[MM-13158] Initial attempt at extracting SVG dimensions (#10332)

* initial attempt at extracting svg dimensions

* rafactor SVG dimensions extraction

* pass SVG parsing errors to calling context

* tweaks to svg parsing placement

- also stopped trying to pre/post process SVG’s as images

* add svg parsing tests

* updates for PR change requests

* code review updates

* correct a conditional typo

app/svg.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"encoding/xml"
+	"io"
+	"regexp"
+	"strconv"
+
+	"github.com/pkg/errors"
+)
+
+type SVGInfo struct {
+	Width  int
+	Height int
+}
+
+func parseSVG(svgReader io.Reader) (SVGInfo, error) {
+	var parsedSVG struct {
+		Width   string `xml:"width,attr,omitempty"`
+		Height  string `xml:"height,attr,omitempty"`
+		ViewBox string `xml:"viewBox,attr,omitempty"`
+	}
+	svgInfo := SVGInfo{
+		Width:  0,
+		Height: 0,
+	}
+	viewBoxPattern := regexp.MustCompile("^([0-9]+)[, ]+([0-9]+)[, ]+([0-9]+)[, ]+([0-9]+)$")
+	dimensionPattern := regexp.MustCompile("(?i)^([0-9]+)(?:px)?$")
+
+	// decode provided SVG
+	if err := xml.NewDecoder(svgReader).Decode(&parsedSVG); err != nil {
+		return svgInfo, err
+	}
+
+	// prefer viewbox for SVG dimensions over width/height
+	if viewBoxMatches := viewBoxPattern.FindStringSubmatch(parsedSVG.ViewBox); len(viewBoxMatches) == 5 {
+		svgInfo.Width, _ = strconv.Atoi(viewBoxMatches[3])
+		svgInfo.Height, _ = strconv.Atoi(viewBoxMatches[4])
+	} else if len(parsedSVG.Width) > 0 && len(parsedSVG.Height) > 0 {
+		widthMatches := dimensionPattern.FindStringSubmatch(parsedSVG.Width)
+		heightMatches := dimensionPattern.FindStringSubmatch(parsedSVG.Height)
+		if len(widthMatches) == 2 && len(heightMatches) == 2 {
+			svgInfo.Width, _ = strconv.Atoi(widthMatches[1])
+			svgInfo.Height, _ = strconv.Atoi(heightMatches[1])
+		}
+	}
+
+	// if width and/or height are still zero, create new error
+	if svgInfo.Width == 0 || svgInfo.Height == 0 {
+		return svgInfo, errors.New("unable to extract SVG dimensions")
+	}
+	return svgInfo, nil
+}