Add Image Diff for SVG files (#14867)

* Added type sniffer.

* Switched content detection from base to typesniffer.

* Added GuessContentType to Blob.

* Moved image info logic to client.
Added support for SVG images in diff.

* Restore old blocked svg behaviour.

* Added missing image formats.

* Execute image diff only when container is visible.

* add margin to spinner

* improve BIN tag on image diffs

* Default to render view.

* Show image diff on incomplete diff.

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>

modules/typesniffer/typesniffer.go

@@ -0,0 +1,96 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package typesniffer
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"regexp"
+	"strings"
+)
+
+// Use at most this many bytes to determine Content Type.
+const sniffLen = 1024
+
+// SvgMimeType MIME type of SVG images.
+const SvgMimeType = "image/svg+xml"
+
+var svgTagRegex = regexp.MustCompile(`(?si)\A\s*(?:(<!--.*?-->|<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg[\s>\/]`)
+var svgTagInXMLRegex = regexp.MustCompile(`(?si)\A<\?xml\b.*?\?>\s*(?:(<!--.*?-->|<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg[\s>\/]`)
+
+// SniffedType contains informations about a blobs type.
+type SniffedType struct {
+	contentType string
+}
+
+// IsText etects if content format is plain text.
+func (ct SniffedType) IsText() bool {
+	return strings.Contains(ct.contentType, "text/")
+}
+
+// IsImage detects if data is an image format
+func (ct SniffedType) IsImage() bool {
+	return strings.Contains(ct.contentType, "image/")
+}
+
+// IsSvgImage detects if data is an SVG image format
+func (ct SniffedType) IsSvgImage() bool {
+	return strings.Contains(ct.contentType, SvgMimeType)
+}
+
+// IsPDF detects if data is a PDF format
+func (ct SniffedType) IsPDF() bool {
+	return strings.Contains(ct.contentType, "application/pdf")
+}
+
+// IsVideo detects if data is an video format
+func (ct SniffedType) IsVideo() bool {
+	return strings.Contains(ct.contentType, "video/")
+}
+
+// IsAudio detects if data is an video format
+func (ct SniffedType) IsAudio() bool {
+	return strings.Contains(ct.contentType, "audio/")
+}
+
+// IsRepresentableAsText returns true if file content can be represented as
+// plain text or is empty.
+func (ct SniffedType) IsRepresentableAsText() bool {
+	return ct.IsText() || ct.IsSvgImage()
+}
+
+// DetectContentType extends http.DetectContentType with more content types. Defaults to text/unknown if input is empty.
+func DetectContentType(data []byte) SniffedType {
+	if len(data) == 0 {
+		return SniffedType{"text/unknown"}
+	}
+
+	ct := http.DetectContentType(data)
+
+	if len(data) > sniffLen {
+		data = data[:sniffLen]
+	}
+
+	if (strings.Contains(ct, "text/plain") || strings.Contains(ct, "text/html")) && svgTagRegex.Match(data) ||
+		strings.Contains(ct, "text/xml") && svgTagInXMLRegex.Match(data) {
+		// SVG is unsupported. https://github.com/golang/go/issues/15888
+		ct = SvgMimeType
+	}
+
+	return SniffedType{ct}
+}
+
+// DetectContentTypeFromReader guesses the content type contained in the reader.
+func DetectContentTypeFromReader(r io.Reader) (SniffedType, error) {
+	buf := make([]byte, sniffLen)
+	n, err := r.Read(buf)
+	if err != nil && err != io.EOF {
+		return SniffedType{}, fmt.Errorf("DetectContentTypeFromReader io error: %w", err)
+	}
+	buf = buf[:n]
+
+	return DetectContentType(buf), nil
+}