osbuild: move osrelease parser into ./util

Reduce the clutter in our pipeline implementation and move the
os-release parser into ./osbuild/util/.

osbuild/util/osrelease.py

@@ -0,0 +1,51 @@
+"""OS-Release Information
+
+This module implements handlers for the `/etc/os-release` type of files. The
+related documentation can be found in `os-release(5)`.
+"""
+
+import os
+
+
+def parse_files(*paths):
+    """Read Operating System Information from `os-release`
+
+    This creates a dictionary with information describing the running operating
+    system. It reads the information from the path array provided as `paths`.
+    The first available file takes precedence. It must be formatted according
+    to the rules in `os-release(5)`.
+    """
+    osrelease = {}
+
+    path = next((p for p in paths if os.path.exists(p)), None)
+    if path:
+        with open(path) as f:
+            for line in f:
+                line = line.strip()
+                if not line:
+                    continue
+                if line[0] == "#":
+                    continue
+                key, value = line.split("=", 1)
+                osrelease[key] = value.strip('"')
+
+    return osrelease
+
+
+def describe_os(*paths):
+    """Read the Operating System Description from `os-release`
+
+    This creates a string describing the running operating-system name and
+    version. It uses `parse_files()` underneath to acquire the requested
+    information.
+
+    The returned string uses the format `${ID}${VERSION_ID}` with all dots
+    stripped.
+    """
+    osrelease = parse_files(*paths)
+
+    # Fetch `ID` and `VERSION_ID`. Defaults are defined in `os-release(5)`.
+    osrelease_id = osrelease.get("ID", "linux")
+    osrelease_version_id = osrelease.get("VERSION_ID", "")
+
+    return osrelease_id + osrelease_version_id.replace(".", "")