test: add tests for FIFO file content waiting logic

Add tests for _wait_for_file_content() to verify it correctly handles
both regular files and FIFOs. The existing FIFO test worked because it
started a writer thread before reading, ensuring the FIFO was ready.

However, with 1Password's asynchronously mounted FIFO files, the FIFO
may exist but not be immediately readable when first accessed. The old
code would open FIFOs directly in text mode, which could block
indefinitely waiting for a writer.

The new _wait_for_file_content() function addresses this by:
- Detecting FIFOs using stat.S_ISFIFO()
- Using select.select() to wait for the FIFO to become readable
- Reading content only when the FIFO is ready

These tests verify:
- Regular files continue to work correctly (no regression)
- FIFOs are properly handled with waiting logic for async-mounted files

Author: lukeocodes

tests/test_fifo_dotenv.py

@@ -6,6 +6,7 @@
 import pytest
 
 from dotenv import load_dotenv
+from dotenv.main import _wait_for_file_content
 
 pytestmark = pytest.mark.skipif(
     sys.platform.startswith("win"), reason="FIFOs are Unix-only"
@@ -31,3 +32,35 @@ def writer():
 
     assert ok is True
     assert os.getenv("MY_PASSWORD") == "pipe-secret"
+
+
+def test_wait_for_file_content_with_regular_file(tmp_path: pathlib.Path):
+    """Test that _wait_for_file_content handles regular files correctly."""
+    regular_file = tmp_path / ".env"
+    regular_file.write_text("KEY=value\n", encoding="utf-8")
+
+    stream = _wait_for_file_content(str(regular_file), encoding="utf-8")
+    content = stream.read()
+
+    assert content == "KEY=value\n"
+    stream.close()
+
+
+def test_wait_for_file_content_with_fifo(tmp_path: pathlib.Path):
+    """Test that _wait_for_file_content handles FIFOs correctly."""
+    fifo = tmp_path / ".env"
+    os.mkfifo(fifo)
+
+    def writer():
+        with open(fifo, "w", encoding="utf-8") as w:
+            w.write("FIFO_KEY=fifo-value\n")
+
+    t = threading.Thread(target=writer)
+    t.start()
+
+    stream = _wait_for_file_content(str(fifo), encoding="utf-8")
+    content = stream.read()
+
+    t.join(timeout=2)
+
+    assert content == "FIFO_KEY=fifo-value\n"