gh-122179: Fix hashlib.file_digest and non-blocking I/O

Lib/hashlib.py

@@ -231,6 +231,8 @@ def file_digest(fileobj, digest, /, *, _bufsize=2**18):
     view = memoryview(buf)
     while True:
         size = fileobj.readinto(buf)
+        if size is None:
+            raise BlockingIOError("I/O operation would block.")
         if size == 0:
             break  # EOF
         digestobj.update(view[:size])

Lib/test/test_hashlib.py

@@ -1185,6 +1185,15 @@ def test_file_digest(self):
             with open(os_helper.TESTFN, "wb") as f:
                 hashlib.file_digest(f, "sha256")
 
+        class NonBlocking:
+            def readinto(self, buf):
+                return None
+            def readable(self):
+                return True
+
+        with self.assertRaises(BlockingIOError):
+            hashlib.file_digest(NonBlocking(), hashlib.sha256)
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2024-07-23-17-08-41.gh-issue-122179.0jZm9h.rst

@@ -0,0 +1,3 @@
+:func:`hashlib.file_digest` now raises :exc:`BlockingIOError` when no data
+is available during non-blocking I/O. Before, it added spurious null bytes
+to the digest.