err := cast.ToStringE(filename)
+	if err != nil {
+		return nil, err
+	}
+
+	filenamestr = filepath.Clean(filenamestr)
+
+	// Resource Get'ing is currently limited to /assets to make it simpler
+	// to control the behaviour of publishing and partial rebuilding.
+	return ns.createClient.Get(ns.deps.BaseFs.Assets.Fs, filenamestr)
+
+}
+
+// Concat concatenates a slice of Resource objects. These resources must
+// (currently) be of the same Media Type.
+func (ns *Namespace) Concat(targetPathIn interface{}, r []interface{}) (resource.Resource, error) {
+	targetPath, err := cast.ToStringE(targetPathIn)
+	if err != nil {
+		return nil, err
+	}
+	rr := make([]resource.Resource, len(r))
+	for i := 0; i < len(r); i++ {
+		rv, ok := r[i].(resource.Resource)
+		if !ok {
+			return nil, fmt.Errorf("cannot concat type %T", rv)
+		}
+		rr[i] = rv
+	}
+	return ns.bundlerClient.Concat(targetPath, rr)
+}
+
+// FromString creates a Resource from a string published to the relative target path.
+func (ns *Namespace) FromString(targetPathIn, contentIn interface{}) (resource.Resource, error) {
+	targetPath, err := cast.ToStringE(targetPathIn)
+	if err != nil {
+		return nil, err
+	}
+	content, err := cast.ToStringE(contentIn)
+	if err != nil {
+		return nil, err
+	}
+
+	return ns.createClient.FromString(targetPath, content)
+}
+
+// ExecuteAsTemplate creates a Resource from a Go template, parsed and executed with
+// the given data, and published to the relative target path.
+func (ns *Namespace) ExecuteAsTemplate(args ...interface{}) (resource.Resource, error) {
+	if len(args) != 3 {
+		return nil, fmt.Errorf("must provide targetPath, the template data context and a Resource object")
+	}
+	targetPath, err := cast.ToStringE(args[0])
+	if err != nil {
+		return nil, err
+	}
+	data := args[1]
+
+	r, ok := args[2].(resource.Resource)
+	if !ok {
+		return nil, fmt.Errorf("type %T not supported in Resource transformations", args[2])
+	}
+
+	return ns.templatesClient.ExecuteAsTemplate(r, targetPath, data)
+}
+
+// Fingerprint transforms the given Resource with a MD5 hash of the content in
+// the RelPermalink and Permalink.
+func (ns *Namespace) Fingerprint(args ...interface{}) (resource.Resource, error) {
+	if len(args) < 1 || len(args) > 2 {
+		return nil, errors.New("must provide a Resource and (optional) crypto algo")
+	}
+
+	var algo string
+	resIdx := 0
+
+	if len(args) == 2 {
+		resIdx = 1
+		var err error
+		algo, err = cast.ToStringE(args[0])
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	r, ok := args[resIdx].(resource.Resource)
+	if !ok {
+		return nil, fmt.Errorf("%T is not a Resource", args[resIdx])
+	}
+
+	return ns.integrityClient.Fingerprint(r, algo)
+}
+
+// Minify minifies the given Resource using the MediaType to pick the correct
+// minifier.
+func (ns *Namespace) Minify(r resource.Resource) (resource.Resource, error) {
+	return ns.minifyClient.Minify(r)
+}
+
+// ToCSS converts the given Resource to CSS. You can optional provide an Options
+// object or a target path (string) as first argument.
+func (ns *Namespace) ToCSS(args ...interface{}) (resource.Resource, error) {
+	var (
+		r          resource.Resource
+		m          map[string]interface{}
+		targetPath string
+		err        error
+		ok         bool
+	)
+
+	r, targetPath, ok = ns.resolveIfFirstArgIsString(args)
+
+	if !ok {
+		r, m, err = ns.resolveArgs(args)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	var options scss.Options
+	if targetPath != "" {
+		options.TargetPath = targetPath
+	} else if m != nil {
+		options, err = scss.DecodeOptions(m)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return ns.scssClient.ToCSS(r, options)
+}
+
+// PostCSS processes the given Resource with PostCSS
+func (ns *Namespace) PostCSS(args ...interface{}) (resource.Resource, error) {
+	r, m, err := ns.resolveArgs(args)
+	if err != nil {
+		return nil, err
+	}
+	var options postcss.Options
+	if m != nil {
+		options, err = postcss.DecodeOptions(m)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return ns.postcssClient.Process(r, options)
+}
+
+// We allow string or a map as the first argument in some cases.
+func (ns *Namespace) resolveIfFirstArgIsString(args []interface{}) (resource.Resource, string, bool) {
+	if len(args) != 2 {
+		return nil, "", false
+	}
+
+	v1, ok1 := args[0].(string)
+	if !ok1 {
+		return nil, "", false
+	}
+	v2, ok2 := args[1].(resource.Resource)
+
+	return v2, v1, ok2
+}
+
+// This roundabout way of doing it is needed to get both pipeline behaviour and options as arguments.
+func (ns *Namespace) resolveArgs(args []interface{}) (resource.Resource, map[string]interface{}, error) {
+	if len(args) == 0 {
+		return nil, nil, errors.New("no Resource provided in transformation")
+	}
+
+	if len(args) == 1 {
+		r, ok := args[0].(resource.Resource)
+		if !ok {
+			return nil, nil, fmt.Errorf("type %T not supported in Resource transformations", args[0])
+		}
+		return r, nil, nil
+	}
+
+	r, ok := args[1].(resource.Resource)
+	if !ok {
+		return nil, nil, fmt.Errorf("type %T not supported in Resource transformations", args[0])
+	}
+
+	m, err := cast.ToStringMapE(args[0])
+	if err != nil {
+		return nil, nil, fmt.Errorf("invalid options type: %s", err)
+	}
+
+	return r, m, nil
+}