Attempt to handle DNL (Define Number of Lines) markers when parsing JPEG images (issue 8614)

Please refer to the specification, found at https://www.w3.org/Graphics/JPEG/itu-t81.pdf#page=49

Given how the JPEG decoder is currently implemented, we need to know the value of the scanLines parameter (among others) *before* parsing of the SOS (Start of Scan) data begins.
Hence the best solution I could come up with here, is to re-parse the image in the *hopefully* rare case of JPEG images that include a DNL (Define Number of Lines) marker.

Fixes 8614.

src/core/jpg.js

@@ -28,6 +28,19 @@ let JpegError = (function JpegErrorClosure() {
   return JpegError;
 })();
 
+let DNLMarkerError = (function DNLMarkerErrorClosure() {
+  function DNLMarkerError(message, scanLines) {
+    this.message = message;
+    this.scanLines = scanLines;
+  }
+
+  DNLMarkerError.prototype = new Error();
+  DNLMarkerError.prototype.name = 'DNLMarkerError';
+  DNLMarkerError.constructor = DNLMarkerError;
+
+  return DNLMarkerError;
+})();
+
 /**
  * This code was forked from https://github.com/notmasteryet/jpgjs.
  * The original version was created by GitHub user notmasteryet.
@@ -112,7 +125,8 @@ var JpegImage = (function JpegImageClosure() {
   }
 
   function decodeScan(data, offset, frame, components, resetInterval,
-                      spectralStart, spectralEnd, successivePrev, successive) {
+                      spectralStart, spectralEnd, successivePrev, successive,
+                      parseDNLMarker = false) {
     var mcusPerLine = frame.mcusPerLine;
     var progressive = frame.progressive;
 
@@ -127,6 +141,14 @@ var JpegImage = (function JpegImageClosure() {
       if (bitsData === 0xFF) {
         var nextByte = data[offset++];
         if (nextByte) {
+          if (nextByte === 0xDC && parseDNLMarker) { // DNL == 0xFFDC
+            offset += 2; // Skip data length.
+            const scanLines = (data[offset++] << 8) | data[offset++];
+            if (scanLines > 0 && scanLines !== frame.scanLines) {
+              throw new DNLMarkerError(
+                'Found DNL marker (0xFFDC) while parsing scan data', scanLines);
+            }
+          }
           throw new JpegError(
             `unexpected marker ${((bitsData << 8) | nextByte).toString(16)}`);
         }
@@ -635,7 +657,7 @@ var JpegImage = (function JpegImageClosure() {
   }
 
   JpegImage.prototype = {
-    parse: function parse(data) {
+    parse(data, { dnlScanLines = null, } = {}) {
 
       function readUint16() {
         var value = (data[offset] << 8) | data[offset + 1];
@@ -685,6 +707,7 @@ var JpegImage = (function JpegImageClosure() {
       var jfif = null;
       var adobe = null;
       var frame, resetInterval;
+      let numSOSMarkers = 0;
       var quantizationTables = [];
       var huffmanTablesAC = [], huffmanTablesDC = [];
       var fileMarker = readUint16();
@@ -781,7 +804,8 @@ var JpegImage = (function JpegImageClosure() {
             frame.extended = (fileMarker === 0xFFC1);
             frame.progressive = (fileMarker === 0xFFC2);
             frame.precision = data[offset++];
-            frame.scanLines = readUint16();
+            const sofScanLines = readUint16();
+            frame.scanLines = dnlScanLines || sofScanLines;
             frame.samplesPerLine = readUint16();
             frame.components = [];
             frame.componentIds = {};
@@ -839,6 +863,12 @@ var JpegImage = (function JpegImageClosure() {
             break;
 
           case 0xFFDA: // SOS (Start of Scan)
+            // A DNL marker (0xFFDC), if it exists, is only allowed at the end
+            // of the first scan segment and may only occur once in an image.
+            // Furthermore, to prevent an infinite loop, do *not* attempt to
+            // parse DNL markers during re-parsing of the JPEG scan data.
+            const parseDNLMarker = (++numSOSMarkers) === 1 && !dnlScanLines;
+
             readUint16(); // scanLength
             var selectorsCount = data[offset++];
             var components = [], component;
@@ -853,11 +883,26 @@ var JpegImage = (function JpegImageClosure() {
             var spectralStart = data[offset++];
             var spectralEnd = data[offset++];
             var successiveApproximation = data[offset++];
-            var processed = decodeScan(data, offset,
-              frame, components, resetInterval,
-              spectralStart, spectralEnd,
-              successiveApproximation >> 4, successiveApproximation & 15);
-            offset += processed;
+            try {
+              var processed = decodeScan(data, offset,
+                frame, components, resetInterval,
+                spectralStart, spectralEnd,
+                successiveApproximation >> 4, successiveApproximation & 15,
+                parseDNLMarker);
+              offset += processed;
+            } catch (ex) {
+              if (ex instanceof DNLMarkerError) {
+                warn('Attempting to re-parse JPEG image using "scanLines" ' +
+                     'parameter found in DNL marker (0xFFDC) segment.');
+                return this.parse(data, { dnlScanLines: ex.scanLines, });
+              }
+              throw ex;
+            }
+            break;
+
+          case 0xFFDC: // DNL (Define Number of Lines)
+            // Ignore the marker, since it's being handled in `decodeScan`.
+            offset += 4;
             break;
 
           case 0xFFFF: // Fill bytes