Additional EL8 build fixes

CMakeLists.txt

@@ -2,6 +2,8 @@
 cmake_minimum_required( VERSION 3.16 )
 project( xrdhttp-pelican )
 
+set( CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake )
+
 find_package( XRootD REQUIRED )
 
 set (CMAKE_CXX_STANDARD 20)
@@ -20,6 +22,8 @@ if(NOT APPLE)
   SET( CMAKE_MODULE_LINKER_FLAGS "-Wl,--no-undefined")
 endif()
 
+find_package( Filesystem REQUIRED )
+
 option(ENABLE_TESTS "Enable unit tests" FALSE)
 if (ENABLE_TESTS)
   find_package(GTest REQUIRED)
@@ -33,7 +37,7 @@ add_subdirectory(tests)
 include_directories(${XRootD_INCLUDE_DIR})
 
 add_library(XrdHttpPelicanObj OBJECT src/XrdHttpPelican.cc src/Prestage.cc)
-target_link_libraries(XrdHttpPelicanObj ${XRootD_HTTP_LIBRARIES} ${XRootD_UTILS_LIBRARIES} Threads::Threads)
+target_link_libraries(XrdHttpPelicanObj ${XRootD_HTTP_LIBRARIES} ${XRootD_UTILS_LIBRARIES} Threads::Threads std::filesystem)
 set_target_properties(XrdHttpPelicanObj PROPERTIES POSITION_INDEPENDENT_CODE ON)
 
 # The test executables cannot link against the normal library on Linux as we hide the exported symbols

cmake/FindFilesystem.cmake

@@ -0,0 +1,45 @@
+
+# Ideas come from
+#
+#  https://gitlab.kitware.com/cmake/cmake/-/issues/17834
+#
+# Basically, upstream CMake claims the fact that a separate library is
+# needed for std::filesystem support is a short-lived fact (of all the
+# platforms we use, only RHEL 8 uses a compiler where this is needed),
+# hence they don't want a standardized way to detect std::filesystem
+
+include(CheckSourceCompiles)
+
+set( CMAKE_REQUIRED_INCLUDES "${XRootD_INCLUDE_DIR}" )
+set( SAMPLE_FILESYSTEM "#include <cstdlib>
+        #include <filesystem>
+
+        int main() {
+            auto cwd = std::filesystem::current_path();
+            return cwd.empty();
+        }")
+
+
+CHECK_SOURCE_COMPILES( CXX "${SAMPLE_FILESYSTEM}" CXX_FILESYSTEM_NO_LINK_NEEDED )
+
+set( _found FALSE )
+if( CXX_FILESYSTEM_NO_LINK_NEEDED )
+  set( _found TRUE )
+else()
+  # Add the libstdc++ flag
+  set( CMAKE_REQUIRED_LIBRARIES "-lstdc++fs" )
+  CHECK_SOURCE_COMPILES( CXX "${SAMPLE_FILESYSTEM}" CXX_FILESYSTEM_STDCPPFS_NEEDED )
+  set( _found TRUE )
+endif()
+
+add_library( std::filesystem INTERFACE IMPORTED )
+#set_property( TARGET std::filesystem APPEND PROPERTY INTERFACE_COMPILE_FEATURES cxx_std_17 )
+
+if( CXX_FILESYSTEM_STDCPPFS_NEEDED )
+  set_property( TARGET std::filesystem APPEND PROPERTY INTERFACE_LINK_LIBRARIES -lstdc++fs )
+endif()
+
+set( Filesystem_FOUND ${_found} CACHE BOOL "TRUE if we can run a program using std::filesystem" FORCE )
+if( Filesystem_FIND_REQUIRED AND NOT Filesystem_FOUND )
+    message( FATAL_ERROR "Cannot run simple program using std::filesystem" )
+endif()

src/Prestage.cc

@@ -22,6 +22,7 @@
 #include <XrdOuc/XrdOucEnv.hh>
 #include <XrdSys/XrdSysError.hh>
 
+#include <algorithm>
 #include <string>
 #include <thread>