feat: check image sources in picture and srcset

Message changes:

- new `MED-007` (`ERROR`) message is reported when an image source
  references a foreign resource (unless on `source` elements which
  `type` attribute specify non-core media type).
- reworded `MED-003`; it now includes the resource path.

Internal changes:

- add a `SourceSet` class to represent an image source set, along with
  a parser folloing the "parsing a srcset attribute" algorithm from HTML.
  See https://html.spec.whatwg.org/#parsing-a-srcset-attribute
- add to new reference types to the `XRefChecker` class, `PICTURE_SOURCE`
  and `PICTURE_SOURCE_FOREIGN`, to identify image source references in
  `picture` elements.
- add tests, for both the `srcset` parser and the new fallback rules.

Fix #781

src/main/java/com/adobe/epubcheck/messages/DefaultSeverities.java

@@ -144,6 +144,7 @@ private void initialize()
  severities.put(MessageId.MED_004, Severity.ERROR);
  severities.put(MessageId.MED_005, Severity.ERROR);
  severities.put(MessageId.MED_006, Severity.USAGE);
+ severities.put(MessageId.MED_007, Severity.ERROR);
 
  // NAV
  severities.put(MessageId.NAV_001, Severity.ERROR);

src/main/java/com/adobe/epubcheck/messages/MessageId.java

@@ -137,6 +137,7 @@ public enum MessageId implements Comparable<MessageId>
  MED_004("MED-004"),
  MED_005("MED-005"),
  MED_006("MED_006"),
+ MED_007("MED_007"),
 
  // Epub3 based table of content errors
  NAV_001("NAV-001"),

src/main/java/com/adobe/epubcheck/opf/XRefChecker.java

@@ -68,7 +68,9 @@ public static enum Type
  REGION_BASED_NAV,
  SEARCH_KEY,
  NAV_TOC_LINK,
- NAV_PAGELIST_LINK;
+ NAV_PAGELIST_LINK,
+ PICTURE_SOURCE,
+ PICTURE_SOURCE_FOREIGN;
  }
 
  private static class Reference
@@ -98,6 +100,7 @@ public Reference(String srcResource, int srcLineNumber, int srcColumnNumber, Str
  private static class Anchor
  {
 
+ @SuppressWarnings("unused")
  public final String id;
  public final Type type;
  public final int position;
@@ -383,19 +386,28 @@ else if (!undeclared.contains(ref.refResource)
  }
  break;
  case IMAGE:
+ case PICTURE_SOURCE:
+ case PICTURE_SOURCE_FOREIGN:
  if (ref.fragment != null && !res.item.getMimeType().equals("image/svg+xml"))
  {
  report.message(MessageId.RSC_009, EPUBLocation.create(ref.source, ref.lineNumber,
  ref.columnNumber, ref.refResource + "#" + ref.fragment));
  return;
  }
  // if mimeType is null, we should have reported an error already
- if (!OPFChecker.isBlessedImageType(res.item.getMimeType()) && !res.hasValidImageFallback)
+ if (!OPFChecker.isBlessedImageType(res.item.getMimeType()))
  {
- report.message(MessageId.MED_003,
- EPUBLocation.create(ref.source, ref.lineNumber, ref.columnNumber),
- res.item.getMimeType());
- return;
+ if (version == EPUBVersion.VERSION_3 && ref.type == Type.PICTURE_SOURCE) {
+ report.message(MessageId.MED_007,
+ EPUBLocation.create(ref.source, ref.lineNumber, ref.columnNumber),
+ ref.refResource, res.item.getMimeType());
+ return;
+ }
+ else if (ref.type == Type.IMAGE && !res.hasValidImageFallback) {
+ report.message(MessageId.MED_003,
+ EPUBLocation.create(ref.source, ref.lineNumber, ref.columnNumber),
+ ref.refResource, res.item.getMimeType());
+ }
  }
  break;
  case SEARCH_KEY:

