resources: Cache Exif data to disk

```bash
name                        old time/op    new time/op    delta
ImageExif/Cold_cache-4         312µs ±28%     355µs ± 7%     ~     (p=0.343 n=4+4)
ImageExif/Cold_cache,_10-4     479µs ± 6%     546µs ± 0%  +13.91%  (p=0.029 n=4+4)
ImageExif/Warm_cache-4         272µs ± 1%      81µs ± 5%  -70.30%  (p=0.029 n=4+4)

name                        old alloc/op   new alloc/op   delta
ImageExif/Cold_cache-4         151kB ± 0%     161kB ± 0%   +6.46%  (p=0.029 n=4+4)
ImageExif/Cold_cache,_10-4     179kB ± 0%     189kB ± 0%   +5.49%  (p=0.029 n=4+4)
ImageExif/Warm_cache-4         151kB ± 0%      13kB ± 0%  -91.52%  (p=0.029 n=4+4)

name                        old allocs/op  new allocs/op  delta
ImageExif/Cold_cache-4         1.03k ± 0%     1.21k ± 0%  +17.78%  (p=0.029 n=4+4)
ImageExif/Cold_cache,_10-4     1.65k ± 0%     1.83k ± 0%  +11.09%  (p=0.029 n=4+4)
ImageExif/Warm_cache-4         1.03k ± 0%     0.28k ± 0%  -72.40%  (p=0.029 n=4+4)
```

Fixes #6291

4 files changed, 161 insertions, 38 deletions

diff --git a/resources/image.go b/resources/image.go
index f335e97da..8242f7119 100644
--- a/resources/image.go
+++ b/resources/image.go
@@ -14,15 +14,20 @@
 package resources
 
 import (
+	"encoding/json"
 	"fmt"
 	"image"
 	"image/draw"
 	_ "image/gif"
 	_ "image/png"
+	"io"
+	"io/ioutil"
 	"os"
+	"path"
 	"strings"
 	"sync"
 
+	"github.com/gohugoio/hugo/cache/filecache"
 	"github.com/gohugoio/hugo/resources/images/exif"
 
 	"github.com/gohugoio/hugo/resources/internal"
@@ -55,43 +60,81 @@ type imageResource struct {
 	// original (first).
 	root *imageResource
 
-	exifInit    sync.Once
-	exifInitErr error
-	exif        *exif.Exif
+	metaInit    sync.Once
+	metaInitErr error
+	meta        *imageMeta
 
 	baseResource
 }
 
+type imageMeta struct {
+	Exif *exif.Exif
+}
+
 func (i *imageResource) Exif() (*exif.Exif, error) {
 	return i.root.getExif()
 }
 
 func (i *imageResource) getExif() (*exif.Exif, error) {
 
-	i.exifInit.Do(func() {
+	i.metaInit.Do(func() {
+
 		supportsExif := i.Format == images.JPEG || i.Format == images.TIFF
 		if !supportsExif {
 			return
+
 		}
 
-		f, err := i.root.ReadSeekCloser()
-		if err != nil {
-			i.exifInitErr = err
-			return
+		key := i.getImageMetaCacheTargetPath()
+
+		read := func(info filecache.ItemInfo, r io.Reader) error {
+			meta := &imageMeta{}
+			data, err := ioutil.ReadAll(r)
+			if err != nil {
+				return err
+			}
+
+			if err = json.Unmarshal(data, &meta); err != nil {
+				return err
+			}
+
+			i.meta = meta
+
+			return nil
 		}
-		defer f.Close()
 
-		x, err := i.getSpec().imaging.DecodeExif(f)
-		if err != nil {
-			i.exifInitErr = err
-			return
+		create := func(info filecache.ItemInfo, w io.WriteCloser) (err error) {
+
+			f, err := i.root.ReadSeekCloser()
+			if err != nil {
+				i.metaInitErr = err
+				return
+			}
+			defer f.Close()
+
+			x, err := i.getSpec().imaging.DecodeExif(f)
+			if err != nil {
+				i.metaInitErr = err
+				return
+			}
+
+			i.meta = &imageMeta{Exif: x}
+
+			// Also write it to cache
+			enc := json.NewEncoder(w)
+			return enc.Encode(i.meta)
+
 		}
 
-		i.exif = x
+		_, i.metaInitErr = i.getSpec().imageCache.fileCache.ReadOrCreate(key, read, create)
 
 	})
 
