Merge topic 'externalproject-update-refactor'

ac6a4d48 ExternalProject: Improve robustness of update step
17c4c8b9 Tests: Prevent the noisy CMP0114 warnings in ExternalProjectUpdate test
1cb65e68 ExternalProject: Prevent the noisy detached head messages on checkout
Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !5389

Modules/ExternalProject-gitupdate.cmake.in

@@ -3,214 +3,275 @@
 
 cmake_minimum_required(VERSION 3.5)
 
-execute_process(
-  COMMAND "@git_EXECUTABLE@" rev-list --max-count=1 HEAD
-  WORKING_DIRECTORY "@work_dir@"
-  RESULT_VARIABLE error_code
-  OUTPUT_VARIABLE head_sha
-  OUTPUT_STRIP_TRAILING_WHITESPACE
+function(get_hash_for_ref ref out_var err_var)
+  execute_process(
+    COMMAND "@git_EXECUTABLE@" rev-parse "${ref}"
+    WORKING_DIRECTORY "@work_dir@"
+    RESULT_VARIABLE error_code
+    OUTPUT_VARIABLE ref_hash
+    ERROR_VARIABLE error_msg
+    OUTPUT_STRIP_TRAILING_WHITESPACE
   )
-if(error_code)
-  message(FATAL_ERROR "Failed to get the hash for HEAD")
+  if(error_code)
+    set(${out_var} "" PARENT_SCOPE)
+  else()
+    set(${out_var} "${ref_hash}" PARENT_SCOPE)
+  endif()
+  set(${err_var} "${error_msg}" PARENT_SCOPE)
+endfunction()
+
+get_hash_for_ref(HEAD head_sha error_msg)
+if(head_sha STREQUAL "")
+  message(FATAL_ERROR "Failed to get the hash for HEAD:\n${error_msg}")
 endif()
 
+
 execute_process(
   COMMAND "@git_EXECUTABLE@" show-ref "@git_tag@"
   WORKING_DIRECTORY "@work_dir@"
   OUTPUT_VARIABLE show_ref_output
-  )
-# If a remote ref is asked for, which can possibly move around,
-# we must always do a fetch and checkout.
-if("${show_ref_output}" MATCHES "remotes")
-  set(is_remote_ref 1)
-else()
-  set(is_remote_ref 0)
-endif()
+)
+if(show_ref_output MATCHES "^[a-z0-9]+[ \\t]+refs/remotes/")
+  # Given a full remote/branch-name and we know about it already. Since
+  # branches can move around, we always have to fetch.
+  set(fetch_required YES)
+  set(checkout_name "@git_tag@")
+
+elseif(show_ref_output MATCHES "^[a-z0-9]+[ \\t]+refs/tags/")
+  # Given a tag name that we already know about. We don't know if the tag we
+  # have matches the remote though (tags can move), so we should fetch.
+  set(fetch_required YES)
+  set(checkout_name "@git_tag@")
+
+  # Special case to preserve backward compatibility: if we are already at the
+  # same commit as the tag we hold locally, don't do a fetch and assume the tag
+  # hasn't moved on the remote.
+  # FIXME: We should provide an option to always fetch for this case
+  get_hash_for_ref("@git_tag@" tag_sha error_msg)
+  if(tag_sha STREQUAL head_sha)
+    message(VERBOSE "Already at requested tag: ${tag_sha}")
+    return()
+  endif()
+
+elseif(show_ref_output MATCHES "^[a-z0-9]+[ \\t]+refs/heads/")
+  # Given a branch name without any remote and we already have a branch by that
+  # name. We might already have that branch checked out or it might be a
+  # different branch. It isn't safe to use a bare branch name without the
+  # remote, so do a fetch and replace the ref with one that includes the remote.
+  set(fetch_required YES)
+  set(checkout_name "@git_remote_name@/@git_tag@")
 
