docs: Add a Meson version policy

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2596

docs/meson-version.md

@@ -0,0 +1,30 @@
+Meson version policy
+===
+
+Aims
+---
+
+ * Stable versions of GLib should be buildable from source on the majority of
+   systems which are still supported by their vendors, without requiring the
+   user to manually build a number of dependencies
+ * Unstable versions of GLib should be able to take advantage of newer build
+   system features where they would make maintenance of GLib easier, without
+   prejudicing the other aims
+
+Policy
+---
+
+ * Stable branches of GLib will not change their Meson dependency after the
+   first release of that stable series
+ * Unstable branches of GLib can bump their Meson dependency if
+   - at least that version of Meson currently available in Debian Testing; or
+   - the Python version required by the new Meson dependency is available in
+     Debian Stable *and* the oldest currently-supported Ubuntu LTS
+ * The version of Meson used by GLib should be pinned and pre-installed in the
+   CI `Dockerfile`s so that GLib is guaranteed to be built against the expected
+   version
+
+The reasoning behind allowing a version bump if the Python which Meson depends
+on is available in Debian Stable is that it’s [straightforward to install a more
+recent Meson version using
+`pip`](https://mesonbuild.com/Getting-meson.html#installing-meson-with-pip).

meson.build

@@ -1,6 +1,6 @@
 project('glib', 'c', 'cpp',
   version : '2.73.0',
-  # NOTE: We keep this pinned at 0.52 because that's what Debian Stable ships
+  # NOTE: See the policy in docs/meson-version.md before changing the Meson dependency
   meson_version : '>= 0.52.0',
   default_options : [
     'buildtype=debugoptimized',