testsuite: add log control-char escaping test (KI-51/52)

Transfers files whose names carry C0 (0x1b) and C1 (0x9b) control bytes via
--log-file and checks the log contains no raw control bytes (only \#NNN
escapes).  RED before the fix: logit() writes the raw filename to the log
(CWE-117), and C1 controls slip through filtered_fwrite.  Skips if the fs
rejects control-char names.

Reported-by: Leonid Bugaev

testsuite/log-control-chars_test.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+# Regression for KI-51/52: the log file must escape control characters in a
+# (possibly attacker-controlled) filename, so an admin who cat's the log can't
+# have terminal escapes injected (CWE-117).  Covers both C0 (< 0x20) and the C1
+# range (0x80-0x9f, incl CSI 0x9b) on the log path.
+
+import os
+
+from rsyncfns import SCRATCHDIR, run_rsync, test_fail, test_skipped
+
+base = SCRATCHDIR / 'logctl'
+src = base / 'src'
+dst = base / 'dst'
+src.mkdir(parents=True, exist_ok=True)
+dst.mkdir(parents=True, exist_ok=True)
+log = base / 'rsync.log'
+
+srcb = os.fsencode(str(src))
+made = 0
+for raw in (b'c0_\x1b_esc', b'c1_\x9b_csi'):
+    try:
+        with open(srcb + b'/' + raw, 'wb') as fh:
+            fh.write(b'x')
+        made += 1
+    except OSError:
+        pass  # a filesystem that rejects control-char names (e.g. Cygwin)
+if made == 0:
+    test_skipped("filesystem rejects control-char filenames")
+
+run_rsync('-rv', f'--log-file={log}', f'{src}/', f'{dst}/')
+
+data = log.read_bytes()
+if b'\x1b' in data:
+    test_fail("raw C0 ESC (0x1b) byte left un-escaped in the log file")
+if b'\x9b' in data:
+    test_fail("raw C1 CSI (0x9b) byte left un-escaped in the log file")
+if b'\\#' not in data:
+    test_fail("expected escaped \\#NNN sequences in the log file, found none")
+
+print(f'log-control-chars: {made} control-char name(s) escaped in the log file')