avoid failing on standard streams that are None. (#10041)

See https://docs.python.org/3/library/sys.html#sys.__stdin__.

> [!Note]
> Under some conditions stdin, stdout and stderr as well as the original values __stdin__, __stdout__ and __stderr__ can be None. It is usually the case for Windows GUI apps that aren’t connected to a console and Python apps started with pythonw.

This is a best-effort solution. External libraries (eg: `tqdm`) and future code that is unaware of this stuff may still fail.
As a console application, `stdout`/`stderr` being `None` is not really something that we should support in general.
Regarding the failure of the daemon, it has been fixed so that it'll always have a stream attached (if not, it falls back to
/dev/null). Today, this is a problem in daemon only if the main process is run with no streams attached, which is not
something that we support or go on length to support.

It is tricky to write tests for this case, and I could not come up with one. Also decided not to test daemon as it should no longer be a problem.

dvc/cli/__init__.py

@@ -165,9 +165,9 @@ def main(argv=None):  # noqa: C901, PLR0912, PLR0915
     # the copied parent's standard file descriptors. If we make any logging
     # calls in this state it will cause an exception due to writing to a closed
     # file descriptor.
-    if sys.stderr.closed:  # pylint: disable=using-constant-test
+    if not sys.stderr or sys.stderr.closed:  # pylint: disable=using-constant-test
         logging.disable()
-    elif sys.stdout.closed:  # pylint: disable=using-constant-test
+    elif not sys.stdout or sys.stdout.closed:  # pylint: disable=using-constant-test
         logging.disable(logging.INFO)
 
     args = None
@@ -199,7 +199,7 @@ def main(argv=None):  # noqa: C901, PLR0912, PLR0915
 
         logger.trace(args)  # type: ignore[attr-defined]
 
-        if not sys.stdout.closed and not args.quiet:
+        if sys.stdout and not sys.stdout.closed and not args.quiet:
             from dvc.ui import ui
 
             ui.enable()

dvc/daemon.py

@@ -129,12 +129,12 @@ def _spawn_posix(
 
     # disconnect from the terminal
     fd = os.open(os.devnull, os.O_RDWR)
-    os.dup2(fd, sys.stdin.fileno())
+    os.dup2(fd, 0)
     os.close(fd)
 
     with open(output_file or os.devnull, "ab") as f:
-        os.dup2(f.fileno(), sys.stdout.fileno())
-        os.dup2(f.fileno(), sys.stderr.fileno())
+        os.dup2(f.fileno(), 1)
+        os.dup2(f.fileno(), 2)
 
     if platform.system() == "Darwin":
         # workaround for MacOS bug

dvc/logger.py

@@ -182,9 +182,11 @@ def set_loggers_level(level: int = logging.INFO) -> None:
 
 
 def setup(level: int = logging.INFO, log_colors: bool = True) -> None:
+    from dvc.utils import isatty
+
     colorama.init()
 
-    formatter = ColorFormatter(log_colors=log_colors and sys.stdout.isatty())
+    formatter = ColorFormatter(log_colors=log_colors and isatty(sys.stdout))
 
     console_info = LoggerHandler(sys.stdout)
     console_info.setLevel(logging.INFO)
@@ -205,7 +207,7 @@ def setup(level: int = logging.INFO, log_colors: bool = True) -> None:
 
     show_traceback = bool(os.environ.get(DVC_SHOW_TRACEBACK))
     err_formatter = ColorFormatter(
-        log_colors=log_colors and sys.stderr.isatty(), show_traceback=show_traceback
+        log_colors=log_colors and isatty(sys.stderr), show_traceback=show_traceback
     )
     console_errors = LoggerHandler(sys.stderr)
     console_errors.setLevel(logging.WARNING)

dvc/prompt.py

@@ -1,7 +1,6 @@
 """Manages user prompts."""
 
 import logging
-import sys
 from getpass import getpass
 from typing import Collection, Optional
 
@@ -9,7 +8,9 @@ logger = logging.getLogger(__name__)
 
 
 def ask(prompt: str, limited_to: Optional[Collection[str]] = None):
-    if not sys.stdout.isatty():
+    from dvc.ui import Console
+
+    if not Console.isatty():
         return None
 
     while True:

dvc/ui/__init__.py

@@ -340,10 +340,13 @@ class Console:
     def status(self, status: str, **kwargs: Any) -> "Status":
         return self.error_console.status(status, **kwargs)
 
-    def isatty(self) -> bool:
+    @staticmethod
+    def isatty() -> bool:
         import sys
 
-        return sys.stdout.isatty()
+        from dvc import utils
+
+        return utils.isatty(sys.stdout)
 
     def open_browser(self, file: "StrPath") -> int:
         import webbrowser

dvc/ui/pager.py

@@ -3,7 +3,6 @@
 import logging
 import os
 import pydoc
-import sys
 
 from rich.pager import Pager
 
@@ -51,7 +50,9 @@ def make_pager(cmd=None):
 
 
 def find_pager():
-    if not sys.stdout.isatty():
+    from . import Console
+
+    if not Console.isatty():
         return None
 
     # pylint: disable=redefined-outer-name

dvc/updater.py

@@ -1,6 +1,5 @@
 import logging
 import os
-import sys
 import time
 from typing import TYPE_CHECKING, Optional
 
@@ -123,7 +122,7 @@ class Updater:
     def _notify(self, latest: str, pkg: Optional[str] = PKG) -> None:
         from dvc.ui import ui
 
-        if not sys.stdout.isatty():
+        if not ui.isatty():
             return
 
         message = self._get_message(latest, pkg=pkg)

dvc/utils/__init__.py

@@ -11,6 +11,8 @@ from typing import TYPE_CHECKING, Dict, Iterable, Iterator, List, Optional, Tupl
 import colorama
 
 if TYPE_CHECKING:
+    from typing import TextIO
+
     from dvc.fs import FileSystem
 
 logger = logging.getLogger(__name__)
@@ -424,3 +426,9 @@ def expand_paths(fs: "FileSystem", paths: Iterable[str]) -> Iterator[str]:
             yield from fs.find(path)
         else:
             yield path
+
+
+def isatty(stream: "Optional[TextIO]") -> bool:
+    if stream is None:
+        return False
+    return stream.isatty()