git_watcher_functions.cmake

macro(PATH_TO_ABSOLUTE var_name)
    get_filename_component(${var_name} "${${var_name}}" ABSOLUTE)
endmacro()

# Check that a required variable is set.
macro(CHECK_REQUIRED_VARIABLE var_name)
    if(NOT DEFINED ${var_name})
        message(FATAL_ERROR "The \"${var_name}\" variable must be defined.")
    endif()
    PATH_TO_ABSOLUTE(${var_name})
endmacro()

# Check that an optional variable is set, or, set it to a default value.
macro(CHECK_OPTIONAL_VARIABLE_NOPATH var_name default_value)
    if(NOT DEFINED ${var_name})
        set(${var_name} ${default_value})
    endif()
endmacro()

# Check that an optional variable is set, or, set it to a default value.
# Also converts that path to an abspath.
macro(CHECK_OPTIONAL_VARIABLE var_name default_value)
    CHECK_OPTIONAL_VARIABLE_NOPATH(${var_name} ${default_value})
    PATH_TO_ABSOLUTE(${var_name})
endmacro()

CHECK_OPTIONAL_VARIABLE(GIT_STATE_FILE "${CMAKE_CURRENT_BINARY_DIR}/git-state-hash")
CHECK_OPTIONAL_VARIABLE(GIT_WORKING_DIR "${CMAKE_SOURCE_DIR}")
CHECK_OPTIONAL_VARIABLE_NOPATH(GIT_FAIL_IF_NONZERO_EXIT TRUE)
CHECK_OPTIONAL_VARIABLE_NOPATH(GIT_IGNORE_UNTRACKED FALSE)

# Check the optional git variable.
# If it's not set, we'll try to find it using the CMake packaging system.
if(NOT DEFINED GIT_EXECUTABLE)
    find_package(Git QUIET REQUIRED)
endif()
CHECK_REQUIRED_VARIABLE(GIT_EXECUTABLE)

set(_state_variable_names
        GIT_RETRIEVED_STATE
        GIT_HEAD_SHA1
        GIT_IS_DIRTY
        GIT_COMMIT_DATE_ISO8601
        GIT_DESCRIBE
        GIT_BRANCH
        GIT_TAG
        # >>>
        # 1. Add the name of the additional git variable you're interested in monitoring
        #    to this list.
)



# Macro: RunGitCommand
# Description: short-hand macro for calling a git function. Outputs are the
#              "exit_code" and "output" variables. The "_permit_git_failure"
#              variable can locally override the exit code checking- use it
#              with caution.
macro(RunGitCommand)
    execute_process(COMMAND
            "${GIT_EXECUTABLE}" ${ARGV}
            WORKING_DIRECTORY "${_working_dir}"
            RESULT_VARIABLE exit_code
            OUTPUT_VARIABLE output
            ERROR_VARIABLE stderr
            OUTPUT_STRIP_TRAILING_WHITESPACE)
    if(NOT exit_code EQUAL 0 AND NOT _permit_git_failure)
        set(ENV{GIT_RETRIEVED_STATE} "false")

        # Issue 26: git info not properly set
        #
        # Check if we should fail if any of the exit codes are non-zero.
        # Most methods have a fall-back default value that's used in case of non-zero
        # exit codes. If you're feeling risky, disable this safety check and use
        # those default values.
        if(GIT_FAIL_IF_NONZERO_EXIT )
            string(REPLACE ";" " " args_with_spaces "${ARGV}")
            message(FATAL_ERROR "${stderr} (${GIT_EXECUTABLE} ${args_with_spaces})")
        endif()
    endif()
endmacro()



# Function: GetGitState
# Description: gets the current state of the git repo.
# Args:
#   _working_dir (in)  string; the directory from which git commands will be executed.
function(GetGitState _working_dir)

    # This is an error code that'll be set to FALSE if the
    # RunGitCommand ever returns a non-zero exit code.
    set(ENV{GIT_RETRIEVED_STATE} "true")

    # Get whether or not the working tree is dirty.
    if (GIT_IGNORE_UNTRACKED)
        set(untracked_flag "-uno")
    else()
        set(untracked_flag "-unormal")
    endif()
    RunGitCommand(status --porcelain ${untracked_flag})
    if(NOT exit_code EQUAL 0)
        set(ENV{GIT_IS_DIRTY} "false")
    else()
        if(NOT "${output}" STREQUAL "")
            set(ENV{GIT_IS_DIRTY} "true")
        else()
            set(ENV{GIT_IS_DIRTY} "false")
        endif()
    endif()

    # There's a long list of attributes grabbed from git show.
    set(object HEAD)
    RunGitCommand(show -s "--format=%H" ${object})
    if(exit_code EQUAL 0)
        set(ENV{GIT_HEAD_SHA1} ${output})
    endif()

    RunGitCommand(show -s "--format=%ci" ${object})
    if(exit_code EQUAL 0)
        set(ENV{GIT_COMMIT_DATE_ISO8601} "${output}")
    endif()

    # Get output of git describe
    RunGitCommand(describe --always ${object})
    if(NOT exit_code EQUAL 0)
        set(ENV{GIT_DESCRIBE} "unknown")
    else()
        set(ENV{GIT_DESCRIBE} "${output}")
    endif()