-	return i.exif, i.exifInitErr
+	if i.metaInitErr != nil {
+		return nil, i.metaInitErr
+	}
+
+	return i.meta.Exif, nil
 }
 
 func (i *imageResource) Clone() resource.Resource {
@@ -271,6 +314,17 @@ func (i *imageResource) setBasePath(conf images.ImageConfig) {
 	i.getResourcePaths().relTargetDirFile = i.relTargetPathFromConfig(conf)
 }
 
+func (i *imageResource) getImageMetaCacheTargetPath() string {
+	const imageMetaVersionNumber = 1 // Increment to invalidate the meta cache
+
+	cfg := i.getSpec().imaging.Cfg
+	df := i.getResourcePaths().relTargetDirFile
+	p1, _ := helpers.FileAndExt(df.file)
+	h, _ := i.hash()
+	idStr := internal.HashString(h, i.size(), imageMetaVersionNumber, cfg)
+	return path.Join(df.dir, fmt.Sprintf("%s%s.json", p1, idStr))
+}
+
 func (i *imageResource) relTargetPathFromConfig(conf images.ImageConfig) dirFile {
 	p1, p2 := helpers.FileAndExt(i.getResourcePaths().relTargetDirFile.file)
 	if conf.Action == "trace" {
diff --git a/resources/image_test.go b/resources/image_test.go
index 6d879d271..5afc4e36a 100644
--- a/resources/image_test.go
+++ b/resources/image_test.go
@@ -15,6 +15,7 @@ package resources
 
 import (
 	"fmt"
+	"math/big"
 	"math/rand"
 	"os"
 	"path/filepath"
@@ -22,6 +23,7 @@ import (
 	"strconv"
 	"sync"
 	"testing"
+	"time"
 
 	"github.com/spf13/afero"
 
@@ -50,6 +52,14 @@ var eq = qt.CmpEquals(
 	cmp.Comparer(func(m1, m2 media.Type) bool {
 		return m1.Type() == m2.Type()
 	}),
+	cmp.Comparer(
+		func(v1, v2 *big.Rat) bool {
+			return v1.RatString() == v2.RatString()
+		},
+	),
+	cmp.Comparer(func(v1, v2 time.Time) bool {
+		return v1.Unix() == v2.Unix()
+	}),
 )
 
 func TestImageTransformBasic(t *testing.T) {
@@ -336,27 +346,35 @@ func TestSVGImageContent(t *testing.T) {
 
 func TestImageExif(t *testing.T) {
 	c := qt.New(t)
-	image := fetchImage(c, "sunset.jpg")
+	fs := afero.NewMemMapFs()
+	spec := newTestResourceSpec(specDescriptor{fs: fs, c: c})
+	image := fetchResourceForSpec(spec, c, "sunset.jpg").(resource.Image)
 
-	x, err := image.Exif()
-	c.Assert(err, qt.IsNil)
-	c.Assert(x, qt.Not(qt.IsNil))
+	getAndCheckExif := func(c *qt.C, image resource.Image) {
+		x, err := image.Exif()
+		c.Assert(err, qt.IsNil)
+		c.Assert(x, qt.Not(qt.IsNil))
 
-	c.Assert(x.Date.Format("2006-01-02"), qt.Equals, "2017-10-27")
+		c.Assert(x.Date.Format("2006-01-02"), qt.Equals, "2017-10-27")
 
-	// Malaga: https://goo.gl/taazZy
-	c.Assert(x.Lat, qt.Equals, float64(36.59744166666667))
-	c.Assert(x.Long, qt.Equals, float64(-4.50846))
+		// Malaga: https://goo.gl/taazZy
+		c.Assert(x.Lat, qt.Equals, float64(36.59744166666667))
+		c.Assert(x.Long, qt.Equals, float64(-4.50846))
 
-	v, found := x.Values["LensModel"]
-	c.Assert(found, qt.Equals, true)
-	lensModel, ok := v.(string)
-	c.Assert(ok, qt.Equals, true)
-	c.Assert(lensModel, qt.Equals, "smc PENTAX-DA* 16-50mm F2.8 ED AL [IF] SDM")
+		v, found := x.Tags["LensModel"]
+		c.Assert(found, qt.Equals, true)
+		lensModel, ok := v.(string)
+		c.Assert(ok, qt.Equals, true)
+		c.Assert(lensModel, qt.Equals, "smc PENTAX-DA* 16-50mm F2.8 ED AL [IF] SDM")
+		resized, _ := image.Resize("300x200")
+		x2, _ := resized.Exif()
+		c.Assert(x2, eq, x)
+	}
 
