ExternalProject/FetchContent: Support relative remote URLs

Teach `ExternalProject_Add` and `FetchContent_Declare` to resolve
relative remote URLs provided via `GIT_REPOSITORY`.  Add policy
CMP0150 to maintain compatibility.

Fixes: #24211
Co-Authored-By: Craig Scott <craig.scott@crascit.com>

3 files changed, 229 insertions, 0 deletions

diff --git a/Modules/ExternalProject.cmake b/Modules/ExternalProject.cmake
index 9a6cbd6bc4..e2cc497a65 100644
--- a/Modules/ExternalProject.cmake
+++ b/Modules/ExternalProject.cmake
@@ -278,6 +278,13 @@ External Project Definition
         URL of the git repository. Any URL understood by the ``git`` command
         may be used.
 
+        .. versionchanged:: 3.27
+          A relative URL will be resolved based on the parent project's
+          remote, subject to :policy:`CMP0150`.  See the policy documentation
+          for how the remote is selected, including conditions where the
+          remote selection can fail.  Local filesystem remotes should
+          always use absolute paths.
+
       ``GIT_TAG <tag>``
         Git branch name, tag or commit hash. Note that branch names and tags
         should generally be specified as remote names (i.e. ``origin/myBranch``
@@ -1188,6 +1195,8 @@ The custom step could then be triggered from the main build like so::
 
 #]=======================================================================]
 
+include(${CMAKE_CURRENT_LIST_DIR}/ExternalProject/shared_internal_commands.cmake)
+
 cmake_policy(PUSH)
 cmake_policy(SET CMP0054 NEW) # if() quoted variables not dereferenced
 cmake_policy(SET CMP0057 NEW) # if() supports IN_LIST
@@ -4159,6 +4168,17 @@ function(ExternalProject_Add name)
     set_property(TARGET ${name} PROPERTY EXCLUDE_FROM_ALL TRUE)
   endif()
 
