testsuite: regression for short-checksum --append-verify s2length

Forces --checksum-choice=xxh64 (an 8-byte transfer checksum) with a
corrupted-prefix --append-verify so the full-checksum redo path runs.
Before the generator capped s2length at MIN(SUM_LENGTH, xfer_sum_len)
this died with "Invalid checksum length 16 [sender]"; the test is RED on
the prior generator and GREEN with the cap.  Reproduces on any build that
has xxhash, so it guards the fix without an old-libxxhash host; skips when
xxh64 is absent (a build without xxhash).

diff --git a/testsuite/append-shortsum_test.py b/testsuite/append-shortsum_test.py
new file mode 100644 (file)
index 0000000..6fa3bdd
--- /dev/null
+++ b/testsuite/append-shortsum_test.py
@@ -0,0 +1,52 @@
+#!/usr/bin/env python3
+"""Regression: a short transfer checksum must not over-state the block s2length.
+
+A full-checksum (--append-verify redo) pass computes the strong block sum length
+(s2length).  The generator used to cap it at SUM_LENGTH (16), the legacy MD4/MD5
+digest size, regardless of the negotiated algorithm.  Since the sum2 array holds
+xfer_sum_len-byte elements and the sender rejects an s2length larger than
+xfer_sum_len, a sub-16-byte transfer checksum -- xxh64 (8 bytes), which is what
+rsync negotiates when the build's libxxhash lacks xxh128/xxh3 (e.g. Ubuntu
+20.04) -- made the sender die with "Invalid checksum length 16 [sender]"
+(protocol incompatibility, code 2).
+
+Forcing --checksum-choice=xxh64 reproduces it on any build that has xxhash, so
+this guards the fix without needing an old-libxxhash host.  Skipped where xxh64
+is unavailable (a build without xxhash).
+"""
+
+import json
+
+from rsyncfns import (
+    FROMDIR, TODIR, assert_same, make_data_file, rmtree, run_rsync,
+    test_skipped,
+)
+
+vv = json.loads(run_rsync('-VV', check=True, capture_output=True).stdout)
+if 'xxh64' not in vv.get('checksum_list', []):
+    test_skipped("xxh64 not in this build's checksum list (no xxhash)")
+
+src, dst = FROMDIR, TODIR
+rmtree(src)
+rmtree(dst)
+src.mkdir(parents=True)
+dst.mkdir(parents=True)
+
+# Source longer than the destination so --append has bytes to add; the dest is a
+# *corrupted* prefix so --append-verify's whole-file check fails and the file is
+# redone with a full checksum -- the csum_length == SUM_LENGTH path that emitted
+# the over-long s2length.
+make_data_file(src / 'f', 40000)
+full = (src / 'f').read_bytes()
+prefix = bytearray(full[:20000])
+prefix[0:64] = b'\x00' * 64
+(dst / 'f').write_bytes(bytes(prefix))
+
+# --no-whole-file forces the delta/checksum path regardless of local-vs-remote.
+# run_rsync(check=True) fails the test on the non-zero exit the bug produced.
+run_rsync('-a', '--append-verify', '--checksum-choice=xxh64', '--no-whole-file',
+          f'{src}/', f'{dst}/')
+assert_same(dst / 'f', src / 'f', label='append-verify xxh64 redo')
+
+print("append-shortsum: --append-verify with an 8-byte (xxh64) checksum no "
+      "longer overflows the block s2length")