modules/funcmap: add funcs to calculate SRI hashes

* correctly handle LiveMode resp. whether or not to set/read embeds

app/app.go

@@ -2,9 +2,11 @@ package app
 
 import (
 	"embed"
+	"io/fs"
 
 	"git.dotya.ml/mirre-mt/pcmt/app/settings"
 	"git.dotya.ml/mirre-mt/pcmt/ent"
+	modfuncmap "git.dotya.ml/mirre-mt/pcmt/modules/funcmap"
 	"git.dotya.ml/mirre-mt/pcmt/slogging"
 	"github.com/labstack/echo/v4"
 	gommonlog "github.com/labstack/gommon/log"
@@ -111,9 +113,17 @@ func (a *App) SetSettings(s *settings.Settings) {
 
 // SetEmbeds saves the embedded files to application state.
 func (a *App) SetEmbeds(templates, assets embed.FS) {
-	a.Logger().Debug("saving embeds in app struct")
-	a.embeds.templates = templates
-	a.embeds.assets = assets
+	if !a.setting.IsLive() {
+		a.Logger().Debug("saving embeds in the app struct")
+
+		a.embeds.templates = templates
+		a.embeds.assets = assets
+
+		var fsfs fs.FS = &assets
+
+		// save pointer to assets to funcmap for integrity calculations.
+		modfuncmap.SetEmbeds(&fsfs)
+	}
 }
 
 // setDevel puts the app in devel mode, which loads a browser-sync script in

modules/funcmap/funcmap.go

@@ -2,10 +2,16 @@ package funcmap
 
 import (
 	"html/template"
+	"io/fs"
 
 	modbluemonday "git.dotya.ml/mirre-mt/pcmt/modules/bluemonday"
 )
 
+var (
+	embedAssets *fs.FS
+	isLive      = true
+)
+
 func FuncMap() template.FuncMap {
 	return template.FuncMap{
 		"ifIE":    func() template.HTML { return template.HTML("<!--[if IE]>") },
@@ -18,9 +24,25 @@ func FuncMap() template.FuncMap {
 		"pageIs": func(want, got string) bool {
 			return want == got
 		},
+		"sha256": func(path string) string {
+			t := New("sha256")
+
+			r, err := t.Integrity(path, isLive)
+			if err != nil {
+				return ""
+			}
+
+			return *r
+		},
 	}
 }
 
+// SetEmbeds saves the pointer to embedded assets (and toggles the isLive var).
+func SetEmbeds(embeds *fs.FS) {
+	embedAssets = embeds
+	isLive = false
+}
+
 // TODO: mimic https://github.com/drone/funcmap/blob/master/funcmap.go
 func setFuncMap(t *template.Template) { //nolint:unused
 	t.Funcs(FuncMap())

modules/funcmap/integrity.go

@@ -0,0 +1,115 @@
+package funcmap
+
+import (
+	"crypto/sha256"
+	"crypto/sha512"
+	"encoding/base64"
+	"fmt"
+	"hash"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+)
+
+// resIntegrity type implements several methods that allow getting a formatted
+// hash digest for the purpose of checking integrity of a web resource using
+// the algo specified.
+type resIntegrity struct {
+	algo string
+}
+
+const defaultHashAlgo = "sha256"
+
+// New returns a new resource integrity (resIntegrity) struct for the
+// particular algorithm.  Supported argument values are sha256, sha384 and
+// sha512.
+func New(algo string) *resIntegrity { //nolint:revive
+	return &resIntegrity{algo: algo}
+}
+
+// Integrity reads the resource at path, optionally from an fs.FS if parameter
+// live is false (i.e. we're checking embedded resources), and returns a
+// pointer to the integrity string in the form <algo>-<digest>.
+func (i *resIntegrity) Integrity(path string, live bool) (*string, error) {
+	r := ""
+
+	// e is an fs.FS created from embed.FS for convenience.
+	var e fs.FS
+
+	if embedAssets != nil {
+		e = *embedAssets
+	}
+
+	h, err := newHash(i.algo)
+	if err != nil {
+		return &r, err
+	}
+
+	if live {
+		path = filepath.Join("assets", "public", path)
+
+		f, err := os.Open(path)
+		if err != nil {
+			return &r, err
+		}
+
+		defer f.Close()
+
+		_, err = io.Copy(h, f)
+		if err != nil {
+			return &r, err
+		}
+	} else {
+		path = filepath.Join("assets", "public", path)
+
+		f, err := e.Open(path)
+		if err != nil {
+			return &r, err
+		}
+
+		defer f.Close()
+
+		_, err = io.Copy(h, f)
+		if err != nil {
+			return &r, err
+		}
+	}
+
+	d, err := digest(h)
+	if err != nil {
+		return &r, err
+	}
+
+	r = integrity(i.algo, d)
+
+	return &r, nil
+}
+
+func newHash(algo string) (hash.Hash, error) {
+	switch algo {
+	case "sha256":
+		return sha256.New(), nil
+	case "sha384":
+		return sha512.New384(), nil
+	case "sha512":
+		return sha512.New(), nil
+	default:
+		return nil, fmt.Errorf("unsupported hash algorithm: %q, use either sha256, sha384 or sha512", algo)
+	}
+}
+
+func integrity(algo string, sum []byte) string {
+	if algo == "" {
+		algo = defaultHashAlgo
+	}
+
+	encoded := base64.StdEncoding.EncodeToString(sum)
+
+	return algo + "-" + encoded
+}
+
+func digest(h hash.Hash) ([]byte, error) {
+	sum := h.Sum(nil)
+	return sum, nil
+}