meson: wire up generation of distribution archive

Meson knows to generate distribution archives via `meson dist`. Despite
generating the archive itself, this target also knows to compile and
execute tests from that archive, which helps to ensure that the result
is an adequate drop-in replacement for the versioned project.

While this already works as-is, one omission is that we don't propagate
the commit that this is built from into the resulting archive. This can
be fixed though by adding a distribution script that propagates the
version into the "version" file, which GIT-VERSION-GEN knows to read if
present.

Use GIT-VERSION-GEN itself to populate that file. There are two smallish
gotchas:

  - The script is executed in the build directory, not in the directory
    where we generate the archive. The target directory is propagated
    via the "MESON_DIST_ROOT" environment variable, but because we don't
    use a shell to execute GIT-VERSION-GEN we cannot populate the envvar
    into its arguments. We thus adapt GIT-VERSION-GEN to handle this for
    us.

  - We use the "GIT-VERSION-FILE.in" template to generate the version,
    which contains a "GIT_VERSION=" prefix that we need to strip after
    reading the "version" file. We could avoid this extra logic if we
    used a template that only had the `@GIT_VERSION@` placeholder, but
    it would require us to add one more template file.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

GIT-VERSION-GEN

@@ -19,6 +19,11 @@ then
 	exit 1
 fi
 
+if test -n "$OUTPUT" && test -n "$MESON_DIST_ROOT"
+then
+    OUTPUT="$MESON_DIST_ROOT/$OUTPUT"
+fi
+
 DEF_VER=$(cat "$SOURCE_DIR"/GIT-VERSION)
 
 # Protect us from reading Git version information outside of the Git directory
@@ -33,7 +38,7 @@ then
 	# then try git-describe, then default.
 	if test -f "$SOURCE_DIR"/version
 	then
-		VN=$(cat "$SOURCE_DIR"/version) || VN="$DEF_VER"
+		VN=$(cat "$SOURCE_DIR"/version) && VN=${VN#GIT_VERSION=} || VN="$DEF_VER"
 	elif {
 			test -d "$SOURCE_DIR/.git" ||
 			test -d "${GIT_DIR:-.git}" ||

meson.build

@@ -1944,6 +1944,18 @@ devenv.set('GIT_BUILD_DIR', meson.current_build_dir())
 devenv.prepend('PATH', meson.current_build_dir() / 'bin-wrappers')
 meson.add_devenv(devenv)
 
+# Generate the 'version' file in the distribution tarball. This is used via
+# `meson dist -C <builddir>` to populate the source archive with the Git
+# version that the archive is being generated from. GIT-VERSION-GEN knows to
+# pick up this file.
+meson.add_dist_script(
+  shell,
+  meson.current_source_dir() / 'GIT-VERSION-GEN',
+  meson.current_source_dir(),
+  meson.current_source_dir() / 'GIT-VERSION-FILE.in',
+  'version',
+)
+
 summary({
   'curl': curl.found(),
   'expat': expat.found(),