ExternalData: Add option to disable use of symlinks

Add an ExternalData_NO_SYMLINKS to enable use of copies instead of
symlinks to populate the real data files behind a DATA{} reference.
This will be useful on UNIX-like systems when the underlying filesystem
does not actually support symbolic links.

Suggested-by: Matt McCormick <matt.mccormick@kitware.com>

Help/release/dev/ExternalData-no-symlinks.rst

@@ -0,0 +1,7 @@
+ExternalData-no-symlinks
+------------------------
+
+* The :module:`ExternalData` module learned a new
+  :variable:`ExternalData_NO_SYMLINKS` option to disable use of
+  symbolic links to populate the real data files and use copies
+  instead.

Modules/ExternalData.cmake

@@ -117,6 +117,13 @@ calling any of the functions provided by this module.
   data fetch rule created for the content link will use the staged
   object if it cannot be found using any URL template.
 
+.. variable:: ExternalData_NO_SYMLINKS
+
+  The real data files named by expanded ``DATA{}`` references may be made
+  available under ``ExternalData_BINARY_ROOT`` using symbolic links on
+  some platforms.  The ``ExternalData_NO_SYMLINKS`` variable may be set
+  to disable use of symbolic links and enable use of copies instead.
+
 .. variable:: ExternalData_OBJECT_STORES
 
   The ``ExternalData_OBJECT_STORES`` variable may be set to a list of local
@@ -842,7 +849,7 @@ function(_ExternalData_link_or_copy src dst)
   file(MAKE_DIRECTORY "${dst_dir}")
   _ExternalData_random(random)
   set(tmp "${dst}.tmp${random}")
-  if(UNIX)
+  if(UNIX AND NOT ExternalData_NO_SYMLINKS)
     # Create a symbolic link.
     set(tgt "${src}")
     if(relative_top)

Modules/ExternalData_config.cmake.in

@@ -2,4 +2,5 @@ set(ExternalData_OBJECT_STORES "@ExternalData_OBJECT_STORES@")
 set(ExternalData_URL_TEMPLATES "@ExternalData_URL_TEMPLATES@")
 set(ExternalData_TIMEOUT_INACTIVITY "@ExternalData_TIMEOUT_INACTIVITY@")
 set(ExternalData_TIMEOUT_ABSOLUTE "@ExternalData_TIMEOUT_ABSOLUTE@")
+set(ExternalData_NO_SYMLINKS "@ExternalData_NO_SYMLINKS@")
 @_ExternalData_CONFIG_CODE@

Tests/Module/ExternalData/CMakeLists.txt

@@ -53,3 +53,4 @@ ExternalData_Add_Target(Data1)
 add_subdirectory(Data2)
 add_subdirectory(Data3)
 add_subdirectory(Data4)
+add_subdirectory(DataNoSymlinks)

Tests/Module/ExternalData/DataNoSymlinks/CMakeLists.txt

@@ -0,0 +1,8 @@
+set(ExternalData_NO_SYMLINKS 1)
+ExternalData_Add_Test(DataNoSymlinks
+  NAME DataNoSymlinksCheck
+  COMMAND ${CMAKE_COMMAND}
+    -D Data=DATA{Data.dat}
+    -P ${CMAKE_CURRENT_SOURCE_DIR}/DataNoSymlinksCheck.cmake
+  )
+ExternalData_Add_Target(DataNoSymlinks)

Tests/Module/ExternalData/DataNoSymlinks/Data.dat.md5

@@ -0,0 +1 @@
+8c018830e3efa5caf3c7415028335a57

Tests/Module/ExternalData/DataNoSymlinks/DataNoSymlinksCheck.cmake

@@ -0,0 +1,6 @@
+if(NOT EXISTS "${Data}")
+  message(SEND_ERROR "Input file:\n  ${Data}\ndoes not exist!")
+endif()
+if(IS_SYMLINK "${Data}")
+  message(SEND_ERROR "Input file:\n  ${Data}\nis a symlink but should not be!")
+endif()