-	resized, _ := image.Resize("300x200")
-	x2, _ := resized.Exif()
-	c.Assert(x2, qt.Equals, x)
+	getAndCheckExif(c, image)
+	image = fetchResourceForSpec(spec, c, "sunset.jpg").(resource.Image)
+	// This will read from file cache.
+	getAndCheckExif(c, image)
 
 }
 
diff --git a/resources/images/exif/exif.go b/resources/images/exif/exif.go
index 7a3c982c1..b5161f770 100644
--- a/resources/images/exif/exif.go
+++ b/resources/images/exif/exif.go
@@ -24,6 +24,8 @@ import (
 	"unicode"
 	"unicode/utf8"
 
+	"github.com/bep/tmc"
+
 	_exif "github.com/rwcarlsen/goexif/exif"
 	"github.com/rwcarlsen/goexif/tiff"
 )
@@ -31,10 +33,10 @@ import (
 const exifTimeLayout = "2006:01:02 15:04:05"
 
 type Exif struct {
-	Lat    float64
-	Long   float64
-	Date   time.Time
-	Values map[string]interface{}
+	Lat  float64
+	Long float64
+	Date time.Time
+	Tags Tags
 }
 
 type Decoder struct {
@@ -139,7 +141,7 @@ func (d *Decoder) Decode(r io.Reader) (ex *Exif, err error) {
 		return
 	}
 
-	ex = &Exif{Lat: lat, Long: long, Date: tm, Values: walker.vals}
+	ex = &Exif{Lat: lat, Long: long, Date: tm, Tags: walker.vals}
 
 	return
 }
@@ -240,3 +242,30 @@ func nullString(in []byte) string {
 
 	return ""
 }
+
+var tcodec *tmc.Codec
+
+func init() {
+	var err error
+	tcodec, err = tmc.New()
+	if err != nil {
+		panic(err)
+	}
+}
+
+type Tags map[string]interface{}
+
+func (v *Tags) UnmarshalJSON(b []byte) error {
+	vv := make(map[string]interface{})
+	if err := tcodec.Unmarshal(b, &vv); err != nil {
+		return err
+	}
+
+	*v = vv
+
+	return nil
+}
+
+func (v Tags) MarshalJSON() ([]byte, error) {
+	return tcodec.Marshal(v)
+}
diff --git a/resources/images/exif/exif_test.go b/resources/images/exif/exif_test.go
index eee60c089..c3cfad1cc 100644
--- a/resources/images/exif/exif_test.go
+++ b/resources/images/exif/exif_test.go
@@ -14,12 +14,15 @@
 package exif
 
 import (
+	"encoding/json"
+	"math/big"
 	"os"
 	"path/filepath"
 	"testing"
 	"time"
 
 	"github.com/gohugoio/hugo/htesting/hqt"
+	"github.com/google/go-cmp/cmp"
 
 	qt "github.com/frankban/quicktest"
 )
@@ -40,16 +43,24 @@ func TestExif(t *testing.T) {
 	c.Assert(x.Lat, qt.Equals, float64(36.59744166666667))
 	c.Assert(x.Long, qt.Equals, float64(-4.50846))
 
-	v, found := x.Values["LensModel"]
+	v, found := x.Tags["LensModel"]
 	c.Assert(found, qt.Equals, true)
 	lensModel, ok := v.(string)
 	c.Assert(ok, qt.Equals, true)
 	c.Assert(lensModel, qt.Equals, "smc PENTAX-DA* 16-50mm F2.8 ED AL [IF] SDM")
 
-	v, found = x.Values["DateTime"]
+	v, found = x.Tags["DateTime"]
 	c.Assert(found, qt.Equals, true)
 	c.Assert(v, hqt.IsSameType, time.Time{})
 
+	// Verify that it survives a round-trip to JSON and back.
+	data, err := json.Marshal(x)
+	c.Assert(err, qt.IsNil)
+	x2 := &Exif{}
+	err = json.Unmarshal(data, x2)
+
+	c.Assert(x2, eq, x)
+
 }
 
 func TestExifPNG(t *testing.T) {
@@ -81,3 +92,14 @@ func BenchmarkDecodeExif(b *testing.B) {
 		f.Seek(0, 0)
 	}
 }
+
+var eq = qt.CmpEquals(
+	cmp.Comparer(
+		func(v1, v2 *big.Rat) bool {
+			return v1.RatString() == v2.RatString()
+		},
+	),
+	cmp.Comparer(func(v1, v2 time.Time) bool {
+		return v1.Unix() == v2.Unix()
+	}),
+)