+  get_property(repo TARGET ${name} PROPERTY _EP_GIT_REPOSITORY)
+  if(NOT repo STREQUAL "")
+    cmake_policy(GET CMP0150 cmp0150
+      PARENT_SCOPE # undocumented, do not use outside of CMake
+    )
+    get_property(source_dir TARGET ${name} PROPERTY _EP_SOURCE_DIR)
+    get_filename_component(work_dir "${source_dir}" PATH)
+    _ep_resolve_git_remote(resolved_git_repository "${repo}" "${cmp0150}" "${work_dir}")
+    set_property(TARGET ${name} PROPERTY _EP_GIT_REPOSITORY ${resolved_git_repository})
+  endif()
+
   # The 'complete' step depends on all other steps and creates a
   # 'done' mark.  A dependent external project's 'configure' step
   # depends on the 'done' mark so that it rebuilds when this project
diff --git a/Modules/ExternalProject/shared_internal_commands.cmake b/Modules/ExternalProject/shared_internal_commands.cmake
new file mode 100644
index 0000000000..ca3cd9fec7
--- /dev/null
+++ b/Modules/ExternalProject/shared_internal_commands.cmake
@@ -0,0 +1,182 @@
+cmake_policy(VERSION 3.25)
+
+# Determine the remote URL of the project containing the working_directory.
+# This will leave output_variable unset if the URL can't be determined.
+function(_ep_get_git_remote_url output_variable working_directory)
+  set("${output_variable}" "" PARENT_SCOPE)
+
+  find_package(Git QUIET REQUIRED)
+
+  execute_process(
+    COMMAND ${GIT_EXECUTABLE} symbolic-ref --short HEAD
+    WORKING_DIRECTORY "${working_directory}"
+    OUTPUT_VARIABLE git_symbolic_ref
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_QUIET
+  )
+
+  if(NOT git_symbolic_ref STREQUAL "")
+    # We are potentially on a branch. See if that branch is associated with
+    # an upstream remote (might be just a local one or not a branch at all).
+    execute_process(
+      COMMAND ${GIT_EXECUTABLE} config branch.${git_symbolic_ref}.remote
+      WORKING_DIRECTORY "${working_directory}"
+      OUTPUT_VARIABLE git_remote_name
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_QUIET
+    )
+  endif()
+
+  if(NOT git_remote_name)
+    # Can't select a remote based on a branch. If there's only one remote,
+    # or we have multiple remotes but one is called "origin", choose that.
+    execute_process(
+      COMMAND ${GIT_EXECUTABLE} remote
+      WORKING_DIRECTORY "${working_directory}"
+      OUTPUT_VARIABLE git_remote_list
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_QUIET
+    )
+    string(REPLACE "\n" ";" git_remote_list "${git_remote_list}")
+    list(LENGTH git_remote_list git_remote_list_length)
+
+    if(git_remote_list_length EQUAL 0)
+      message(FATAL_ERROR "Git remote not found in parent project.")
+    elseif(git_remote_list_length EQUAL 1)
+      list(GET git_remote_list 0 git_remote_name)
+    else()
+      set(base_warning_msg "Multiple git remotes found for parent project")
+      if("origin" IN_LIST git_remote_list)
+        message(WARNING "${base_warning_msg}, defaulting to origin.")
+        set(git_remote_name "origin")
+      else()
+        message(FATAL_ERROR "${base_warning_msg}, none of which are origin.")
+      endif()
+    endif()
+  endif()
+
+  if(GIT_VERSION VERSION_LESS 1.7.5)
+    set(_git_remote_url_cmd_args config remote.${git_remote_name}.url)
+  elseif(GIT_VERSION VERSION_LESS 2.7)
+    set(_git_remote_url_cmd_args ls-remote --get-url ${git_remote_name})
+  else()
+    set(_git_remote_url_cmd_args remote get-url ${git_remote_name})
+  endif()
+
+  execute_process(
+    COMMAND ${GIT_EXECUTABLE} ${_git_remote_url_cmd_args}
+    WORKING_DIRECTORY "${working_directory}"
+    OUTPUT_VARIABLE git_remote_url
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    COMMAND_ERROR_IS_FATAL LAST
+    ENCODING UTF-8   # Needed to handle non-ascii characters in local paths
+  )
+
+  set("${output_variable}" "${git_remote_url}" PARENT_SCOPE)
+endfunction()
+
+function(_ep_is_relative_git_remote output_variable remote_url)
+  if(remote_url MATCHES "^\\.\\./")
+    set("${output_variable}" TRUE PARENT_SCOPE)
+  else()
+    set("${output_variable}" FALSE PARENT_SCOPE)
+  endif()
+endfunction()
+
+# Return an absolute remote URL given an existing remote URL and relative path.
+# The output_variable will be set to an empty string if an absolute URL
+# could not be computed (no error message is output).
+function(_ep_resolve_relative_git_remote
+  output_variable
+  parent_remote_url
+  relative_remote_url
+)
+  set("${output_variable}" "" PARENT_SCOPE)
+
+  if(parent_remote_url STREQUAL "")
+    return()
+  endif()
+
+  string(REGEX MATCH
+    "^(([A-Za-z0-9][A-Za-z0-9+.-]*)://)?(([^/@]+)@)?(\\[[A-Za-z0-9:]+\\]|[^/:]+)?([/:]/?)(.+(\\.git)?/?)$"
+    git_remote_url_components
+    "${parent_remote_url}"
+  )
+
+  set(protocol "${CMAKE_MATCH_1}")
+  set(auth "${CMAKE_MATCH_3}")
+  set(host "${CMAKE_MATCH_5}")
+  set(separator "${CMAKE_MATCH_6}")
+  set(path "${CMAKE_MATCH_7}")
+
+  string(REPLACE "/" ";" remote_path_components "${path}")
+  string(REPLACE "/" ";" relative_path_components "${relative_remote_url}")
+
+  foreach(relative_path_component IN LISTS relative_path_components)
+    if(NOT relative_path_component STREQUAL "..")
+      break()
+    endif()
+
+    list(LENGTH remote_path_components remote_path_component_count)
+
+    if(remote_path_component_count LESS 1)
+      return()
+    endif()
+
+    list(POP_BACK remote_path_components)
+    list(POP_FRONT relative_path_components)
+  endforeach()
+
+  list(APPEND final_path_components ${remote_path_components} ${relative_path_components})
+  list(JOIN final_path_components "/" path)
+
+  set("${output_variable}" "${protocol}${auth}${host}${separator}${path}" PARENT_SCOPE)
+endfunction()
+
+# The output_variable will be set to the original git_repository if it
+# could not be resolved (no error message is output). The original value is
+# also returned if it doesn't need to be resolved.
+function(_ep_resolve_git_remote
+  output_variable
+  git_repository
+  cmp0150
+  cmp0150_old_base_dir
+)
+  if(git_repository STREQUAL "")
+    set("${output_variable}" "" PARENT_SCOPE)
+    return()
+  endif()
+
+  _ep_is_relative_git_remote(_git_repository_is_relative "${git_repository}")
+
+  if(NOT _git_repository_is_relative)
+    set("${output_variable}" "${git_repository}" PARENT_SCOPE)
+    return()
+  endif()
+
+  if(cmp0150 STREQUAL "NEW")
+    _ep_get_git_remote_url(_parent_git_remote_url "${CMAKE_CURRENT_SOURCE_DIR}")
+    _ep_resolve_relative_git_remote(_resolved_git_remote_url "${_parent_git_remote_url}" "${git_repository}")
+
+    if(_resolved_git_remote_url STREQUAL "")
+      message(FATAL_ERROR
+        "Failed to resolve relative git remote URL:\n"
+        "  Relative URL: ${git_repository}\n"
+        "  Parent URL:   ${_parent_git_remote_url}"
+      )
+    endif()
+    set("${output_variable}" "${_resolved_git_remote_url}" PARENT_SCOPE)
+    return()
+  elseif(cmp0150 STREQUAL "")
+    cmake_policy(GET_WARNING CMP0150 _cmp0150_warning)
+    message(AUTHOR_WARNING
+      "${_cmp0150_warning}\n"
+      "A relative GIT_REPOSITORY path was detected. "
+      "This will be interpreted as a local path to where the project is being cloned. "
+      "Set GIT_REPOSITORY to an absolute path or set policy CMP0150 to NEW to avoid "
+      "this warning."
+    )
+  endif()
+
+  set("${output_variable}" "${cmp0150_old_base_dir}/${git_repository}" PARENT_SCOPE)
+endfunction()
diff --git a/Modules/FetchContent.cmake b/Modules/FetchContent.cmake
index dd5f617acd..74ac8aab2a 100644
--- a/Modules/FetchContent.cmake
+++ b/Modules/FetchContent.cmake
@@ -1076,6 +1076,8 @@ current working directory.
 
