FetchContent: Respect the CMP0135 policy setting

Fixes: #23560

Modules/FetchContent.cmake

@@ -1159,9 +1159,12 @@ function(FetchContent_Declare contentName)
 
   set(options "")
   set(oneValueArgs
+    SVN_REPOSITORY
+    DOWNLOAD_NO_EXTRACT
+    DOWNLOAD_EXTRACT_TIMESTAMP
+    URL
     BINARY_DIR
     SOURCE_DIR
-    SVN_REPOSITORY
   )
   set(multiValueArgs "")
 
@@ -1188,13 +1191,47 @@ function(FetchContent_Declare contentName)
     string(SHA1 urlSHA ${ARG_SVN_REPOSITORY})
     string(SUBSTRING ${urlSHA} 0 7 urlSHA)
     string(APPEND ARG_SOURCE_DIR "-${urlSHA}")
-    list(PREPEND ARG_UNPARSED_ARGUMENTS SVN_REPOSITORY "${ARG_SVN_REPOSITORY}")
   endif()
 
-  list(PREPEND ARG_UNPARSED_ARGUMENTS
-    SOURCE_DIR "${ARG_SOURCE_DIR}"
-    BINARY_DIR "${ARG_BINARY_DIR}"
-  )
+  # The ExternalProject_Add() call in the sub-build won't see the CMP0135
+  # policy setting of our caller. Work out if that policy will be needed and
+  # explicitly set the relevant option if not already provided. The condition
+  # here is essentially an abbreviated version of the logic in
+  # ExternalProject's _ep_add_download_command() function.
+  if(ARG_URL AND
+     NOT IS_DIRECTORY "${ARG_URL}" AND
+     NOT ARG_DOWNLOAD_NO_EXTRACT AND
+     NOT DEFINED ARG_DOWNLOAD_EXTRACT_TIMESTAMP)
+    cmake_policy(GET CMP0135 _FETCHCONTENT_CMP0135
+      PARENT_SCOPE # undocumented, do not use outside of CMake
+    )
+    if(_FETCHCONTENT_CMP0135 STREQUAL "")
+      message(AUTHOR_WARNING
+        "The DOWNLOAD_EXTRACT_TIMESTAMP option was not given and policy "
+        "CMP0135 is not set. The policy's OLD behavior will be used. "
+        "When using a URL download, the timestamps of extracted files "
+        "should preferably be that of the time of extraction, otherwise "
+        "code that depends on the extracted contents might not be "
+        "rebuilt if the URL changes. The OLD behavior preserves the "
+        "timestamps from the archive instead, but this is usually not "
+        "what you want. Update your project to the NEW behavior or "
+        "specify the DOWNLOAD_EXTRACT_TIMESTAMP option with a value of "
+        "true to avoid this robustness issue."
+      )
+      set(ARG_DOWNLOAD_EXTRACT_TIMESTAMP TRUE)
+    elseif(_FETCHCONTENT_CMP0135 STREQUAL "NEW")
+      set(ARG_DOWNLOAD_EXTRACT_TIMESTAMP FALSE)
+    else()
+      set(ARG_DOWNLOAD_EXTRACT_TIMESTAMP TRUE)
+    endif()
+  endif()
+
+  # Add back in the keyword args we pulled out and potentially tweaked/added
+  foreach(key IN LISTS oneValueArgs)
+    if(DEFINED ARG_${key})
+      list(PREPEND ARG_UNPARSED_ARGUMENTS ${key} "${ARG_${key}}")
+    endif()
+  endforeach()
 
   set(__argsQuoted)
   foreach(__item IN LISTS ARG_UNPARSED_ARGUMENTS)

