Try to prevent sideeffects during macro expansion

Macro expansion can have sideeffects and modify other macros,
which becomes a problem when the spec file is processed line by line
to determine line validity. Even lines in false condition branches
are expanded, and if the expansion has sideeffects it can affect
further processing and condition evaluation.

Try to prevent that by temporarily overriding macros that can affect
other macros, making them noop during expansion.

Signed-off-by: Nikola Forró <nforro@redhat.com>

Author: nforro

specfile/macros.py

@@ -2,6 +2,7 @@
 # SPDX-License-Identifier: MIT
 
 import collections
+import functools
 import re
 from enum import IntEnum
 from typing import List, Optional
@@ -131,24 +132,36 @@ def dump(cls) -> List[Macro]:
         return cls._parse([line.decode() for line in stderr])
 
     @staticmethod
-    def expand(expression: str) -> str:
+    def expand(expression: str, safe: bool = True) -> str:
         """
         Expands an expression in the global context.
 
         Args:
             expression: Expression to expand.
+            safe: Whether to disable macro manipulation during expansion.
 
         Returns:
             Expanded expression.
 
         Raises:
             RPMException: If expansion error occurs.
         """
+        cleanups = []
+        if safe:
+            # override macros that can affect other macros
+            # this should avoid any side-effects caused by the expansion
+            for macro in ("global", "define", "undefine"):
+                rpm.addMacro(macro, "%{nil}")
+                cleanups.append(functools.partial(rpm.delMacro, macro))
         try:
-            with capture_stderr() as stderr:
-                return rpm.expandMacro(expression)
-        except rpm.error as e:
-            raise RPMException(stderr=stderr) from e
+            try:
+                with capture_stderr() as stderr:
+                    return rpm.expandMacro(expression)
+            except rpm.error as e:
+                raise RPMException(stderr=stderr) from e
+        finally:
+            while cleanups:
+                cleanups.pop(0)()
 
     @classmethod
     def remove(cls, macro: str) -> None:
@@ -169,7 +182,10 @@ def remove(cls, macro: str) -> None:
         while retry < MAX_REMOVAL_RETRIES:
             rpm.delMacro(macro)
             try:
-                if cls.expand(f"%{{{macro}}}") == f"%{{{macro.replace('%%', '%')}}}":
+                if (
+                    cls.expand(f"%{{{macro}}}", safe=False)
+                    == f"%{{{macro.replace('%%', '%')}}}"
+                ):
                     break
             except RPMException:
                 # the macro can't be expanded, but it still exists

tests/unit/test_macros.py

@@ -114,3 +114,20 @@ def test_macros_define():
 def test_macros_reinit():
     Macros.reinit(MacroLevel.BUILTIN)
     assert all(m.level == MacroLevel.BUILTIN for m in Macros.dump())
+
+
+@pytest.mark.xfail(
+    rpm.__version__ <= "4.16.1.3",
+    reason="rpm <= 4.16.1.3 treats builtin macros specially and overriding them has no effect",
+)
+def test_macros_sideeffects():
+    rpm.reloadConfig()
+    rpm.addMacro("with_feature", "1")
+    assert rpm.expandMacro("%with_feature") == "1"
+    Macros.expand("%{expand:%global with_feature 0}", safe=False)
+    assert rpm.expandMacro("%with_feature") == "0"
+    rpm.reloadConfig()
+    rpm.addMacro("with_feature", "1")
+    assert rpm.expandMacro("%with_feature") == "1"
+    Macros.expand("%{expand:%global with_feature 0}", safe=True)
+    assert rpm.expandMacro("%with_feature") == "1"