-# Tag is in the form <remote>/<tag> (i.e. origin/master) we must strip
-# the remote from the tag.
-if("${show_ref_output}" MATCHES "refs/remotes/@git_tag@")
-  string(REGEX MATCH "^([^/]+)/(.+)$" _unused "@git_tag@")
-  set(git_remote "${CMAKE_MATCH_1}")
-  set(git_tag "${CMAKE_MATCH_2}")
 else()
-  set(git_remote "@git_remote_name@")
-  set(git_tag "@git_tag@")
+  get_hash_for_ref("@git_tag@" tag_sha error_msg)
+  if(tag_sha STREQUAL head_sha)
+    # Have the right commit checked out already
+    message(VERBOSE "Already at requested ref: ${tag_sha}")
+    return()
+
+  elseif(tag_sha STREQUAL "")
+    # We don't know about this ref yet, so we have no choice but to fetch.
+    # We deliberately swallow any error message at the default log level
+    # because it can be confusing for users to see a failed git command.
+    # That failure is being handled here, so it isn't an error.
+    set(fetch_required YES)
+    set(checkout_name "@git_tag@")
+    if(NOT error_msg STREQUAL "")
+      message(VERBOSE "${error_msg}")
+    endif()
+
+  else()
+    # We have the commit, so we know we were asked to find a commit hash
+    # (otherwise it would have been handled further above), but we don't
+    # have that commit checked out yet
+    set(fetch_required NO)
+    set(checkout_name "@git_tag@")
+    if(NOT error_msg STREQUAL "")
+      message(WARNING "${error_msg}")
+    endif()
+
+  endif()
 endif()
 
-# This will fail if the tag does not exist (it probably has not been fetched
-# yet).
-execute_process(
-  COMMAND "@git_EXECUTABLE@" rev-list --max-count=1 "${git_tag}"
-  WORKING_DIRECTORY "@work_dir@"
-  RESULT_VARIABLE error_code
-  OUTPUT_VARIABLE tag_sha
-  OUTPUT_STRIP_TRAILING_WHITESPACE
+if(fetch_required)
+  message(VERBOSE "Fetching latest from the remote @git_remote_name@")
+  execute_process(
+    COMMAND "@git_EXECUTABLE@" fetch --tags --force "@git_remote_name@"
+    WORKING_DIRECTORY "@work_dir@"
+    COMMAND_ERROR_IS_FATAL ANY
   )
+endif()
 
-# Is the hash checkout out that we want?
-if(error_code OR is_remote_ref OR NOT ("${tag_sha}" STREQUAL "${head_sha}"))
+set(git_update_strategy "@git_update_strategy@")
+if(git_update_strategy STREQUAL "")
+  # Backward compatibility requires REBASE as the default behavior
+  set(git_update_strategy REBASE)
+endif()
+
+if(git_update_strategy MATCHES "^REBASE(_CHECKOUT)?$")
+  # Asked to potentially try to rebase first, maybe with fallback to checkout.
+  # We can't if we aren't already on a branch and we shouldn't if that local
+  # branch isn't tracking the one we want to checkout.
   execute_process(
-    COMMAND "@git_EXECUTABLE@" fetch
+    COMMAND "@git_EXECUTABLE@" symbolic-ref -q HEAD
     WORKING_DIRECTORY "@work_dir@"
-    RESULT_VARIABLE error_code
-    )
-  if(error_code)
-    message(FATAL_ERROR "Failed to fetch repository '@git_repository@'")
-  endif()
+    OUTPUT_VARIABLE current_branch
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    # Don't test for an error. If this isn't a branch, we get a non-zero error
+    # code but empty output.
+  )
 
-  if(is_remote_ref)
-    # Check if stash is needed
+  if(current_branch STREQUAL "")
+    # Not on a branch, checkout is the only sensible option since any rebase
+    # would always fail (and backward compatibility requires us to checkout in
+    # this situation)
+    set(git_update_strategy CHECKOUT)
+
+  else()
     execute_process(
-      COMMAND "@git_EXECUTABLE@" status --porcelain
+      COMMAND "@git_EXECUTABLE@" for-each-ref "--format='%(upstream:short)'" "${current_branch}"
       WORKING_DIRECTORY "@work_dir@"
-      RESULT_VARIABLE error_code
-      OUTPUT_VARIABLE repo_status
-      )
-    if(error_code)
-      message(FATAL_ERROR "Failed to get the status")
+      OUTPUT_VARIABLE upstream_branch
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      COMMAND_ERROR_IS_FATAL ANY  # There is no error if no upstream is set
+    )
+    if(NOT upstream_branch STREQUAL checkout_name)
+      # Not safe to rebase when asked to checkout a different branch to the one
+      # we are tracking. If we did rebase, we could end up with arbitrary
+      # commits added to the ref we were asked to checkout if the current local
+      # branch happens to be able to rebase onto the target branch. There would
+      # be no error message and the user wouldn't know this was occurring.
+      set(git_update_strategy CHECKOUT)
     endif()
-    string(LENGTH "${repo_status}" need_stash)
 
