AI junk

CHANGES.md

@@ -14,6 +14,9 @@ Unreleased
   prompt when the output stream does not support them, matching `echo()`.
   This stripping was lost in `8.4.0` when {pr}`2969` began writing the
   prompt with `input()` directly. {issue}`3572` {pr}`3653`
+- Lazy read-mode {class}`File` parameters no longer eagerly open FIFO paths,
+  which could consume a writer's data before the command accesses the file.
+  {issue}`2645`
 - Fix test failures when using pytest >= 9.1. {pr}`3656`
 - {class}`Path` with `allow_dash=True` no longer triggers a `BytesWarning`,
   an error under `python -bb`, when checking a value against the `-`

src/click/utils.py

@@ -3,6 +3,7 @@
 import collections.abc as cabc
 import os
 import re
+import stat
 import sys
 import typing as t
 from functools import update_wrapper
@@ -146,10 +147,20 @@ def __init__(
         if self.name == "-":
             self._f, self.should_close = open_stream(filename, mode, encoding, errors)
         else:
+            is_fifo = False
+
             if "r" in mode:
+                try:
+                    is_fifo = stat.S_ISFIFO(os.stat(filename).st_mode)
+                except OSError:
+                    pass
+
+            if "r" in mode and not is_fifo:
                 # Open and close the file in case we're opening it for
                 # reading so that we can catch at least some errors in
-                # some cases early.
+                # some cases early. Do not do this for FIFOs because an
+                # eager read can consume the writer's data before the lazy
+                # file is accessed.
                 open(filename, mode).close()
             self._f = None
             self.should_close = True

tests/test_utils/test_LazyFile.py

@@ -1,3 +1,8 @@
+import builtins
+import os
+
+import pytest
+
 import click
 
 
@@ -9,3 +14,18 @@ def test_iter_lazyfile(tmpdir):
         with click.utils._LazyFile(f.name) as lf:
             for e_line, a_line in zip(expected, lf, strict=False):
                 assert e_line == a_line.strip()
+
+
+@pytest.mark.skipif(not hasattr(os, "mkfifo"), reason="FIFOs are not supported.")
+def test_lazyfile_does_not_eagerly_open_fifo(tmp_path, monkeypatch):
+    """Issue #2645: lazy read-mode files must not consume FIFO input early."""
+    path = tmp_path / "input"
+    os.mkfifo(path)
+
+    def unexpected_open(*args, **kwargs):
+        raise AssertionError("lazy FIFO setup should not open the file")
+
+    monkeypatch.setattr(builtins, "open", unexpected_open)
+    lazy_file = click.utils._LazyFile(path, "rb")
+
+    assert lazy_file._f is None