fix: ask nothing when the output goes to a file

install.py > log.txt at a terminal wrote the menu into the file and
waited for an answer nobody saw. A question now needs stdin and stdout on
a terminal, otherwise the run goes on as with --no-input and says so.

install.py

@@ -8933,6 +8933,14 @@ def _unknown_target(target: str, plat: str, targets: dict) -> None:
         print(f"  {plat} publishes no target list", file=sys.stderr)
 
 
+def _is_terminal(stream: TextIO) -> bool:
+    isatty = getattr(stream, "isatty", None)
+    try:
+        return bool(isatty and isatty())
+    except (OSError, ValueError):
+        return False
+
+
 def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
     """One run of the installer, once the options are read."""
     KEPT.reset()
@@ -8940,6 +8948,13 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
     interactive = sys.stdin.isatty() and not args.no_input and not args.quiet
     listing = (args.list_targets or args.list_systems or args.list_cores
                or args.list_emulators)
+    if interactive and not _is_terminal(real_stdout):
+        # A question goes to stdout: sent to a file, it waited on an answer
+        # 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)
     print(f"{term.bold('RetroBIOS')} {term.dim(installer_id())}\n")
 
     os_type = detect_os()

tests/test_install_experience.py

@@ -566,6 +566,30 @@ class AFolderThisUserCannotEnter(unittest.TestCase):
         self.assertEqual(state, "wrong_hash")
 
 
+class OutputToAFileAsksNothing(unittest.TestCase):
+    """`install.py > log.txt` at a terminal: the menu went into the file
+    and the run waited on an answer to a prompt nobody could see."""
+
+    @unittest.skipIf(os.name == "nt", "a pseudo-terminal for stdin")
+    def test_the_run_goes_on_as_with_no_input(self):
+        import pty
+
+        master, slave = pty.openpty()
+        try:
+            with FakeRepository() as repo:
+                proc = subprocess.run(
+                    repo.command(), env=repo.env(), stdin=slave, capture_output=True,
+                    text=True, timeout=60, check=False)
+                installed = repo.installed()
+        finally:
+            os.close(master)
+            os.close(slave)
+        self.assertEqual(proc.returncode, 0, proc.stdout + proc.stderr)
+        self.assertIn("nothing is asked", proc.stderr)
+        self.assertNotIn("Enter) install everything", proc.stdout)
+        self.assertEqual(len(installed), len(FakeRepository.PAYLOADS))
+
+
 class _Tty(io.StringIO):
     def isatty(self) -> bool:
         return True

wiki/installer.md

@@ -701,7 +701,7 @@ what the options name, everything by default.
 | <span id="option-list-cores"></span>`--list-cores` | Print the cores whose extra files a platform installs |
 | <span id="option-list-emulators"></span>`--list-emulators` | Print every name `--emulator` accepts, each buildbot name beside the emulator it stands for |
 | <span id="option-romm-firmware"></span>`--romm-firmware TEMPLATE` | RomM: where each platform's firmware goes in the library, as `filesystem.structure.firmware` in RomM's `config.yml` says (`{platform}/bios` for a library laid out as `{platform}/roms`). Read from `config.yml` beside the library when it is there. See [RomM firmware layout](#romm-firmware-layout) |
-| <span id="option-no-input"></span>`--no-input` | Never ask a question, even at a terminal |
+| <span id="option-no-input"></span>`--no-input` | Never ask a question, even at a terminal. A run whose output goes to a file or a pipe asks nothing either, and says so on stderr |
 | <span id="option-check"></span>`--check` | Report and exit without writing |
 | <span id="option-dry-run"></span>`--dry-run` | Show what would be downloaded, the totals first, then by system and with sizes, without writing anything. A plan taller than the screen names the ten largest systems, and `--verbose` lists every system and file |
 | <span id="option-yes"></span>`--yes`, `-y` | Do not ask before a download of 1 GB or more |