-    # If not in clean state, stash changes in order to be able to perform a
-    # rebase or checkout without losing those changes permanently
-    if(need_stash)
-      execute_process(
-        COMMAND "@git_EXECUTABLE@" stash save @git_stash_save_options@
-        WORKING_DIRECTORY "@work_dir@"
-        RESULT_VARIABLE error_code
-        )
-      if(error_code)
-        message(FATAL_ERROR "Failed to stash changes")
-      endif()
-    endif()
+  endif()
+elseif(NOT git_update_strategy STREQUAL "CHECKOUT")
+  message(FATAL_ERROR "Unsupported git update strategy: ${git_update_strategy}")
+endif()
 
-    if("@git_update_strategy@" STREQUAL "CHECKOUT")
-      execute_process(
-        COMMAND "@git_EXECUTABLE@" checkout "${git_remote}/${git_tag}"
-        WORKING_DIRECTORY "@work_dir@"
-        RESULT_VARIABLE error_code
-        )
-      if(error_code)
-        message(FATAL_ERROR "Failed to checkout tag: '${git_remote}/${git_tag}'")
-      endif()
-    else()
-      # Pull changes from the remote branch
-      execute_process(
-        COMMAND "@git_EXECUTABLE@" rebase "${git_remote}/${git_tag}"
-        WORKING_DIRECTORY "@work_dir@"
-        RESULT_VARIABLE error_code
-        OUTPUT_VARIABLE rebase_output
-        ERROR_VARIABLE  rebase_output
-        )
-      if(error_code)
-        # Rebase failed, undo the rebase attempt before continuing
-        execute_process(
-          COMMAND "@git_EXECUTABLE@" rebase --abort
-          WORKING_DIRECTORY "@work_dir@"
-        )
-
-        if(NOT "@git_update_strategy@" STREQUAL "REBASE_CHECKOUT")
-          # Not allowed to do a checkout as a fallback, so cannot proceed
-          if(need_stash)
-            execute_process(
-              COMMAND "@git_EXECUTABLE@" stash pop --index --quiet
-              WORKING_DIRECTORY "@work_dir@"
-              )
-          endif()
-          message(FATAL_ERROR "\nFailed to rebase in: '@work_dir@'."
-                              "\nOutput from the attempted rebase follows:"
-                              "\n${rebase_output}"
-                              "\n\nYou will have to resolve the conflicts manually")
-        endif()
-
-        # Fall back to checkout. We create an annotated tag so that the user
-        # can manually inspect the situation and revert if required.
-        # We can't log the failed rebase output because MSVC sees it and
-        # intervenes, causing the build to fail even though it completes.
-        # Write it to a file instead.
-        string(TIMESTAMP tag_timestamp "%Y%m%dT%H%M%S" UTC)
-        set(tag_name _cmake_ExternalProject_moved_from_here_${tag_timestamp}Z)
-        set(error_log_file ${CMAKE_CURRENT_LIST_DIR}/rebase_error_${tag_timestamp}Z.log)
-        file(WRITE ${error_log_file} "${rebase_output}")
-        message(WARNING "Rebase failed, output has been saved to ${error_log_file}"
-                        "\nFalling back to checkout, previous commit tagged as ${tag_name}")
-        execute_process(
-          COMMAND "@git_EXECUTABLE@" tag -a
-                  -m "ExternalProject attempting to move from here to ${git_remote}/${git_tag}"
-                  ${tag_name}
-          WORKING_DIRECTORY "@work_dir@"
-          RESULT_VARIABLE error_code
-        )
-        if(error_code)
-          message(FATAL_ERROR "Failed to add marker tag")
-        endif()
 
-        execute_process(
-          COMMAND "@git_EXECUTABLE@" checkout "${git_remote}/${git_tag}"
-          WORKING_DIRECTORY "@work_dir@"
-          RESULT_VARIABLE error_code
-        )
-        if(error_code)
-          message(FATAL_ERROR "Failed to checkout : '${git_remote}/${git_tag}'")
-        endif()
+# Check if stash is needed
+execute_process(
+  COMMAND "@git_EXECUTABLE@" status --porcelain
+  WORKING_DIRECTORY "@work_dir@"
+  RESULT_VARIABLE error_code
+  OUTPUT_VARIABLE repo_status
+)
+if(error_code)
+  message(FATAL_ERROR "Failed to get the status")
+endif()
+string(LENGTH "${repo_status}" need_stash)
 
