Copy files instead of hard-linking on Windows

Fixes an issue on Windows where when source and build directory are on different drives hard-linking
to files or directory fails as it doesn't work across filesystem boundaries. Note that symlinking is also
not possible because it requires administrator privileges on Windows.

The solution copies the files using the built-in cmake `configure_file(src dest COPYONLY)` command.
As this command only operates on files, if a directory is specified the files will be globbed recursively
and through symlinks.

Signed-off-by: Dominik Gschwind <dominik.gschwind99@gmail.com>

CMakeLists.txt

@@ -120,35 +120,33 @@ endif()
 
 # Create a symbolic link from ${base_name} in the binary directory
 # to the corresponding path in the source directory.
+# Note: Copies the file(s) on Windows.
 function(link_to_source base_name)
-    # Get OS dependent path to use in `execute_process`
-    if (CMAKE_HOST_WIN32)
-        #mklink is an internal command of cmd.exe it can only work with \
-        string(REPLACE "/" "\\" link "${CMAKE_CURRENT_BINARY_DIR}/${base_name}")
-        string(REPLACE "/" "\\" target "${CMAKE_CURRENT_SOURCE_DIR}/${base_name}")
-    else()
-        set(link "${CMAKE_CURRENT_BINARY_DIR}/${base_name}")
-        set(target "${CMAKE_CURRENT_SOURCE_DIR}/${base_name}")
-    endif()
+    set(link "${CMAKE_CURRENT_BINARY_DIR}/${base_name}")
+    set(target "${CMAKE_CURRENT_SOURCE_DIR}/${base_name}")
 
-    if (NOT EXISTS ${link})
+    # Linking to non-existent file is not desirable. At best you will have a
+    # dangling link, but when building in tree, this can create a symbolic link
+    # to itself.
+    if (EXISTS ${target} AND NOT EXISTS ${link})
         if (CMAKE_HOST_UNIX)
-            set(command ln -s ${target} ${link})
+            execute_process(COMMAND ln -s ${target} ${link}
+                RESULT_VARIABLE result
+                ERROR_VARIABLE output)
+
+            if (NOT ${result} EQUAL 0)
+                message(FATAL_ERROR "Could not create symbolic link for: ${target} --> ${output}")
+            endif()
         else()
             if (IS_DIRECTORY ${target})
-                set(command cmd.exe /c mklink /j ${link} ${target})
+                file(GLOB_RECURSE files FOLLOW_SYMLINKS LIST_DIRECTORIES false RELATIVE ${target} "${target}/*")
+                foreach(file IN LISTS files)
+                    configure_file("${target}/${file}" "${link}/${file}" COPYONLY)
+                endforeach(file)
             else()
-                set(command cmd.exe /c mklink /h ${link} ${target})
+                configure_file(${target} ${link} COPYONLY)
             endif()
         endif()
-
-        execute_process(COMMAND ${command}
-            RESULT_VARIABLE result
-            ERROR_VARIABLE output)
-
-        if (NOT ${result} EQUAL 0)
-            message(FATAL_ERROR "Could not create symbolic link for: ${target} --> ${output}")
-        endif()
     endif()
 endfunction(link_to_source)
 

ChangeLog.d/fix_hard_link_across_drives

@@ -0,0 +1,3 @@
+Bugfix
+   * Fix a build issue on Windows where the source and build directory could not be on
+     different drives (#5751).