Bug 1666964 - Retrieve Printer Default Media Through IPP Attribute r=hiro

cupsGetDestMediaDefault seems to not be returning the correct defaults,
so this patch retrieves the default media through the IPP attribute
instead, in hopes that this will provide more accurate defaults.

Differential Revision: https://phabricator.services.mozilla.com/D91376

widget/nsCUPSShim.h

@@ -34,11 +34,13 @@ class nsCUPSShim {
   X(cupsCopyDest)              \
   X(cupsCopyDestInfo)          \
   X(cupsDoRequest)             \
+  X(cupsFindDestDefault)       \
   X(cupsFreeDestInfo)          \
   X(cupsFreeDests)             \
   X(cupsGetDestMediaDefault)   \
   X(cupsGetDestMediaCount)     \
   X(cupsGetDestMediaByIndex)   \
+  X(cupsGetDestMediaByName)    \
   X(cupsGetDest)               \
   X(cupsGetDests)              \
   X(cupsGetNamedDest)          \

widget/nsPrinterCUPS.cpp

@@ -96,9 +96,23 @@ PrintSettingsInitializer nsPrinterCUPS::DefaultSettings() const {
 
   cups_size_t media;
 
+// cupsGetDestMediaDefault appears to return more accurate defaults on macOS,
+// and the IPP attribute appears to return more accurate defaults on Linux.
+#ifdef XP_MACOSX
   bool hasDefaultMedia =
       mShim.cupsGetDestMediaDefault(CUPS_HTTP_DEFAULT, mPrinter, printerInfo,
                                     CUPS_MEDIA_FLAGS_DEFAULT, &media);
+#else
+  ipp_attribute_t* defaultMediaIPP = mShim.cupsFindDestDefault(
+      CUPS_HTTP_DEFAULT, mPrinter, printerInfo, "media");
+
+  const char* defaultMediaName =
+      mShim.ippGetString(defaultMediaIPP, 0, nullptr);
+
+  bool hasDefaultMedia = mShim.cupsGetDestMediaByName(
+      CUPS_HTTP_DEFAULT, mPrinter, printerInfo, defaultMediaName,
+      CUPS_MEDIA_FLAGS_DEFAULT, &media);
+#endif
 
   if (!hasDefaultMedia) {
     return PrintSettingsInitializer{