fix: say a pipe when output goes to a pipe

| tee log.txt was told its output went to a file. The line now names a pipe or a file, and says how to keep the run or answer.

install.py

@@ -9023,6 +9023,18 @@ def _is_terminal(stream: TextIO) -> bool:
         return False
 
 
+def _output_kind(stream: TextIO) -> str:
+    """"pipe" or "file" for what stream writes to, "" when neither is known.
+    `| tee log.txt` shows every line, `> log.txt` none of them."""
+    try:
+        mode = os.fstat(stream.fileno()).st_mode
+    except (OSError, ValueError, AttributeError, io.UnsupportedOperation):
+        return ""
+    if stat.S_ISFIFO(mode) or stat.S_ISSOCK(mode):
+        return "pipe"
+    return "file" if stat.S_ISREG(mode) else ""
+
+
 def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
     """One run of the installer, once the options are read."""
     KEPT.reset()
@@ -9035,8 +9047,12 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
         # to a prompt nobody could see.
         interactive = False
         if not listing and not args.check:
-            print("Output goes to a file, so nothing is asked: the run goes on as "
-                  "with --no-input.", file=sys.stderr)
+            kind = _output_kind(real_stdout)
+            where = f"Output goes to a {kind}" if kind else "Output is not a terminal"
+            undo = "the pipe" if kind == "pipe" else "the redirection"
+            print(f"{where}, so nothing is asked: the run goes on as with --no-input. "
+                  f"Add --no-input to say so, or run it without {undo} to answer.",
+                  file=sys.stderr)
     print(f"{term.bold('RetroBIOS')} {term.dim(installer_id())}\n")
 
     os_type = detect_os()

tests/test_install_experience.py

@@ -618,6 +618,40 @@ class OutputToAFileAsksNothing(unittest.TestCase):
         self.assertNotIn("Enter) install everything", proc.stdout)
         self.assertEqual(len(installed), len(FakeRepository.PAYLOADS))
 
+    @unittest.skipIf(os.name == "nt", "a pseudo-terminal for stdin")
+    def test_a_pipe_is_named_a_pipe(self):
+        """`install.py | tee log.txt` was told its output went to a file."""
+        import pty
+
+        master, slave = pty.openpty()
+        self.addCleanup(os.close, master)
+        self.addCleanup(os.close, slave)
+        with FakeRepository() as repo:
+            piped = subprocess.run(
+                [*repo.command(), "--dry-run"], env=repo.env(), stdin=slave,
+                capture_output=True, text=True, timeout=60, check=False)
+            log = repo.root / "log.txt"
+            with open(log, "w", encoding="utf-8") as handle:
+                filed = subprocess.run(
+                    [*repo.command(), "--dry-run"], env=repo.env(), stdin=slave,
+                    stdout=handle, stderr=subprocess.PIPE, text=True, timeout=60,
+                    check=False)
+        self.assertIn("Output goes to a pipe, so nothing is asked", piped.stderr)
+        self.assertIn("run it without the pipe to answer", piped.stderr)
+        self.assertIn("Output goes to a file, so nothing is asked", filed.stderr)
+        self.assertIn("run it without the redirection to answer", filed.stderr)
+        for proc in (piped, filed):
+            self.assertIn("Add --no-input to say so", proc.stderr)
+
+    def test_the_kind_of_output(self):
+        self.assertEqual(install._output_kind(io.StringIO()), "")
+        read, write = os.pipe()
+        with open(read, "rb") as reader, open(write, "w") as writer:
+            self.assertEqual(install._output_kind(writer), "pipe")
+            del reader
+        with tempfile.TemporaryFile("w", dir=REPO_ROOT / "tmp") as handle:
+            self.assertEqual(install._output_kind(handle), "file")
+
 
 class _Tty(io.StringIO):
     def isatty(self) -> bool: