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tpl: Add imageConfig function

Add imageConfig function which calls image.DecodeConfig and returns the height, width and color mode of the image. (#2677)

This allows for more advanced image shortcodes and templates such as those required by AMP.

layouts/shortcodes/amp-img.html
```
{{ $src := .Get "src" }}
{{ $config := imageConfig (printf "/static/%s" $src) }}

<amp-img src="{{$src}}"
           height="{{$config.Height}}"
           width="{{$config.Width}}"
           layout="responsive">
</amp-img>
```

--- a/docs/content/templates/functions.md

+++ b/docs/content/templates/functions.md

@@ -356,7 +356,7 @@

        {{ .Content }}

     {{ end }}

-## Files    

+## Files

 ### readDir

@@ -371,6 +371,16 @@

  So, if you have a file with the name `README.txt` in the root of your project with the content `Hugo Rocks!`:

  `{{readFile "README.txt"}}` → `"Hugo Rocks!"`

+

+### imageConfig

+Parses the image and returns the height, width and color model.

+

+e.g.

+```

+{{ with (imageConfig "favicon.ico") }}

+favicon.ico: {{.Width}} x {{.Height}}

+{{ end }}

+```

 ## Math

--- a/hugolib/hugo_sites.go

+++ b/hugolib/hugo_sites.go

@@ -120,12 +120,14 @@

 	return Nodes{}

 }

-// Reset resets the sites, making it ready for a full rebuild.

+// Reset resets the sites and template caches, making it ready for a full rebuild.

 func (h *HugoSites) reset() {

 	h.nodeMap = make(map[string]Nodes)

 	for i, s := range h.Sites {

 		h.Sites[i] = s.reset()

 	}

+

+	tpl.ResetCaches()

 }

 func (h *HugoSites) reCreateFromConfig() error {

--- a/tpl/template_funcs.go

+++ b/tpl/template_funcs.go

@@ -26,6 +26,7 @@

 	"fmt"

 	"html"

 	"html/template"

+	"image"

 	"math/rand"

 	"net/url"

 	"os"

@@ -45,6 +46,11 @@

 	"github.com/spf13/hugo/hugofs"

 	jww "github.com/spf13/jwalterweatherman"

 	"github.com/spf13/viper"

+

+	// Importing image codecs for image.DecodeConfig

+	_ "image/gif"

+	_ "image/jpeg"

+	_ "image/png"

 )

 var (

@@ -364,6 +370,63 @@

 	}

 }

+// ResetCaches resets all caches that might be used during build.

+func ResetCaches() {

+	resetImageConfigCache()

+}

+

+// imageConfigCache is a lockable cache for image.Config objects. It must be

+// locked before reading or writing to config.

+var imageConfigCache struct {

+	sync.RWMutex

+	config map[string]image.Config

+}

+

+// resetImageConfigCache initializes and resets the imageConfig cache for the

+// imageConfig template function. This should be run once before every batch of

+// template renderers so the cache is cleared for new data.

+func resetImageConfigCache() {

+	imageConfigCache.Lock()

+	defer imageConfigCache.Unlock()

+

+	imageConfigCache.config = map[string]image.Config{}

+}

+

+// imageConfig returns the image.Config for the specified path relative to the

+// working directory. resetImageConfigCache must be run beforehand.

+func imageConfig(path interface{}) (image.Config, error) {

+	filename, err := cast.ToStringE(path)

+	if err != nil {

+		return image.Config{}, err

+	}

+

+	if filename == "" {

+		return image.Config{}, errors.New("imageConfig needs a filename")

+	}

+

+	// Check cache for image config.