    # Convert HEAD to a symbolic ref. This can fail, in which case we just
    # set that variable to HEAD.
    set(_permit_git_failure ON)
    RunGitCommand(symbolic-ref --short -q ${object})
    unset(_permit_git_failure)
    if(NOT exit_code EQUAL 0)
        set(ENV{GIT_BRANCH} "${object}")
    else()
        set(ENV{GIT_BRANCH} "${output}")
    endif()

    # Get output of git describe --tags
    set(_permit_git_failure ON)
    RunGitCommand(describe --tags --dirty)
    unset(_permit_git_failure)
    if(NOT exit_code EQUAL 0)
        set(ENV{GIT_TAG} "")
    else()
        set(ENV{GIT_TAG} "${output}")
    endif()

    # >>>
    # 2. Additional git properties can be added here via the
    #    "execute_process()" command. Be sure to set them in
    #    the environment using the same variable name you added
    #    to the "_state_variable_names" list.

endfunction()



# Function: GitStateChangedAction
# Description: this function is executed when the state of the git
#              repository changes (e.g. a commit is made).
function(GitStateChangedAction)
    foreach(var_name ${_state_variable_names})
        set(${var_name} $ENV{${var_name}})
    endforeach()
    configure_file("${PRE_CONFIGURE_FILE}" "${POST_CONFIGURE_FILE}" @ONLY)
endfunction()



# Function: HashGitState
# Description: loop through the git state variables and compute a unique hash.
# Args:
#   _state (out)  string; a hash computed from the current git state.
function(HashGitState _state)
    set(ans "")
    foreach(var_name ${_state_variable_names})
        string(SHA256 ans "${ans}$ENV{${var_name}}")
    endforeach()
    set(${_state} ${ans} PARENT_SCOPE)
endfunction()



# Function: CheckGit
# Description: check if the git repo has changed. If so, update the state file.
# Args:
#   _working_dir    (in)  string; the directory from which git commands will be ran.
#   _state_changed (out)    bool; whether or no the state of the repo has changed.
function(CheckGit _working_dir _state_changed)

    # Get the current state of the repo.
    GetGitState("${_working_dir}")

    # Convert that state into a hash that we can compare against
    # the hash stored on-disk.
    HashGitState(state)

    # Issue 14: post-configure file isn't being regenerated.
    #
    # Update the state to include the SHA256 for the pre-configure file.
    # This forces the post-configure file to be regenerated if the
    # pre-configure file has changed.
    file(SHA256 ${PRE_CONFIGURE_FILE} preconfig_hash)
    string(SHA256 state "${preconfig_hash}${state}")

    # Check if the state has changed compared to the backup on disk.
    if(EXISTS "${GIT_STATE_FILE}")
        file(READ "${GIT_STATE_FILE}" OLD_HEAD_CONTENTS)
        if(OLD_HEAD_CONTENTS STREQUAL "${state}")
            # State didn't change.
            set(${_state_changed} "false" PARENT_SCOPE)
            return()
        endif()
    endif()

    # The state has changed.
    # We need to update the state file on disk.
    # Future builds will compare their state to this file.
    file(WRITE "${GIT_STATE_FILE}" "${state}")
    set(${_state_changed} "true" PARENT_SCOPE)
endfunction()



# Function: SetupGitMonitoring
# Description: this function sets up custom commands that make the build system
#              check the state of git before every build. If the state has
#              changed, then a file is configured.
function(SetupGitMonitoring)
    add_custom_target(check_git
            ALL
            DEPENDS ${PRE_CONFIGURE_FILE}
            BYPRODUCTS
            ${POST_CONFIGURE_FILE}
            ${GIT_STATE_FILE}
            COMMENT "Checking the git repository for changes..."
            COMMAND
            ${CMAKE_COMMAND}
            -D_BUILD_TIME_CHECK_GIT=TRUE
            -DGIT_WORKING_DIR=${GIT_WORKING_DIR}
            -DGIT_EXECUTABLE=${GIT_EXECUTABLE}
            -DGIT_STATE_FILE=${GIT_STATE_FILE}
            -DPRE_CONFIGURE_FILE=${PRE_CONFIGURE_FILE}
            -DPOST_CONFIGURE_FILE=${POST_CONFIGURE_FILE}
            -DGIT_FAIL_IF_NONZERO_EXIT=${GIT_FAIL_IF_NONZERO_EXIT}
            -DGIT_IGNORE_UNTRACKED=${GIT_IGNORE_UNTRACKED}
            -P "${CMAKE_CURRENT_LIST_DIR}/git_watcher.cmake")
endfunction()



# Function: Main
# Description: primary entry-point to the script. Functions are selected based
#              on whether it's configure or build time.
function(Main)
    CHECK_REQUIRED_VARIABLE(PRE_CONFIGURE_FILE)
    CHECK_REQUIRED_VARIABLE(POST_CONFIGURE_FILE)
    if(_BUILD_TIME_CHECK_GIT)
        # Check if the repo has changed.
        # If so, run the change action.
        CheckGit("${GIT_WORKING_DIR}" changed)
        if(changed OR NOT EXISTS "${POST_CONFIGURE_FILE}")
            GitStateChangedAction()
        endif()
    else()
        # >> Executes at configure time.
        SetupGitMonitoring()
    endif()
endfunction()