-      endif()
-    endif()
+# If not in clean state, stash changes in order to be able to perform a
+# rebase or checkout without losing those changes permanently
+if(need_stash)
+  execute_process(
+    COMMAND "@git_EXECUTABLE@" stash save @git_stash_save_options@
+    WORKING_DIRECTORY "@work_dir@"
+    COMMAND_ERROR_IS_FATAL ANY
+  )
+endif()
 
-    if(need_stash)
-      execute_process(
-        COMMAND "@git_EXECUTABLE@" stash pop --index --quiet
-        WORKING_DIRECTORY "@work_dir@"
-        RESULT_VARIABLE error_code
-        )
-      if(error_code)
-        # Stash pop --index failed: Try again dropping the index
-        execute_process(
-          COMMAND "@git_EXECUTABLE@" reset --hard --quiet
-          WORKING_DIRECTORY "@work_dir@"
-          RESULT_VARIABLE error_code
-          )
+if(git_update_strategy STREQUAL "CHECKOUT")
+  execute_process(
+    COMMAND "@git_EXECUTABLE@" checkout "${checkout_name}"
+    WORKING_DIRECTORY "@work_dir@"
+    COMMAND_ERROR_IS_FATAL ANY
+  )
+else()
+  execute_process(
+    COMMAND "@git_EXECUTABLE@" rebase "${checkout_name}"
+    WORKING_DIRECTORY "@work_dir@"
+    RESULT_VARIABLE error_code
+    OUTPUT_VARIABLE rebase_output
+    ERROR_VARIABLE  rebase_output
+  )
+  if(error_code)
+    # Rebase failed, undo the rebase attempt before continuing
+    execute_process(
+      COMMAND "@git_EXECUTABLE@" rebase --abort
+      WORKING_DIRECTORY "@work_dir@"
+    )
+
+    if(NOT git_update_strategy STREQUAL "REBASE_CHECKOUT")
+      # Not allowed to do a checkout as a fallback, so cannot proceed
+      if(need_stash)
         execute_process(
-          COMMAND "@git_EXECUTABLE@" stash pop --quiet
+          COMMAND "@git_EXECUTABLE@" stash pop --index --quiet
           WORKING_DIRECTORY "@work_dir@"
-          RESULT_VARIABLE error_code
-          )
-        if(error_code)
-          # Stash pop failed: Restore previous state.
-          execute_process(
-            COMMAND "@git_EXECUTABLE@" reset --hard --quiet ${head_sha}
-            WORKING_DIRECTORY "@work_dir@"
-          )
-          execute_process(
-            COMMAND "@git_EXECUTABLE@" stash pop --index --quiet
-            WORKING_DIRECTORY "@work_dir@"
           )
-          message(FATAL_ERROR "\nFailed to unstash changes in: '@work_dir@'."
-                              "\nYou will have to resolve the conflicts manually")
-        endif()
       endif()
+      message(FATAL_ERROR "\nFailed to rebase in: '@work_dir@'."
+                          "\nOutput from the attempted rebase follows:"
+                          "\n${rebase_output}"
+                          "\n\nYou will have to resolve the conflicts manually")
     endif()
-  else()
+
+    # Fall back to checkout. We create an annotated tag so that the user
+    # can manually inspect the situation and revert if required.
+    # We can't log the failed rebase output because MSVC sees it and
+    # intervenes, causing the build to fail even though it completes.
+    # Write it to a file instead.
+    string(TIMESTAMP tag_timestamp "%Y%m%dT%H%M%S" UTC)
+    set(tag_name _cmake_ExternalProject_moved_from_here_${tag_timestamp}Z)
+    set(error_log_file ${CMAKE_CURRENT_LIST_DIR}/rebase_error_${tag_timestamp}Z.log)
+    file(WRITE ${error_log_file} "${rebase_output}")
+    message(WARNING "Rebase failed, output has been saved to ${error_log_file}"
+                    "\nFalling back to checkout, previous commit tagged as ${tag_name}")
     execute_process(
-      COMMAND "@git_EXECUTABLE@" checkout "${git_tag}"
+      COMMAND "@git_EXECUTABLE@" tag -a
+              -m "ExternalProject attempting to move from here to ${checkout_name}"
+              ${tag_name}
       WORKING_DIRECTORY "@work_dir@"
-      RESULT_VARIABLE error_code
-      )
-    if(error_code)
-      message(FATAL_ERROR "Failed to checkout tag: '${git_tag}'")
-    endif()
+      COMMAND_ERROR_IS_FATAL ANY
+    )
+
+    execute_process(
+      COMMAND "@git_EXECUTABLE@" checkout "${checkout_name}"
+      WORKING_DIRECTORY "@work_dir@"
+      COMMAND_ERROR_IS_FATAL ANY
+    )
   endif()
+endif()
 
