fix I/O operation on closed file error with CliRunner and echo_via_pager

CHANGES.rst

@@ -11,6 +11,7 @@ Unreleased
 -   Shell completion of `Choice` `Enum` values produces a valid completion
     result. :issue:`3015`
 -   Fix empty byte-string handling in echo. :issue:`3487`
+-   Fix closed file error with `echo_via_pager`. :issue:`3449`
 
 
 Version 8.4.0

src/click/_termui_impl.py

@@ -606,15 +606,35 @@ def _tempfilepager(
         os.unlink(f.name)
 
 
+class _SkipClose:
+    def __init__(self, stream: t.IO[t.Any]) -> None:
+        self.stream = stream
+
+    def __getattr__(self, name: str) -> t.Any:
+        return getattr(self.stream, name)
+
+    @property
+    def buffer(self) -> t.BinaryIO:
+        return _SkipClose(self.stream.buffer)  # type: ignore[attr-defined, return-value]
+
+    def close(self) -> None:
+        pass
+
+
 @contextlib.contextmanager
 def _nullpager(
     stream: t.TextIO, color: bool | None = None
 ) -> t.Iterator[tuple[t.TextIO, str, bool]]:
-    """Simply print unformatted text.  This is the ultimate fallback."""
+    """Simply print unformatted text. This is the ultimate fallback. Don't close the
+    output stream in this case, since it's coming from elsewhere rather than our
+    internal helpers.
+    """
     encoding = get_best_encoding(stream)
+
     if color is None:
         color = False
-    yield stream, encoding, color
+
+    yield _SkipClose(stream), encoding, color  # type: ignore[misc]
 
 
 class Editor:

tests/test_termui.py

@@ -798,24 +798,18 @@ def test_get_pager_file_nullpager_wraps_textio_stream(
 def test_get_pager_file_nullpager_keeps_stringio_stream(monkeypatch):
     """The no-stdout fallback should keep a text-only stream and set ``.color``."""
 
-    created = []
-
-    def make_stringio():
-        stream = io.StringIO()
-        created.append(stream)
-        return stream
-
+    stream = io.StringIO()
     monkeypatch.setattr(sys, "stdout", None)
-    monkeypatch.setattr(click._termui_impl, "StringIO", make_stringio)
+    monkeypatch.setattr(click._termui_impl, "StringIO", lambda: stream)
     monkeypatch.setattr(click._termui_impl, "isatty", lambda _: False)
 
     styled_text = click.style("hello", fg="red")
 
     with click.get_pager_file(color=False) as pager:
-        assert pager is created[0]
         pager.write(styled_text)
 
-    assert created[0].getvalue() == styled_text
+    assert not stream.closed
+    assert stream.getvalue() == styled_text
 
 
 def test_get_pager_file_flushes_stream_on_exception(monkeypatch):

tests/test_testing.py

@@ -256,6 +256,17 @@ def cli():
     assert result.output == ""
 
 
+def test_with_echo_via_pager():
+    @click.command()
+    def cli():
+        click.echo_via_pager("Hello, Click!")
+
+    runner = CliRunner()
+    result = runner.invoke(cli)
+    assert not result.exception
+    assert result.output == "Hello, Click!\n"
+
+
 def test_exit_code_and_output_from_sys_exit():
     # See issue #362
     @click.command()