Revert ExternalProject and FetchContent refactoring

Refactoring of the ExternalProject and FetchContent modules moved
the commands into CMake scripts. This broke custom commands that
used shell redirection or special build tool variables of the form
$(MakeVar). Undo the sequence of commits that performed this
refactoring and follow-up fixes associated with it.

The following commits are reverted by this change:

4f3d1abbb4 (ExternalProject: Refactor pre-configure steps to support
no-target uses, 2021-02-05)

17e5516e60 (FetchContent: Invoke steps directly and avoid a separate
sub-build, 2021-01-29)

bd876f3849 (FetchContent: Restore patch command support,
2021-02-18)

404cddb7bb (ExternalProject: Fix misuse of IS_NEWER_THAN in
timestamp checks, 2021-02-21)

b0da671243 (FetchContent: Don't update timestamps if files don't
change, 2021-02-18)

Fixes: #21892

1 files changed, 277 insertions, 0 deletions

diff --git a/Modules/ExternalProject-gitupdate.cmake.in b/Modules/ExternalProject-gitupdate.cmake.in
new file mode 100644
index 0000000000..7033918056
--- /dev/null
+++ b/Modules/ExternalProject-gitupdate.cmake.in
@@ -0,0 +1,277 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+cmake_minimum_required(VERSION 3.5)
+
+function(get_hash_for_ref ref out_var err_var)
+  execute_process(
+    COMMAND "@git_EXECUTABLE@" rev-parse "${ref}"
+    WORKING_DIRECTORY "@work_dir@"
+    RESULT_VARIABLE error_code
+    OUTPUT_VARIABLE ref_hash
+    ERROR_VARIABLE error_msg
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+  )
+  if(error_code)
+    set(${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(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@")
+
+else()
+  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 "${error_msg}")
+    endif()
+
+  endif()
+endif()
+
+if(fetch_required)
+  message(VERBOSE "Fetching latest from the remote @git_remote_name@")
+  execute_process(
+    COMMAND "@git_EXECUTABLE@" fetch --tags --force "@git_remote_name@"
+    WORKING_DIRECTORY "@work_dir@"
+    COMMAND_ERROR_IS_FATAL ANY
+  )
+endif()
+
+set(git_update_strategy "@git_update_strategy@")
+if(git_update_strategy STREQUAL "")
+  # Backward compatibility requires REBASE as the default behavior
+  set(git_update_strategy REBASE)
+endif()
+
+if(git_update_strategy MATCHES "^REBASE(_CHECKOUT)?$")
+  # 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@" symbolic-ref -q HEAD
+    WORKING_DIRECTORY "@work_dir@"
+    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(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@" for-each-ref "--format='%(upstream:short)'" "${current_branch}"
+      WORKING_DIRECTORY "@work_dir@"
+      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()
+
+  endif()
+elseif(NOT git_update_strategy STREQUAL "CHECKOUT")
+  message(FATAL_ERROR "Unsupported git update strategy: ${git_update_strategy}")
+endif()
+
+
+# Check if stash is needed
+execute_process(
+  COMMAND "@git_EXECUTABLE@" status --porcelain
+  WORKING_DIRECTORY "@work_dir@"
+  RESULT_VARIABLE error_code
+  OUTPUT_VARIABLE repo_status
+)
+if(error_code)
+  message(FATAL_ERROR "Failed to get the status")
+endif()
+string(LENGTH "${repo_status}" need_stash)
+
+# 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(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 --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 ${checkout_name}"
+              ${tag_name}
+      WORKING_DIRECTORY "@work_dir@"
+      COMMAND_ERROR_IS_FATAL ANY
+    )
+
+    execute_process(
+      COMMAND "@git_EXECUTABLE@" checkout "${checkout_name}"
+      WORKING_DIRECTORY "@work_dir@"
+      COMMAND_ERROR_IS_FATAL ANY
+    )
+  endif()
+endif()
+
+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@" 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()
+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()