add _forge_util.lua module to simplify macro lookups

We'll rely on this until we can drop support for RPM 4.16 and RHEL 9.
The function have similar behavior to the macros table in the way that
they handle undefined macros.

forge-srpm-macros.spec

@@ -61,6 +61,7 @@ export MACRO_LUA_DIR="%{buildroot}%{_rpmluadir}"
 %doc README.md NEWS.md
 %{_rpmmacrodir}/macros.forge
 %{_rpmluadir}/fedora/srpm/forge.lua
+%{_rpmluadir}/fedora/srpm/_forge_util.lua
 
 
 %changelog

rpm/lua/fedora/srpm/_forge_util.lua

@@ -0,0 +1,45 @@
+-- Utilities for evaluating macros without having to construct rpm.expand() statements.
+-- Uses the macros table on RPM >= 4.17.
+
+--- Return true if a macro is defined and false otherwise.
+local function is_defined(name)
+  if macros then
+    return macros[name] ~= nil
+  end
+  return rpm.expand("%{?" .. name .. ":1}") == "1"
+end
+
+--- Expand a macro. Return nil if the macro is undefined.
+local function get_macro(name)
+  if macros then
+    return macros[name]
+  end
+  if not is_defined(name) then
+    return nil
+  end
+  return rpm.expand("%{" .. name .. "}")
+end
+
+-- Get a flag. Pass true to has_arg if the flag accepts an argument.
+-- For flags with arguments, return the flag argument or nil.
+-- For flags without arguments, return a truthy value for defined or nil if the
+-- flag was not passed.
+local function get_flag(name, has_arg)
+  if opt then
+    return opt[name]
+  end
+  if not is_defined("-" .. name) then
+    return nil
+  end
+  local ender = "}"
+  if has_arg then
+    ender = "*" .. ender
+  end
+  return rpm.expand("%{-" .. name .. ender)
+end
+
+return {
+  is_defined = is_defined,
+  get_macro = get_macro,
+  get_flag = get_flag,
+}