Allow substitution of user-defined variables in RPM preamble

pkg/private/util.bzl

@@ -82,5 +82,14 @@ def substitute_package_variables(ctx, attribute_value):
 
     # Map $(var) to {x} and then use format for substitution.
     # This is brittle and I hate it. We should have template substitution
-    # in the Starlark runtime.
-    return attribute_value.replace("$(", "{").replace(")", "}").format(**vars)
+    # in the Starlark runtime.  This loop compensates for mismatched counts
+    # of $(foo) so that we don't try replace things like (bar) because we
+    # have no regex matching
+    for _ in range(attribute_value.count("$(")):
+        if attribute_value.find(")") == -1:
+            fail("mismatched variable declaration")
+
+        attribute_value = attribute_value.replace("$(", "{", 1)
+        attribute_value = attribute_value.replace(")", "}", 1)
+
+    return attribute_value.format(**vars)

pkg/rpm_pfg.bzl

@@ -33,7 +33,7 @@ load(
     "PackageSymlinkInfo",
     "PackageVariablesInfo",
 )
-load("//pkg/private:util.bzl", "setup_output_files")
+load("//pkg/private:util.bzl", "setup_output_files", "substitute_package_variables")
 
 rpm_filetype = [".rpm"]
 
@@ -349,7 +349,7 @@ def _pkg_rpm_impl(ctx):
     )
     ctx.actions.write(
         output = preamble_file,
-        content = "\n".join(preamble_pieces),
+        content = substitute_package_variables(ctx, "\n".join(preamble_pieces)),
     )
     files.append(preamble_file)
     args.append("--preamble=" + preamble_file.path)