build: Automatically parse version from tag

The build will now parse tags so the git tag is now the single source
of truth for a version. We do not need to manually maintain the
version header.

The VERSION file uses git's export-subst feature. When git archive
creates a snapshot (e.g. cgit), git substitutes the $Format:...$
placeholder with the output of git describe, embedding the version
string directly in the archive. This requires the export-subst
attribute to be set for the VERSION file in the repository's
.gitattributes.

The version is resolved in the following order:
 - git describe (inside a git repository with tags)
 - the VERSION file (archives/snapshots)
 - 0.0.0 (fallback)

The build system will warn when it can't set a correct version, such
as in a git repo without tags, or an archive without VERSION.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

1 files changed, 28 insertions, 4 deletions

diff --git a/cmake/utils/GetGitHash.cmake b/cmake/utils/GetGitHash.cmake
index 8a9be41d..e399216d 100644
--- a/cmake/utils/GetGitHash.cmake
+++ b/cmake/utils/GetGitHash.cmake
@@ -1,15 +1,39 @@
 function(get_git_hash WORKING_DIR VERSION_MAJ VERSION_MIN VERSION_PAT OUTPUT_VAR)
   execute_process(
-    COMMAND git describe --always --dirty
+    COMMAND git describe --tags --always --dirty
+      --match "[0-9]*.[0-9]*.[0-9]*"
     WORKING_DIRECTORY ${WORKING_DIR}
     OUTPUT_VARIABLE _hash
     OUTPUT_STRIP_TRAILING_WHITESPACE
     ERROR_QUIET
   )
 
-  if(NOT _hash)
-    message(WARNING "Could not determine git hash")
-    set(_hash "${VERSION_MAJ}.${VERSION_MIN}.${VERSION_PAT}-custom")
+  if(_hash MATCHES "^[0-9]+\\.[0-9]+\\.[0-9]+")
+    # git describe returned a tag-based version string
+  elseif(_hash)
+    # No version tag found, construct full version string
+    execute_process(
+      COMMAND git rev-list --count HEAD
+      WORKING_DIRECTORY ${WORKING_DIR}
+      OUTPUT_VARIABLE _count
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_QUIET
+    )
+    execute_process(
+      COMMAND git describe --always --dirty
+      WORKING_DIRECTORY ${WORKING_DIR}
+      OUTPUT_VARIABLE _desc
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_QUIET
+    )
+    set(_hash
+      "${VERSION_MAJ}.${VERSION_MIN}.${VERSION_PAT}-${_count}-g${_desc}")
+  elseif(EXISTS "${WORKING_DIR}/VERSION")
+    # No git, use VERSION file (git archive / cgit snapshot)
+    file(READ "${WORKING_DIR}/VERSION" _hash)
+    string(STRIP "${_hash}" _hash)
+  else()
+    set(_hash "${VERSION_MAJ}.${VERSION_MIN}.${VERSION_PAT}")
   endif()
 
   set(${OUTPUT_VAR} "${_hash}" PARENT_SCOPE)