FetchContent: Don't update timestamps if files don't change

The refactoring in 17e5516e60 (FetchContent: Invoke steps directly and
avoid a separate sub-build, 2021-01-29) uses a different way of writing
out the step scripts and updating time stamps when steps are executed.
That inadvertently always wrote out the scripts for custom commands,
even when the contents didn't change. This caused their timestamp to
always be updated, resulting in those steps always being seen as
out-of-date and needing to be re-executed.

The way timestamps were checked to determine whether to re-execute
a step also did not adequately account for file systems which only have
second-resolution timestamps. The IS_NEWER_THAN if condition also
returns true when timestamps are the same, so one needs to use the
negative form to get a true "is newer than" test.

ExternalProject is not susceptible to this problem because it uses
file(GENERATE) to write out the script files and that only updates the file's
timestamp if the contents change. It also mostly leaves timestamp
checking to the build tool.

5 files changed, 85 insertions, 0 deletions

diff --git a/Tests/RunCMake/FetchContent/RunCMakeTest.cmake b/Tests/RunCMake/FetchContent/RunCMakeTest.cmake
index 13013fa..d7fd009 100644
--- a/Tests/RunCMake/FetchContent/RunCMakeTest.cmake
+++ b/Tests/RunCMake/FetchContent/RunCMakeTest.cmake
@@ -27,6 +27,36 @@ run_cmake_with_options(ManualSourceDirectoryRelative
   -D "FETCHCONTENT_SOURCE_DIR_WITHPROJECT:STRING=WithProject"
 )
 
+function(run_FetchContent_TimeStamps)
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/TimeStamps)
+  file(REMOVE_RECURSE "${RunCMake_TEST_BINARY_DIR}")
+  file(MAKE_DIRECTORY "${RunCMake_TEST_BINARY_DIR}")
+
+  # First run should execute the commands
+  run_cmake(TimeStamps)
+
+  # Ensure that the file checks we use in the TimeStampsRerun-check.cmake script
+  # will not be defeated by file systems with only one second resolution.
+  # The IS_NEWER_THAN check returns TRUE if the timestamps of the two files are
+  # the same, which has been observed where filesystems only have one second
+  # resolution.
+  set(cmpTimeStamp   ${RunCMake_TEST_BINARY_DIR}/cmpTimeStamp.txt)
+  set(checkTimeStamp ${RunCMake_TEST_BINARY_DIR}/cmpTimeStampCheck.txt)
+  file(TOUCH ${cmpTimeStamp})
+  file(TOUCH ${checkTimeStamp})
+  if("${cmpTimeStamp}" IS_NEWER_THAN "${checkTimeStamp}")
+    execute_process(
+      COMMAND ${CMAKE_COMMAND} -E sleep 1.125
+      COMMAND_ERROR_IS_FATAL LAST
+    )
+  endif()
+
+  # Run again with no changes, no commands should re-execute
+  set(RunCMake_TEST_NO_CLEAN 1)
+  run_cmake(TimeStampsRerun)
+endfunction()
+run_FetchContent_TimeStamps()
+
 function(run_FetchContent_DirOverrides)
   set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/DirOverrides-build)
   file(REMOVE_RECURSE "${RunCMake_TEST_BINARY_DIR}")
diff --git a/Tests/RunCMake/FetchContent/TimeStamps-stdout.txt b/Tests/RunCMake/FetchContent/TimeStamps-stdout.txt
new file mode 100644
index 0000000..2ba1ff4
--- /dev/null
+++ b/Tests/RunCMake/FetchContent/TimeStamps-stdout.txt
@@ -0,0 +1,2 @@
+.* *download executed
+.* *patch executed
diff --git a/Tests/RunCMake/FetchContent/TimeStamps.cmake b/Tests/RunCMake/FetchContent/TimeStamps.cmake
new file mode 100644
index 0000000..33874f9
--- /dev/null
+++ b/Tests/RunCMake/FetchContent/TimeStamps.cmake
@@ -0,0 +1,14 @@
+include(FetchContent)
+
+# Do nothing for an update because it would result in always re-running the
+# patch step. We want to test that a patch step that only depends on the
+# download step is not re-run unnecessarily.
+FetchContent_Declare(customCommands
+  PREFIX            ${CMAKE_CURRENT_BINARY_DIR}
+  DOWNLOAD_COMMAND  "${CMAKE_COMMAND}" -E echo "download executed"
+  UPDATE_COMMAND    ""
+  PATCH_COMMAND     "${CMAKE_COMMAND}" -E echo "patch executed"
+)
+
+set(FETCHCONTENT_QUIET FALSE)
+FetchContent_MakeAvailable(customCommands)
diff --git a/Tests/RunCMake/FetchContent/TimeStampsRerun-check.cmake b/Tests/RunCMake/FetchContent/TimeStampsRerun-check.cmake
new file mode 100644
index 0000000..c12a5f4
--- /dev/null
+++ b/Tests/RunCMake/FetchContent/TimeStampsRerun-check.cmake
@@ -0,0 +1,38 @@
+set(cmpFile   ${RunCMake_TEST_BINARY_DIR}/cmpTimeStamp.txt)
+set(scriptDir ${RunCMake_TEST_BINARY_DIR}/tmp)
+set(stampDir  ${RunCMake_TEST_BINARY_DIR}/src/customcommands-stamp)
+
+set(errorMessages)
+if(NOT EXISTS "${cmpFile}")
+  list(APPEND errorMessages "  ${cmpFile} is missing")
+else()
+  foreach(script IN ITEMS mkdirs download patch)
+    set(scriptFile "${scriptDir}/customcommands-${script}.cmake")
+    if(NOT EXISTS "${scriptFile}")
+      list(APPEND errorMessages "  ${scriptFile} is missing")
+    elseif(NOT "${cmpFile}" IS_NEWER_THAN "${scriptFile}")
+      list(APPEND errorMessages "  ${scriptFile} was unexectedly updated")
+    endif()
+  endforeach()
+
+  # special case, not a script, has different extension
+  set(repoInfoFile "${scriptDir}/customcommands-download-repoinfo.txt")
+  if(NOT EXISTS "${repoInfoFile}")
+    list(APPEND errorMessages "  ${repoInfoFile} is missing")
+  elseif(NOT "${cmpFile}" IS_NEWER_THAN "${repoInfoFile}")
+    list(APPEND errorMessages "  ${repoInfoFile} was unexectedly updated")
+  endif()
+
+  foreach(step IN ITEMS download patch)
+    set(stampFile "${stampDir}/customcommands-${step}")
+    if(NOT EXISTS "${stampFile}")
+      list(APPEND errorMessages "  ${stampFile} is missing")
+    elseif(NOT "${cmpFile}" IS_NEWER_THAN "${stampFile}")
+      list(APPEND errorMessages "  ${stampFile} was unexectedly updated")
+    endif()
+  endforeach()
+endif()
+
+if(errorMessages)
+  list(JOIN errorMessages "\n" RunCMake_TEST_FAILED)
+endif()
diff --git a/Tests/RunCMake/FetchContent/TimeStampsRerun.cmake b/Tests/RunCMake/FetchContent/TimeStampsRerun.cmake
new file mode 100644
index 0000000..e13667a
--- /dev/null
+++ b/Tests/RunCMake/FetchContent/TimeStampsRerun.cmake
@@ -0,0 +1 @@
+include(${CMAKE_CURRENT_LIST_DIR}/TimeStamps.cmake)