Fix primary JPEG XMP merging across namespace prefixes and reencoding

CMakeLists.txt

@@ -823,6 +823,7 @@ if(UHDR_BUILD_TESTS)
   target_include_directories(ultrahdr_unit_test PRIVATE
     ${PRIVATE_INCLUDE_DIR}
     ${GTEST_INCLUDE_DIRS}
+    "${THIRD_PARTY_DIR}/image_io/includes"
   )
   if(UHDR_BUILD_FUZZERS)
     target_link_options(ultrahdr_unit_test PRIVATE -fsanitize=fuzzer-no-link)

lib/include/ultrahdr/jpegr.h

@@ -357,8 +357,11 @@ class JpegR : public UltraHdr {
    * \param[in]       metadata                 gainmap metadata descriptor
    * \param[in, out]  dest                     output image descriptor to store compressed ultrahdr
    *                                           image
+   * \param[in]       pXmp                     optional primary-image XMP packet to preserve and
+   *                                           merge when XMP metadata is enabled
    *
    * \return uhdr_error_info_t #UHDR_CODEC_OK if operation succeeds, uhdr_codec_err_t otherwise.
+   *         An unsupported or unmergeable XMP packet returns #UHDR_CODEC_INVALID_PARAM in XMP mode.
    */
   uhdr_error_info_t appendGainMap(uhdr_compressed_image_t* sdr_intent_compressed,
                                   uhdr_compressed_image_t* gainmap_compressed,

lib/include/ultrahdr/jpegrutils.h

@@ -134,7 +134,10 @@ std::string generateStandardXmpWithExtendedGuid(const std::string& guid);
  * </x:xmpmeta>
  *
  * @param secondary_image_length length of secondary image
- * @return XMP metadata in type of string
+ * @param user_xmp optional XMP packet from the primary image. Its unrelated XML is preserved while
+ *                  the encoder-owned primary-image gain-map description is replaced.
+ * @return XMP metadata in type of string. An empty string means that a supplied packet could not be
+ *         safely merged.
  */
 std::string generateXmpForPrimaryImage(size_t secondary_image_length,
                                        uhdr_gainmap_metadata_ext_t& metadata,

lib/src/jpegr.cpp

@@ -1292,6 +1292,14 @@ uhdr_error_info_t JpegR::appendGainMap(uhdr_compressed_image_t* sdr_intent_compr
       uhdr_mem_block_t user_block{const_cast<char*>(user_xmp_str.data()), user_xmp_str.size(),
                                   user_xmp_str.size()};
       xmp_primary_str = generateXmpForPrimaryImage(secondary_image_size, *metadata, &user_block);
+      if (xmp_primary_str.empty()) {
+        uhdr_error_info_t status;
+        status.error_code = UHDR_CODEC_INVALID_PARAM;
+        status.has_detail = 1;
+        snprintf(status.detail, sizeof status.detail,
+                 "unable to safely merge supplied XMP metadata into the primary image");
+        return status;
+      }
     } else {
       xmp_primary_str = user_xmp_str;
     }