tests: test_elf: fixed readelf exit code 1

Due to a recent code change in readelf, when no section was found,
readelf returned code 1 instead of the previous code 0. This behaviour
change caused diffoscope to not append details whenever a section wasn't
found, including in our own tests.

The modification was made in version 2.29 of binutils and – although was
quickly fixed in version 2.29.1 – leaves a version hole where diffoscope
might not work as expected without further information.

This patch adds a version checker for readelf where a few tests are
ignored if someone happens to be using binutils 2.29.

For more information on the change:
binutils/readelf.c:get_section_contents

Fixes #877728

Signed-off-by: Juliana Oliveira Rodrigues <juliana.orod@gmail.com>

tests/comparators/test_elf.py

@@ -19,6 +19,7 @@
 
 import pytest
 import os.path
+import subprocess
 
 from diffoscope.config import Config
 from diffoscope.comparators.elf import ElfFile, StaticLibFile
@@ -29,12 +30,22 @@ from diffoscope.comparators.utils.specialize import specialize
 
 from ..utils.data import data, load_fixture, get_data
 from ..utils.tools import skip_unless_tools_exist, \
-    skip_if_binutils_does_not_support_x86, skip_unless_module_exists
+    skip_if_binutils_does_not_support_x86, skip_unless_module_exists, \
+    skip_if_tool_version_is
+
 
 obj1 = load_fixture('test1.o')
 obj2 = load_fixture('test2.o')
 
 
+def readelf_version():
+    try:
+        out = subprocess.check_output(['readelf', '--version'])
+    except subprocess.CalledProcessError as e:
+        out = e.output
+    return out.decode('UTF-8').splitlines()[0].split()[-1].strip()
+
+
 def test_obj_identification(obj1):
     assert isinstance(obj1, ElfFile)
 
@@ -50,6 +61,7 @@ def obj_differences(obj1, obj2):
 
 
 @skip_unless_tools_exist('readelf')
+@skip_if_tool_version_is('readelf', readelf_version, '2.29')
 @skip_if_binutils_does_not_support_x86()
 def test_obj_compare_non_existing(monkeypatch, obj1):
     monkeypatch.setattr(Config(), 'new_file', True)
@@ -59,6 +71,7 @@ def test_obj_compare_non_existing(monkeypatch, obj1):
 
 
 @skip_unless_tools_exist('readelf')
+@skip_if_tool_version_is('readelf', readelf_version, '2.29')
 @skip_if_binutils_does_not_support_x86()
 def test_diff(obj_differences):
     assert len(obj_differences) == 1
@@ -95,6 +108,7 @@ def lib_differences(lib1, lib2):
 
 
 @skip_unless_tools_exist('readelf', 'objdump')
+@skip_if_tool_version_is('readelf', readelf_version, '2.29')
 @skip_if_binutils_does_not_support_x86()
 def test_lib_differences(lib_differences):
     assert len(lib_differences) == 2
@@ -107,6 +121,7 @@ def test_lib_differences(lib_differences):
 
 
 @skip_unless_tools_exist('readelf', 'objdump')
+@skip_if_tool_version_is('readelf', readelf_version, '2.29')
 @skip_if_binutils_does_not_support_x86()
 def test_lib_compare_non_existing(monkeypatch, lib1):
     monkeypatch.setattr(Config(), 'new_file', True)

tests/utils/tools.py

@@ -37,6 +37,15 @@ def skip_unless_tools_exist(*required):
         reason="requires {}".format(" and ".join(required)),
     )
 
+def skip_if_tool_version_is(tool, actual_ver, target_ver, vcls=LooseVersion):
+    if tools_missing(tool):
+        return pytest.mark.skipif(True, reason="requires {}".format(tool))
+    if callable(actual_ver):
+        actual_ver = actual_ver()
+    return pytest.mark.skipif(
+        vcls(str(actual_ver)) == vcls(str(target_ver)),
+        reason="requires {} != {} ({} detected)".format(tool, target_ver, actual_ver)
+    )
 
 def skip_unless_tool_is_at_least(tool, actual_ver, min_ver, vcls=LooseVersion):
     if tools_missing(tool):