cmake: Set source epoch from the HEAD commit

Allow to set the source epoch of the build with
PROJECT_VERSION_TIMESTAMP.

When unset fetch the commit timestamp of the current HEAD automatically
instead of leaving the source as 0. This fixes the firmware DATE
information in the support menu.

CMakeLists.txt

@@ -19,6 +19,10 @@ set(BUILD_NUMBER
     ""
     CACHE STRING "Build number of the firmware. Resolved automatically if not specified."
     )
+set(PROJECT_VERSION_TIMESTAMP
+    ""
+    CACHE STRING "Timestamp for the build. Resolved automatically if not specified."
+    )
 
 include(cmake/ProjectVersion.cmake)
 resolve_version_variables()
@@ -229,8 +233,8 @@ list(TRANSFORM AVR_SOURCES PREPEND ${PRUSA_BOARDS_DIR}/cores/prusa_einsy_rambo/)
 # Target configuration
 #
 if(CMAKE_CROSSCOMPILING)
-  # TODO: get date from the last git commit to set as epoch
-  set_source_epoch(0)
+  # set source epoch
+  set_source_epoch(${PROJECT_VERSION_TIMESTAMP})
 
   # default optimization flags
   set(CMAKE_CXX_FLAGS_DEBUG "-Og -g")

cmake/GetGitRevisionDescription.cmake

@@ -29,6 +29,10 @@
 # I don't know if get_git_head_revision() must be called internally or not, as reason of calling it
 # is not clear for me also in git_local_changes().
 #
+# git_head_commit_timestamp(<var>)
+#
+# Returns the timestamp of the HEAD commit.
+#
 # Requires CMake 2.6 or newer (uses the 'function' command)
 #
 # Original Author: 2009-2010 Ryan Pavlik <rpavlik@iastate.edu> <abiryan@ryand.net>
@@ -230,3 +234,51 @@ function(git_count_parent_commits _var)
   endif()
 
 endfunction()
+
+function(git_get_commit_timestamp _var hash)
+  execute_process(
+    COMMAND "${GIT_EXECUTABLE}" show -s "--format=%ct" "${hash}"
+    WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
+    RESULT_VARIABLE res
+    OUTPUT_VARIABLE out
+    ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE
+    )
+  if(res EQUAL 0)
+    set(${_var}
+        "${out}"
+        PARENT_SCOPE
+        )
+  else()
+    set(${_var}
+        "0"
+        PARENT_SCOPE
+        )
+  endif()
+
+endfunction()
+
+function(git_head_commit_timestamp _var)
+  if(NOT GIT_FOUND)
+    find_package(Git QUIET)
+  endif()
+  get_git_head_revision(refspec hash)
+  if(NOT GIT_FOUND)
+    set(${_var}
+        "GIT-NOTFOUND"
+        PARENT_SCOPE
+        )
+    return()
+  endif()
+  if(NOT hash)
+    set(${_var}
+        "HEAD-HASH-NOTFOUND"
+        PARENT_SCOPE
+        )
+    return()
+  endif()
+  git_get_commit_timestamp(timestamp ${hash})
+  set(${_var}
+      "${timestamp}"
+      PARENT_SCOPE
+      )
+endfunction()

cmake/ProjectVersion.cmake

@@ -7,6 +7,7 @@
 # PROJECT_VERSION_FULL (4.0.3-BETA+1035.PR111.B4)
 # PROJECT_VERSION_SUFFIX (-BETA+1035.PR111.B4)
 # PROJECT_VERSION_SUFFIX_SHORT (+1035)
+# PROJECT_VERSION_TIMESTAMP (unix timestamp)
 #
 # The `PROJECT_VERSION` variable is set as soon as the file is included.
 # To set the rest, the function `resolve_version_variables` has to be called.
@@ -67,4 +68,14 @@ function(resolve_version_variables)
       "${PROJECT_VERSION}${PROJECT_VERSION_SUFFIX}"
       PARENT_SCOPE
       )
+
+  # PROJECT_VERSION_TIMESTAMP
+  if(NOT PROJECT_VERSION_TIMESTAMP)
+    git_head_commit_timestamp(timestamp)
+    set(PROJECT_VERSION_TIMESTAMP
+        "${timestamp}"
+        PARENT_SCOPE
+        )
+  endif()
+
 endfunction()