Tests/RunCMake/CMP0135/CMP0135-Common.cmake

@@ -1,7 +1,8 @@
+#==============================================================================
+# ExternalProject
+#==============================================================================
+set(stamp_dir "${CMAKE_CURRENT_BINARY_DIR}/stamps-ep")
 include(ExternalProject)
-
-set(stamp_dir "${CMAKE_CURRENT_BINARY_DIR}/stamps")
-
 ExternalProject_Add(fake_ext_proj
   # We don't actually do a build, so we never try to download from this URL
   URL https://example.com/something.zip
@@ -12,7 +13,33 @@ ExternalProject_Add(fake_ext_proj
 set(extraction_script "${stamp_dir}/extract-fake_ext_proj.cmake")
 file(STRINGS "${extraction_script}" results REGEX "--touch")
 if("${results}" STREQUAL "")
-  message(STATUS "Using timestamps from archive")
+  message(STATUS "ExternalProject: Using timestamps from archive")
 else()
-  message(STATUS "Using extraction time for the timestamps")
+  message(STATUS "ExternalProject: Using extraction time for the timestamps")
+endif()
+
+#==============================================================================
+# FetchContent
+#==============================================================================
+set(stamp_dir "${CMAKE_CURRENT_BINARY_DIR}/stamps-fc")
+set(archive_file ${CMAKE_CURRENT_BINARY_DIR}/test_archive.7z)
+file(ARCHIVE_CREATE
+  OUTPUT ${archive_file}
+  PATHS ${CMAKE_CURRENT_LIST_DIR}
+  FORMAT 7zip
+)
+include(FetchContent)
+FetchContent_Declare(fake_fc_proj
+  URL file://${archive_file}
+  STAMP_DIR ${stamp_dir}
+)
+FetchContent_MakeAvailable(fake_fc_proj)
+
+# Report whether the --touch option was added to the extraction script
+set(extraction_script "${stamp_dir}/extract-fake_fc_proj-populate.cmake")
+file(STRINGS "${extraction_script}" results REGEX "--touch")
+if("${results}" STREQUAL "")
+  message(STATUS "FetchContent: Using timestamps from archive")
+else()
+  message(STATUS "FetchContent: Using extraction time for the timestamps")
 endif()

Tests/RunCMake/CMP0135/CMP0135-NEW-stdout.txt

@@ -1 +1,2 @@
-Using extraction time for the timestamps
+-- ExternalProject: Using extraction time for the timestamps
+-- FetchContent: Using extraction time for the timestamps

Tests/RunCMake/CMP0135/CMP0135-OLD-stdout.txt

@@ -1 +1,2 @@
-Using timestamps from archive
+-- ExternalProject: Using timestamps from archive
+-- FetchContent: Using timestamps from archive

Tests/RunCMake/CMP0135/CMP0135-WARN-stderr.txt

@@ -8,3 +8,14 @@ CMake Warning \(dev\) at .*/Modules/ExternalProject.cmake:[0-9]+ \(message\):
   what you want\.  Update your project to the NEW behavior or specify the
   DOWNLOAD_EXTRACT_TIMESTAMP option with a value of true to avoid this
   robustness issue\.
+.*
+CMake Warning \(dev\) at .*/Modules/FetchContent.cmake:[0-9]+ \(message\):
+  The DOWNLOAD_EXTRACT_TIMESTAMP option was not given and policy CMP0135 is
+  not set\.  The policy's OLD behavior will be used\.  When using a URL
+  download, the timestamps of extracted files should preferably be that of
+  the time of extraction, otherwise code that depends on the extracted
+  contents might not be rebuilt if the URL changes\.  The OLD behavior
+  preserves the timestamps from the archive instead, but this is usually not
+  what you want\.  Update your project to the NEW behavior or specify the
+  DOWNLOAD_EXTRACT_TIMESTAMP option with a value of true to avoid this
+  robustness issue\.

Tests/RunCMake/CMP0135/CMP0135-WARN-stdout.txt

@@ -1 +1,2 @@
-Using timestamps from archive
+-- ExternalProject: Using timestamps from archive
+-- FetchContent: Using timestamps from archive