-  set(init_submodules "@init_submodules@")
-  if(init_submodules)
+if(need_stash)
+  # Put back the stashed changes
+  execute_process(
+    COMMAND "@git_EXECUTABLE@" stash pop --index --quiet
+    WORKING_DIRECTORY "@work_dir@"
+    RESULT_VARIABLE error_code
+    )
+  if(error_code)
+    # Stash pop --index failed: Try again dropping the index
+    execute_process(
+      COMMAND "@git_EXECUTABLE@" reset --hard --quiet
+      WORKING_DIRECTORY "@work_dir@"
+    )
     execute_process(
-      COMMAND "@git_EXECUTABLE@" submodule update @git_submodules_recurse@ --init @git_submodules@
+      COMMAND "@git_EXECUTABLE@" stash pop --quiet
       WORKING_DIRECTORY "@work_dir@"
       RESULT_VARIABLE error_code
+    )
+    if(error_code)
+      # Stash pop failed: Restore previous state.
+      execute_process(
+        COMMAND "@git_EXECUTABLE@" reset --hard --quiet ${head_sha}
+        WORKING_DIRECTORY "@work_dir@"
       )
+      execute_process(
+        COMMAND "@git_EXECUTABLE@" stash pop --index --quiet
+        WORKING_DIRECTORY "@work_dir@"
+      )
+      message(FATAL_ERROR "\nFailed to unstash changes in: '@work_dir@'."
+                          "\nYou will have to resolve the conflicts manually")
+    endif()
   endif()
-  if(error_code)
-    message(FATAL_ERROR "Failed to update submodules in: '@work_dir@'")
-  endif()
+endif()
+
+set(init_submodules "@init_submodules@")
+if(init_submodules)
+  execute_process(
+    COMMAND "@git_EXECUTABLE@" submodule update @git_submodules_recurse@ --init @git_submodules@
+    WORKING_DIRECTORY "@work_dir@"
+    COMMAND_ERROR_IS_FATAL ANY
+  )
 endif()

Modules/ExternalProject.cmake

@@ -2671,6 +2671,10 @@ function(_ep_add_download_command name)
     get_property(git_progress TARGET ${name} PROPERTY _EP_GIT_PROGRESS)
     get_property(git_config TARGET ${name} PROPERTY _EP_GIT_CONFIG)
 
+    # Make checkouts quiet when checking out a git hash (this avoids the
+    # very noisy detached head message)
+    list(PREPEND git_config advice.detachedHead=false)
+
     # For the download step, and the git clone operation, only the repository
     # should be recorded in a configured RepositoryInfo file. If the repo
     # changes, the clone script should be run again. But if only the tag

Tests/ExternalProjectUpdate/CMakeLists.txt

@@ -2,6 +2,11 @@ cmake_minimum_required(VERSION 2.8.12)
 project(ExternalProjectUpdateTest NONE)
 if(CMAKE_XCODE_BUILD_SYSTEM VERSION_GREATER_EQUAL 12)
   cmake_policy(SET CMP0114 NEW)
+else()
+  # This test is very noisy with warnings about this policy if we don't
+  # explicitly set it. Projects shouldn't do this, but for test code this
+  # is reasonable.
+  cmake_policy(SET CMP0114 OLD)
 endif()
 cmake_policy(GET CMP0114 cmp0114)
 

Tests/ExternalProjectUpdate/ExternalProjectUpdateTest.cmake

@@ -185,7 +185,6 @@ if(do_git_tests)
   # 'git fetch'
   check_a_tag(tag1          d1970730310fe8bc07e73f15dc570071f9f9654a 0 REBASE)
   check_a_tag(tag2          5842b503ba4113976d9bb28d57b5aee1ad2736b7 1 REBASE)
-  check_a_tag(d19707303     d1970730310fe8bc07e73f15dc570071f9f9654a 1 REBASE)
   check_a_tag(d19707303     d1970730310fe8bc07e73f15dc570071f9f9654a 0 REBASE)
   check_a_tag(origin/master b5752a26ae448410926b35c275af3c192a53722e 1 REBASE)
   # This is a remote symbolic ref, so it will always trigger a 'git fetch'