+	imageConfigCache.RLock()

+	config, ok := imageConfigCache.config[filename]

+	imageConfigCache.RUnlock()

+

+	if ok {

+		return config, nil

+	}

+

+	f, err := hugofs.WorkingDir().Open(filename)

+	if err != nil {

+		return image.Config{}, err

+	}

+

+	config, _, err = image.DecodeConfig(f)

+

+	imageConfigCache.Lock()

+	imageConfigCache.config[filename] = config

+	imageConfigCache.Unlock()

+

+	return config, err

+}

+

 // in returns whether v is in the set l.  l may be an array or slice.

 func in(l interface{}, v interface{}) bool {

 	lv := reflect.ValueOf(l)

@@ -1991,6 +2054,7 @@

 		"htmlEscape":    htmlEscape,

 		"htmlUnescape":  htmlUnescape,

 		"humanize":      humanize,

+		"imageConfig":   imageConfig,

 		"in":            in,

 		"index":         index,

 		"int":           func(v interface{}) (int, error) { return cast.ToIntE(v) },

--- a/tpl/template_funcs_test.go

+++ b/tpl/template_funcs_test.go

@@ -19,6 +19,9 @@

 	"errors"

 	"fmt"

 	"html/template"

+	"image"

+	"image/color"

+	"image/png"

 	"math/rand"

 	"path"

 	"path/filepath"

@@ -593,6 +596,109 @@

 				t.Errorf("[%d] got %v but expected %v", i, r, this.expectedValue)

 			}

 		}

+	}

+}

+

+func blankImage(width, height int) []byte {

+	var buf bytes.Buffer

+	img := image.NewRGBA(image.Rect(0, 0, width, height))

+	if err := png.Encode(&buf, img); err != nil {

+		panic(err)

+	}

+	return buf.Bytes()

+}

+

+func TestImageConfig(t *testing.T) {

+	viper.Reset()

+	defer viper.Reset()

+

+	workingDir := "/home/hugo"

+

+	viper.Set("workingDir", workingDir)

+

+	fs := &afero.MemMapFs{}

+	hugofs.InitFs(fs)

+

+	for i, this := range []struct {

+		resetCache bool

+		path       string

+		input      []byte

+		expected   image.Config

+	}{

+		{

+			resetCache: true,

+			path:       "a.png",

+			input:      blankImage(10, 10),

+			expected: image.Config{

+				Width:      10,

+				Height:     10,

+				ColorModel: color.NRGBAModel,

+			},

+		},

+		{

+			resetCache: false,

+			path:       "b.png",

+			input:      blankImage(20, 15),

+			expected: image.Config{

+				Width:      20,

+				Height:     15,

+				ColorModel: color.NRGBAModel,

+			},

+		},

+		{

+			resetCache: false,

+			path:       "a.png",

+			input:      blankImage(20, 15),

+			expected: image.Config{

+				Width:      10,

+				Height:     10,

+				ColorModel: color.NRGBAModel,

+			},

+		},

+		{

+			resetCache: true,

+			path:       "a.png",

+			input:      blankImage(20, 15),

+			expected: image.Config{

+				Width:      20,

+				Height:     15,

+				ColorModel: color.NRGBAModel,

+			},

+		},

+	} {

+		afero.WriteFile(fs, filepath.Join(workingDir, this.path), this.input, 0755)

+

+		if this.resetCache {

+			resetImageConfigCache()

+		}

+

+		result, err := imageConfig(this.path)

+		if err != nil {

+			t.Errorf("imageConfig returned error: %s", err)

+		}

+

+		if !reflect.DeepEqual(result, this.expected) {

+			t.Errorf("[%d] imageConfig: expected '%v', got '%v'", i, this.expected, result)

+		}

+

+		if len(imageConfigCache.config) == 0 {

+			t.Error("imageConfigCache should have at least 1 item")

+		}

+	}

+

+	if _, err := imageConfig(t); err == nil {

+		t.Error("Expected error from imageConfig when passed invalid path")

+	}

+

+	if _, err := imageConfig("non-existant.png"); err == nil {

+		t.Error("Expected error from imageConfig when passed non-existant file")

+	}

+

+	// test cache clearing

+	ResetCaches()

+

+	if len(imageConfigCache.config) != 0 {

+		t.Error("ResetCaches should have cleared imageConfigCache")

 	}

 }