Set gpkg as BINPKG_FORMAT default

cnf/make.globals

@@ -38,9 +38,9 @@ PORTAGE_TMPDIR="/var/tmp"
 # existing installs where bzip2 is used for backward compatibility.
 BINPKG_COMPRESS="zstd"
 
-# The format used for binary packages. The default is use old "xpak" format.
-# Set to "gpkg" to use new gentoo binary package format.
-BINPKG_FORMAT="xpak"
+# The format used for binary packages. The default is to use the new "gpkg" format.
+# Set to "xpak" to use the old gentoo binary package format.
+BINPKG_FORMAT="gpkg"
 
 # The binary package default GPG signing command.
 # flock is used to avoid a racing condition of gnupg

lib/portage/_compat_upgrade/binpkg_format.py

@@ -0,0 +1,51 @@
+# Copyright 2020 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+import re
+
+import portage
+from portage import os
+from portage.const import GLOBAL_CONFIG_PATH
+
+COMPAT_BINPKG_FORMAT = "xpak"
+
+
+def main():
+    """
+    If the current installation is still configured to use the old
+    default BINPKG_FORMAT=xpak setting, then patch make.globals
+    inside ${ED} to maintain backward compatibility, ensuring that
+    binary package consumers are not caught off guard. This is
+    intended to be called from the ebuild as follows:
+
+    pkg_preinst() {
+            python_setup
+            env -u BINPKG_FORMAT
+                    PYTHONPATH="${D%/}$(python_get_sitedir)${PYTHONPATH:+:${PYTHONPATH}}" \
+                    "${PYTHON}" -m portage._compat_upgrade.binpkg_format || die
+    }
+    """
+    if (
+        portage.settings.get("BINPKG_FORMAT", COMPAT_BINPKG_FORMAT)
+        == COMPAT_BINPKG_FORMAT
+    ):
+        config_path = os.path.join(
+            os.environ["ED"], GLOBAL_CONFIG_PATH.lstrip(os.sep), "make.globals"
+        )
+        with open(config_path) as f:
+            content = f.read()
+            compat_setting = f'BINPKG_FORMAT="{COMPAT_BINPKG_FORMAT}"'
+            portage.output.EOutput().einfo(
+                "Setting make.globals default {} for backward compatibility".format(
+                    compat_setting
+                )
+            )
+            content = re.sub(
+                "^BINPKG_FORMAT=.*$", compat_setting, content, flags=re.MULTILINE
+            )
+        with open(config_path, "w") as f:
+            f.write(content)
+
+
+if __name__ == "__main__":
+    main()

lib/portage/_compat_upgrade/meson.build

@@ -1,6 +1,7 @@
 py.install_sources(
     [
         'binpkg_compression.py',
+	'binpkg_format.py',
         'binpkg_multi_instance.py',
         'default_locations.py',
         '__init__.py',