testsuite: regression for #880 --mkpath --dry-run file-to-file

Covers both halves: a --mkpath file-to-file --dry-run must succeed and
match the real run (the #880 abort), and a plain file-to-file --dry-run
onto an existing differing destination must still itemize the real change
rather than report it as brand new.  Both compare "--dry-run -i" output
against the real run.

Co-authored-by: Stiliyan Tonev (Bark) <stiliyan21@gmail.com>

testsuite/file-to-file-mkpath-dry-run_test.py

@@ -0,0 +1,62 @@
+#!/usr/bin/env python3
+# Regression test for issue #880 (and the dry-run itemize regression that the
+# first proposed fix, PR #952, would have introduced).
+#
+# (1) Copying file-to-file with --mkpath and --dry-run used to abort with
+#     "change_dir#3 ... failed", because make_path() only *reports* (does not
+#     create) directories in a dry run, so the later chdir found no parent.
+#
+# (2) The fix must stay scoped to the missing-parent case: a plain
+#     file-to-file --dry-run onto an *existing*, differing destination must
+#     still itemize the real change, not report the file as brand new (PR #952
+#     bumped dry_run unconditionally, which broke this).
+#
+# In both cases a "--dry-run -i" must produce the same itemized output as the
+# real run.  Based on the test from PR #952 by Stiliyan Tonev.
+
+import os
+import subprocess
+
+from rsyncfns import SCRATCHDIR, makepath, rmtree, rsync_argv, test_fail
+
+
+def itemize(*args):
+    p = subprocess.run(rsync_argv('-ai', *args), capture_output=True, text=True)
+    return p.returncode, p.stdout + p.stderr
+
+
+# (1) --mkpath file-to-file: the dry run must succeed and match the real run.
+mk = SCRATCHDIR / 'mk'
+rmtree(mk)
+makepath(mk / 'from')
+(mk / 'from' / 'src').write_text("payload\n")
+
+drc, dry = itemize('--dry-run', '--mkpath',
+                   str(mk / 'from' / 'src'), str(mk / 'dndir' / 'dst'))
+rc, real = itemize('--mkpath', str(mk / 'from' / 'src'), str(mk / 'rdir' / 'dst'))
+if drc != 0:
+    print(dry)
+    test_fail("--mkpath file-to-file --dry-run failed (#880)")
+if not (mk / 'rdir' / 'dst').exists():
+    test_fail("--mkpath real run did not create the file")
+if dry.replace('dndir', 'X') != real.replace('rdir', 'X'):
+    test_fail(f"--mkpath dry-run output differs from the real run:\n"
+              f" dry : {dry!r}\n real: {real!r}")
+
+# (2) Plain file-to-file onto an existing, differing destination: the dry run
+# must itemize the same change as the real run (a/dst and b/dst share the
+# basename 'dst', so the itemized lines are directly comparable).
+ex = SCRATCHDIR / 'ex'
+rmtree(ex)
+makepath(ex / 'a')
+makepath(ex / 'b')
+(ex / 'src').write_text("brand new content\n")
+for d in ('a', 'b'):
+    (ex / d / 'dst').write_text("old\n")
+    os.utime(ex / d / 'dst', (0, 0))        # make size + mtime differ
+
+_, dry2 = itemize('--dry-run', str(ex / 'src'), str(ex / 'a' / 'dst'))
+_, real2 = itemize(str(ex / 'src'), str(ex / 'b' / 'dst'))
+if dry2 != real2:
+    test_fail(f"file-to-file --dry-run misreports an existing destination:\n"
+              f" dry : {dry2!r}\n real: {real2!r}")