COMP: Promote ITK header paths for name mangling expat.h

When linking against ITK's expat library, the expat.h must
be found from the ITK supplied expat.h so that name mangling
is properly asserted.

The source layout changes in 8ec19eb
allowed the /usr/include/expat.h (i.e. without name mangling) to be
found first and resulted in link errors due to name mangling.

Simulate previous ITKv5 behavior for header path ordering by including
an expat.h that does itk name mangling from the ITK include directories.

- Fixed outstanding todo items.
- Simplified configure_file paths
- Changed itk install directory for third-party exap includes

This resolves an issue with building SlicerExecutionModel against
the ITK v6.0b03+ versions which requires using ITK expat.h and linking
against ITK mangled expat libraries.

Author: hjmjohnson

Modules/ThirdParty/Expat/CMakeLists.txt

@@ -17,9 +17,9 @@ if(ITK_USE_SYSTEM_EXPAT)
 else()
   set(
     ITKExpat_INCLUDE_DIRS
-    ${ITKExpat_BINARY_DIR}/src
-    ${ITKExpat_BINARY_DIR}/src/itkexpat
-    ${ITKExpat_SOURCE_DIR}/src
+    ${ITKExpat_BINARY_DIR} # <- for itk_expat.h
+    ${ITKExpat_BINARY_DIR}/src/itkexpat # <- for expat_config.h itk_expat_mangle.h
+    ${ITKExpat_SOURCE_DIR}/src # <- for itkexpat/lib/expat.h in build tree
   )
   set(ITKExpat_LIBRARIES ITKEXPAT)
   set(
@@ -43,6 +43,18 @@ else()
   )
 endif()
 
-configure_file(src/itk_expat.h.in src/itk_expat.h)
+configure_file(itk_expat.h.in ${ITKExpat_BINARY_DIR}/itk_expat.h)
+if(NOT ITK_USE_SYSTEM_EXPAT AND NOT ITK_FUTURE_LEGACY_REMOVE)
+  # Configure a file with the name <expat.h> as was the default in ITKv5 and earlier
+  # so that "#include <expat.h>" finds the itk mangled symbols during linkage stage.
+  # This is only needed to preserve behaviors of ITKv5 and earlier. ITKv6 and greater
+  # should #include "itk_expat.h" to get the correctly mangled symbols.
+  configure_file(itkv5_expat.h.in ${ITKExpat_BINARY_DIR}/expat.h)
+else()
+  # Remove file if configuration changes occur
+  if(EXISTS ${ITKExpat_BINARY_DIR}/expat.h)
+    file(REMOVE ${ITKExpat_BINARY_DIR}/expat.h)
+  endif()
+endif()
 
 itk_module_impl()

…ules/ThirdParty/Expat/src/itk_expat.h.in Modules/ThirdParty/Expat/itk_expat.h.inModules/ThirdParty/Expat/src/itk_expat.h.in renamed to Modules/ThirdParty/Expat/itk_expat.h.in Modules/ThirdParty/Expat/src/itk_expat.h.in renamed to Modules/ThirdParty/Expat/itk_expat.h.in

@@ -0,0 +1,44 @@
+/*=========================================================================
+ *
+ *  Copyright NumFOCUS
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0.txt
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *=========================================================================*/
+
+#ifndef itkv5_expat_h
+#define itkv5_expat_h
+
+/* Use the expat library configured for ITK.  */
+#cmakedefine ITK_USE_SYSTEM_EXPAT
+#ifdef ITK_USE_SYSTEM_EXPAT
+# error "This ITKv5 backward compatible stub file should never be used if requesting ITK_USE_SYSTEM_EXPAT=ON"
+#elif defined(ITK_FUTURE_LEGACY_REMOVE)
+# error "This ITKv5 backward compatible stub file should never be used if requesting ITK_FUTURE_LEGACY_REMOVE=ON"
+#else
+/* ITKv5 used the `#include <expat.h>` with itk mangled symbols and depends on compiler
+ * include file order to find the ITKv5 version of expat.h before the system version.
+ *
+ * ITKv6 moves the itk mangled symbols and expects ITK dependant exapt to use
+ * `#include "itk_expat.h"` to find the itk mangled symbols, or divert to `#include <expat.h>`
+ * if ITK_USE_SYSTEM_EXPAT.
+ *
+ * This file is intended for ITKv5 drop in compatibility by generating a
+ * the file that is found via include path precidence rules called "#include <expat.h>"
+ * by placing an itk generated file "expat.h" in the ITK included files path to superceed
+ * the system file when ITK_USE_SYSTEM_EXPAT=False and ITK_FUTURE_LEGACY_REMOVE=False.
+ */
+# include "itk_expat.h"
+#endif
+
+#endif

Modules/ThirdParty/Expat/itkv5_expat.h.in

@@ -502,12 +502,24 @@ install(TARGETS ITKEXPAT
   ARCHIVE DESTINATION ${ITK3P_INSTALL_ARCHIVE_DIR} COMPONENT Development
   )
 
+install(FILES
+  ${ITKExpat_BINARY_DIR}/itk_expat.h
+  DESTINATION ${ITK3P_INSTALL_INCLUDE_DIR}
+  COMPONENT Development)
+
+if(NOT ITK_USE_SYSTEM_EXPAT AND NOT ITK_FUTURE_LEGACY_REMOVE)
+  install(FILES
+    ${ITKExpat_BINARY_DIR}/expat.h # <- The ITKv5 backward compatibile work-around-file to route to itk_expat.h
+    DESTINATION ${ITK3P_INSTALL_INCLUDE_DIR}
+    COMPONENT Development)
+endif()
+
 install(FILES
   ${ITK3P_EXPAT_BINARY_DIR}/itk_expat_mangle.h
   ${ITK3P_EXPAT_BINARY_DIR}/expat_config.h
   ${ITK3P_EXPAT_SOURCE_DIR}/lib/expat.h
   ${ITK3P_EXPAT_SOURCE_DIR}/lib/expat_external.h
-  DESTINATION ${ITK3P_INSTALL_INCLUDE_DIR} # TODO: itk_expat.h #include "itkexpat/expat.h"
+  DESTINATION ${ITK3P_INSTALL_INCLUDE_DIR}/itkexpat/lib
   COMPONENT Development)
 # ITK --stop