Tests/RunCMake/ExternalProject/FetchGitTags.cmake

+find_package(Git QUIET REQUIRED)
+
+include(ExternalProject)
+
+set(srcRepo ${CMAKE_CURRENT_BINARY_DIR}/srcRepo)
+set(srcDir  ${CMAKE_CURRENT_BINARY_DIR}/src)
+set(binDir  ${CMAKE_CURRENT_BINARY_DIR}/build)
+file(MAKE_DIRECTORY ${srcRepo})
+file(MAKE_DIRECTORY ${srcDir})
+
+file(GLOB entries ${srcRepo}/*)
+file(REMOVE_RECURSE ${entries} ${binDir})
+file(TOUCH ${srcRepo}/firstFile.txt)
+configure_file(${CMAKE_CURRENT_LIST_DIR}/FetchGitTags/CMakeLists.txt
+    ${srcDir}/CMakeLists.txt COPYONLY)
+
+function(execGitCommand)
+  execute_process(
+    WORKING_DIRECTORY ${srcRepo}
+    COMMAND ${GIT_EXECUTABLE} ${ARGN}
+    COMMAND_ECHO STDOUT
+    COMMAND_ERROR_IS_FATAL ANY
+  )
+endfunction()
+
+function(configureAndBuild tag)
+  execute_process(COMMAND ${CMAKE_COMMAND}
+      -G ${CMAKE_GENERATOR} -T "${CMAKE_GENERATOR_TOOLSET}"
+      -A "${CMAKE_GENERATOR_PLATFORM}"
+      -D repoDir:PATH=${srcRepo}
+      -D gitTag:STRING=${tag}
+      -B ${binDir}
+      -S ${srcDir}
+    COMMAND_ECHO STDOUT
+    COMMAND_ERROR_IS_FATAL ANY
+  )
+
+  execute_process(COMMAND ${CMAKE_COMMAND} --build ${binDir} --target fetcher
+    WORKING_DIRECTORY ${binDir}
+    COMMAND_ECHO STDOUT
+    COMMAND_ERROR_IS_FATAL ANY
+  )
+endfunction()
+
+# Setup a fresh source repo with a predictable default branch across all
+# git versions
+execGitCommand(-c init.defaultBranch=master init)
+execGitCommand(config --add user.email "testauthor@cmake.org")
+execGitCommand(config --add user.name testauthor)
+
+# Create the initial repo structure
+execGitCommand(add firstFile.txt)
+execGitCommand(commit -m "First file")
+
+message(STATUS "First configure-and-build")
+configureAndBuild(master)
+
+# Create a tagged commit that is not on any branch. With git 2.20 or later,
+# this commit won't be fetched without the --tags option.
+file(TOUCH ${srcRepo}/secondFile.txt)
+execGitCommand(add secondFile.txt)
+execGitCommand(commit -m "Second file")
+execGitCommand(tag -a -m "Adding tag" tag_of_interest)
+execGitCommand(reset --hard HEAD~1)
+
+message(STATUS "Second configure-and-build")
+configureAndBuild(tag_of_interest)

Tests/RunCMake/ExternalProject/FetchGitTags/CMakeLists.txt

+cmake_minimum_required(VERSION 3.19)
+project(FetchTags LANGUAGES NONE)
+
+include(ExternalProject)
+
+# repoDir and gitTag are expected to be set as cache variables
+
+ExternalProject_Add(fetcher
+    GIT_REPOSITORY ${repoDir}
+    GIT_TAG ${gitTag}
+    GIT_REMOTE_UPDATE_STRATEGY CHECKOUT
+    CONFIGURE_COMMAND ""
+    BUILD_COMMAND ""
+    INSTALL_COMMAND ""
+)

Tests/RunCMake/ExternalProject/RunCMakeTest.cmake

@@ -181,3 +181,12 @@ endfunction()
 if(NOT RunCMake_GENERATOR MATCHES "Visual Studio 9 ")
   __ep_test_CONFIGURE_HANDLED_BY_BUILD()
 endif()
+
+find_package(Git QUIET)
+if(GIT_EXECUTABLE)
+  # Note that there appear to be differences in where git writes its output to
+  # on some platforms. It may go to stdout or stderr, so force it to be merged.
+  set(RunCMake_TEST_OUTPUT_MERGE TRUE)
+  run_cmake(FetchGitTags)
+  set(RunCMake_TEST_OUTPUT_MERGE FALSE)
+endif()