 #]=======================================================================]
 
+include(${CMAKE_CURRENT_LIST_DIR}/ExternalProject/shared_internal_commands.cmake)
+
 #=======================================================================
 # Recording and retrieving content details for later population
 #=======================================================================
@@ -1223,6 +1225,7 @@ function(FetchContent_Declare contentName)
   # cannot check for multi-value arguments with this method. We will have to
   # handle the URL keyword differently.
   set(oneValueArgs
+    GIT_REPOSITORY
     SVN_REPOSITORY
     DOWNLOAD_NO_EXTRACT
     DOWNLOAD_EXTRACT_TIMESTAMP
@@ -1242,6 +1245,30 @@ function(FetchContent_Declare contentName)
     set(ARG_SOURCE_DIR "${FETCHCONTENT_BASE_DIR}/${contentNameLower}-src")
   endif()
 
+  if(ARG_GIT_REPOSITORY)
+    # We resolve the GIT_REPOSITORY here so that we get the right parent in the
+    # remote selection logic. In the sub-build, ExternalProject_Add() would see
+    # the private sub-build directory as the parent project, but the parent
+    # project should be the one that called FetchContent_Declare(). We resolve
+    # a relative repo here so that the sub-build's ExternalProject_Add() only
+    # ever sees a non-relative repo.
+    # Since these checks may be non-trivial on some platforms (notably Windows),
+    # don't perform them if we won't be using these details. This also allows
+    # projects to override calls with relative URLs when they have checked out
+    # the parent project in an unexpected way, such as from a mirror or fork.
+    set(savedDetailsPropertyName "_FetchContent_${contentNameLower}_savedDetails")
+    get_property(alreadyDefined GLOBAL PROPERTY ${savedDetailsPropertyName} DEFINED)
+    if(NOT alreadyDefined)
+      cmake_policy(GET CMP0150 cmp0150
+        PARENT_SCOPE # undocumented, do not use outside of CMake
+      )
+      _ep_resolve_git_remote(_resolved_git_repository
+        "${ARG_GIT_REPOSITORY}" "${cmp0150}" "${FETCHCONTENT_BASE_DIR}"
+      )
+      set(ARG_GIT_REPOSITORY "${_resolved_git_repository}")
+    endif()
+  endif()
+
   if(ARG_SVN_REPOSITORY)
     # Add a hash of the svn repository URL to the source dir. This works
     # around the problem where if the URL changes, the download would