util: add new checksum related utilities

Small convenience functions to work with checksums, specifically
working with files and their checksums.

osbuild/util/checksum.py

@@ -0,0 +1,50 @@
+"""Checksum Utilities
+
+Small convenience functions to work with checksums.
+"""
+import hashlib
+import os
+
+from .types import PathLike
+
+
+# How many bytes to read in one go. Taken from coreutils/gnulib
+BLOCKSIZE = 32768
+
+
+def hexdigest_file(path: PathLike, algorithm: str) -> str:
+    """Return the hexdigest of the file at `path` using `algorithm`
+
+    Will stream the contents of file to the hash `algorithm` and
+    return the hexdigest. If the specified `algorithm` is not
+    supported a `ValueError` will be raised.
+    """
+    hasher = hashlib.new(algorithm)
+
+    with open(path, "rb") as f:
+
+        os.posix_fadvise(f.fileno(), 0, 0, os.POSIX_FADV_SEQUENTIAL)
+
+        while True:
+            data = f.read(BLOCKSIZE)
+            if not data:
+                break
+
+            hasher.update(data)
+
+    return hasher.hexdigest()
+
+
+def verify_file(path: PathLike, checksum: str) -> bool:
+    """Hash the file and return if the specified `checksum` matches
+
+    Uses `hexdigest_file` to hash the contents of the file at
+    `path` and return if the hexdigest matches the one specified
+    in `checksum`, where `checksum` consist of the algorithm used
+    and the digest joined via `:`, e.g. `sha256:abcd...`.
+    """
+    algorithm, want = checksum.split(":", 1)
+
+    have = hexdigest_file(path, algorithm)
+
+    return have == want