* YoutubeDL:validate_destination_filename(): Bugfix of the previous commit when -o contains a directory.

* test_YoutubeDL:test_prepare_outtmpl_and_filename(): Prevents bogus test failure.

test/test_YoutubeDL.py

@@ -742,6 +742,7 @@ def test(tmpl, expected, *, info=None, **params):
             params['outtmpl'] = tmpl
             ydl = FakeYDL(params)
             ydl._num_downloads = 1
+            ydl.validate_destination_filename = lambda *args: None
             self.assertEqual(ydl.validate_outtmpl(tmpl), None)
 
             out = ydl.evaluate_outtmpl(tmpl, info or self.outtmpl_info)

yt_dlp/YoutubeDL.py

@@ -12,6 +12,7 @@
 import locale
 import operator
 import os
+import pathlib
 import random
 import re
 import shutil
@@ -1481,27 +1482,39 @@ def validate_destination_filename(self, filename):
 any error is simply raised, independent of the cause.
 
 Without this, download would fail only after the entire file is downloaded."""
-        cwd = os.getcwd()
-        with tempfile.TemporaryDirectory() as d:
-            os.chdir(d)
-            try:
-                with open(filename, 'w') as f:
-                    f.close()
-            except OSError as e:
-                if (os.name == 'nt' and e.errno == 206) or (e.errno == errno.ENAMETOOLONG):
-                    # The first condition is for windows,
-                    # and the second for unix-ish systems.
-
         # An improvement idea:
         # by default, retry (exec yt-dlp itself) by
         # -o "%(id)s.%(ext)s" --write-info-json,
         # but respect the directory from --output of the original call.
+
+        with tempfile.TemporaryDirectory() as d:
+            try:
+                # To make sure it's confined under tmpdir
+                tmpfn = '.' + os.sep + os.path.splitdrive(filename)[1]
+
+                parentDirStr = os.sep + '..' + os.sep
+                # This may contain '../' so remove them.
+                while parentDirStr in tmpfn:
+                    tmpfn = tmpfn.replace(parentDirStr, os.sep)
+                tmpfn = os.path.join(d, tmpfn)
+                pathlib.Path(os.path.dirname(tmpfn)).mkdir(parents=True, exist_ok=True)
+                open(tmpfn, 'w').close()
+            except OSError as e:
+                if (os.name == 'nt' and e.errno == 206) or (os.name != 'nt' and e.errno == errno.ENAMETOOLONG):
+                    # For Win, 206 means filename length exceeds MAX_PATH.
+
+                    e.filename = filename
                     self.to_screen('''[Notice] The file name to be saved is too long, exceeding the OS limit.
 [Notice] Consider options --trim-filenames or -o (--output).''')
 
+                elif os.name == 'nt' and e.errno == 22:
+                    # Even when MAX_PATH is disabled, 255 chars is the limit, resulting in 22.
+                    # https://github.com/python/cpython/issues/126929#issuecomment-2483684861
+                    e.filename = filename
+                    self.to_screen(f'''[Notice] Attempt to create file {filename} resulted in Errno 22.
+This is often caused e.g. by too long filename or forbidden characters.''')
+
                 raise
-            finally:
-                os.chdir(cwd)
 
     def prepare_filename(self, info_dict, dir_type='', *, outtmpl=None, warn=False):
         """Generate the output filename"""