Honors /usr/lib/os-release when /etc/os-release is not available

README.md

@@ -86,7 +86,7 @@ The `distro` package implements a robust and inclusive way of retrieving the
 information about a distribution based on new standards and old methods,
 namely from these data sources (from high to low precedence):
 
-* The os-release file `/etc/os-release`, if present.
+* The os-release file `/etc/os-release` if present, with a fall-back on `/usr/lib/os-release` if needed.
 * The output of the `lsb_release` command, if available.
 * The distro release file (`/etc/*(-|_)(release|version)`), if present.
 * The `uname` command for BSD based distrubtions.

distro.py

@@ -70,6 +70,7 @@
 
 
 _UNIXCONFDIR = os.environ.get("UNIXCONFDIR", "/etc")
+_UNIXUSRLIBDIR = os.environ.get("UNIXUSRLIBDIR", "/usr/lib")
 _OS_RELEASE_BASENAME = "os-release"
 
 #: Translation table for normalizing the "ID" attribute defined in os-release
@@ -714,9 +715,27 @@ def __init__(
         """
         self.root_dir = root_dir
         self.etc_dir = os.path.join(root_dir, "etc") if root_dir else _UNIXCONFDIR
-        self.os_release_file = os_release_file or os.path.join(
-            self.etc_dir, _OS_RELEASE_BASENAME
+        self.usr_lib_dir = (
+            os.path.join(root_dir, "usr/lib") if root_dir else _UNIXUSRLIBDIR
         )
+
+        if os_release_file:
+            self.os_release_file = os_release_file
+        else:
+            etc_dir_os_release_file = os.path.join(self.etc_dir, _OS_RELEASE_BASENAME)
+            usr_lib_os_release_file = os.path.join(
+                self.usr_lib_dir, _OS_RELEASE_BASENAME
+            )
+
+            # NOTE: The idea is to respect order **and** have it set
+            #       at all times for API backwards compatibility.
+            if os.path.isfile(etc_dir_os_release_file) or not os.path.isfile(
+                usr_lib_os_release_file
+            ):
+                self.os_release_file = etc_dir_os_release_file
+            else:
+                self.os_release_file = usr_lib_os_release_file
+
         self.distro_release_file = distro_release_file or ""  # updated later
         self.include_lsb = include_lsb
         self.include_uname = include_uname

docs/index.rst

@@ -32,7 +32,7 @@ If you want to add test data for more distributions, please
 create an issue in the `distro issue tracker`_
 and provide the following information in the issue:
 
-* The content of the `/etc/os-release` file, if any.
+* The os-release file `/etc/os-release` if present, with a fall-back on `/usr/lib/os-release` if needed.
 * The file names and content of the `/etc/*release` and `/etc/*version` files, if any.
 * The output of the command: `lsb_release -a`, if available.
 * The file names and content of any other files you are aware of that provide
@@ -184,7 +184,8 @@ Os-release file
 ===============
 
 The os-release file is looked up using the path name ``/etc/os-release``. Its
-optional additional location ``/usr/lib/os-release`` is ignored.
+optional additional location ``/usr/lib/os-release`` would be looked up for if
+the former couldn't be read.
 
 The os-release file is expected to be encoded in UTF-8.
 

tests/resources/testdistros/distro/usrlibosreleaseonly/usr/lib/os-release

@@ -0,0 +1,3 @@
+NAME="usr_lib_os-release_only"
+ID=usrlibosreleaseonly
+PRETTY_NAME="/usr/lib/os-release only"

tests/test_distro.py

@@ -39,6 +39,7 @@
     import distro
 
     RELATIVE_UNIXCONFDIR = distro._UNIXCONFDIR[1:]
+    RELATIVE_UNIXUSRLIBDIR = distro._UNIXUSRLIBDIR[1:]
     MODULE_DISTRO = distro._distro
 
 
@@ -118,17 +119,20 @@ def setup_method(self, test_method):
         # changes it:
         self._saved_path = os.environ["PATH"]
         self._saved_UNIXCONFDIR = distro._UNIXCONFDIR
+        self._saved_UNIXUSRLIBDIR = distro._UNIXUSRLIBDIR
 
     def teardown_method(self, test_method):
         os.environ["PATH"] = self._saved_path
         distro._UNIXCONFDIR = self._saved_UNIXCONFDIR
+        distro._UNIXUSRLIBDIR = self._saved_UNIXUSRLIBDIR
 
     def _setup_for_distro(self, distro_root):
         distro_bin = os.path.join(distro_root, "bin")
         # We don't want to pick up a possibly present lsb_release in the
         # distro that runs this test, so we use a PATH with only one entry:
         os.environ["PATH"] = distro_bin
         distro._UNIXCONFDIR = os.path.join(distro_root, RELATIVE_UNIXCONFDIR)
+        distro._UNIXUSRLIBDIR = os.path.join(distro_root, RELATIVE_UNIXUSRLIBDIR)
 
 
 @pytest.mark.skipif(not IS_LINUX, reason="Irrelevant on non-linux")
@@ -708,6 +712,19 @@ def test_bad_uname(self):
         assert self.distro.uname_attr("name") == ""
         assert self.distro.uname_attr("release") == ""
 
+    def test_usrlibosreleaseonly(self):
+        self._setup_for_distro(
+            os.path.join(TESTDISTROS, "distro", "usrlibosreleaseonly")
+        )
+        self.distro = distro.LinuxDistribution()
+
+        desired_outcome = {
+            "id": "usrlibosreleaseonly",
+            "name": "usr_lib_os-release_only",
+            "pretty_name": "/usr/lib/os-release only",
+        }
+        self._test_outcome(desired_outcome)
+
 
 @pytest.mark.skipif(not IS_LINUX, reason="Irrelevant on non-linux")
 class TestDistroRelease:
@@ -2113,7 +2130,7 @@ def test_repr(self):
         repr_str = repr(distro._distro)
         assert "LinuxDistribution" in repr_str
         for attr in MODULE_DISTRO.__dict__.keys():
-            if attr in ("root_dir", "etc_dir"):
+            if attr in ("root_dir", "etc_dir", "usr_lib_dir"):
                 continue
             assert attr + "=" in repr_str