src/main/java/com/adobe/epubcheck/ops/OPSHandler30.java

@@ -6,6 +6,7 @@
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
+import java.util.TreeSet;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -19,6 +20,7 @@
 import com.adobe.epubcheck.util.EpubConstants;
 import com.adobe.epubcheck.util.FeatureEnum;
 import com.adobe.epubcheck.util.PathUtil;
+import com.adobe.epubcheck.util.SourceSet;
 import com.adobe.epubcheck.vocab.AggregateVocab;
 import com.adobe.epubcheck.vocab.AltStylesheetVocab;
 import com.adobe.epubcheck.vocab.ComicsVocab;
@@ -70,6 +72,7 @@ public class OPSHandler30 extends OPSHandler
 
  protected boolean inVideo = false;
  protected boolean inAudio = false;
+ protected boolean inPicture = false;
  protected boolean hasValidFallback = false;
 
  protected int imbricatedObjects = 0;
@@ -134,6 +137,59 @@ public OPSHandler30(ValidationContext context, XMLParser parser)
  .contains(EpubCheckVocab.VOCAB.get(EpubCheckVocab.PROPERTIES.NON_LINEAR));
  }
 
+ protected void checkImage(XMLElement e, String attrNS, String attr)
+ {
+ // if it's an SVG image, fall back to super's logic
+ String ns = e.getNamespace();
+ if ("http://www.w3.org/2000/svg".equals(ns))
+ {
+ super.checkImage(e, attrNS, attr);
+ }
+ // else process image source sets in HTML
+ else if (xrefChecker.isPresent())
+ {
+ String src = e.getAttribute("src");
+ String srcset = e.getAttribute("srcset");
+ // if we're in a 'picture' element
+ if (inPicture)
+ {
+ String type = e.getAttribute("type");
+ // if in a 'source' element specifying a foreign MIME type,
+ // register as foreign picture source
+ if ("source".equals(e.getName()) && type != null && !OPFChecker.isBlessedImageType(type))
+ {
+ registerImageSources(src, srcset, XRefChecker.Type.PICTURE_SOURCE_FOREIGN);
+ }
+ // else register as regular picture source (must be a CMT)
+ else
+ // register as picture source
+ {
+ registerImageSources(src, srcset, XRefChecker.Type.PICTURE_SOURCE);
+ }
+ }
+ // register as regular image sources (must be a CMT or have a manifest fallback
+ else
+ {
+ registerImageSources(src, srcset, XRefChecker.Type.IMAGE);
+ }
+ }
+ }
+
+ protected void registerImageSources(String src, String srcset, XRefChecker.Type type)
+ {
+ // compute a list of URLs to register
+ Set<String> urls = new TreeSet<>();
+ if (src != null) urls.add(src);
+ urls.addAll(SourceSet.parse(srcset).getImageURLs());
+ // register all the URLs
+ for (String url : urls)
+ {
+ xrefChecker.get().registerReference(path, parser.getLineNumber(), parser.getColumnNumber(),
+ PathUtil.resolveRelativeReference(base, url), type);
+ }
+
+ }
+
  protected void checkType(XMLElement e, String type)
  {
  if (type == null)
@@ -305,6 +361,14 @@ else if (name.equals("annotation-xml"))
  {
  hasAltorAnnotation = true;
  }
+ else if (name.equals("picture"))
+ {
+ inPicture = true;
+ }
+ else if (name.equals("source"))
+ {
+ if (inPicture) checkImage(e, null, null);
+ }
  else if ("http://www.w3.org/2000/svg".equals(e.getNamespace()) && name.equals("title"))
  {
  hasTitle = true;
@@ -709,6 +773,10 @@ else if (name.equals("nav") && inRegionBasedNav)
  {
  inRegionBasedNav = false;
  }
+ else if (name.equals("picture"))
+ {
+ inPicture = false;
+ }
  else if (name.equals("svg"))
  {
  inSvg = false;