Basic REPL hangs when ioctls can't be written to TTY

[ambv]

This is related to #134466, but is regarding the basic REPL.

If you remove the ability to write ioctls to tty stdio, for example on macOS by doing

PYTHON_BASIC_REPL=1 sandbox-exec -p '(version 1) (import "system.sb") (allow process-exec) (allow file-read* (subpath "/"))' python3.13

then the REPL never shows the prompt and doesn't allow interaction, but hangs forever.

Linked PRs

• gh-139027: readline doesn't degrade gracefully when tcsetattr() fails. #139989

[yihong0618]

PYTHON_BASIC_REPL=1 sandbox-exec -p '(version 1) (import "system.sb") (allow process-exec) (allow file-read* (subpath "/"))' ./python.exe

not hang in mac main

[glyph]

@yihong0618 @ambv would a repro be helpful? not sure what versions of everything (most saliently, I guess, the operating system?) y'all are testing with

[ambv]

Well, I can reproduce the problem with the latest commit on main as of now (a15aeec). I'm running on M1 Max with macOS 15 Sequoia. I tested on WezTerm, iTerm, and Apple Terminal. I tested with Xonsh, Fish, Zsh, and Bash.

[glyph]

OK, sounds like it's pretty reliable then :). Perhaps yihong was on a branch for an older version or something.

[yihong0618]

my env
for default on mac

➜  cpython git:(main) sandbox-exec -p '(version 1) (import "system.sb") (allow process-exec) (allow file-read* (subpath "/"))' ./python.exe
Python 3.15.0a0 (heads/main:d6dd64ac65, Oct 12 2025, 12:06:06) [Clang 17.0.0 (clang-1700.0.13.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
warning: can't use pyrepl: termios failure (Operation not permitted)

for basic one

➜  cpython git:(main) PYTHON_BASIC_REPL=1 sandbox-exec -p '(version 1) (import "system.sb") (allow process-exec) (allow file-read* (subpath "/"))' ./python.exe
Python 3.15.0a0 (heads/main:d6dd64ac65, Oct 12 2025, 12:06:06) [Clang 17.0.0 (clang-1700.0.13.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

[yihong0618]

some digging cc @ambv @glyph

This is quite an interesting issue really like it

TLDR: its not succeed in readline

some debug process:

lldb to the hang pid:

➜  cpython git:(main) ✗ lldb -p 75093
(lldb) process attach --pid 75093
Process 75093 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP
    frame #0: 0x0000000197245fbc libsystem_kernel.dylib`__select + 8
libsystem_kernel.dylib`__select:
->  0x197245fbc <+8>:  b.lo   0x197245fdc    ; <+40>
    0x197245fc0 <+12>: pacibsp 
    0x197245fc4 <+16>: stp    x29, x30, [sp, #-0x10]!
    0x197245fc8 <+20>: mov    x29, sp
Target 0: (python.exe) stopped.
Executable binary set to "/Users/yihong/repos/cpython/python.exe".
Architecture set to: arm64-apple-macosx-.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP
  * frame #0: 0x0000000197245fbc libsystem_kernel.dylib`__select + 8
    frame #1: 0x0000000100a20b2c readline.cpython-315-darwin.so`call_readline [inlined] readline_until_enter_or_signal(prompt=<unavailable>, signal=<unavailable>) at readline.c:1477:25 [opt]
    frame #2: 0x0000000100a20a3c readline.cpython-315-darwin.so`call_readline(sys_stdin=<unavailable>, sys_stdout=<unavailable>, prompt=<unavailable>) at readline.c:1530:9 [opt]
    frame #3: 0x00000001001b6ab8 python.exe`PyOS_Readline(sys_stdin=0x0000000200bf3c08, sys_stdout=0x0000000200bf3ca0, prompt=">>> ") at myreadline.c:414:14 [opt]
    frame #4: 0x00000001001b43d0 python.exe`tok_underflow_interactive(tok=0x000000013f80a800) at file_tokenizer.c:198:20 [opt]
    frame #5: 0x00000001001b0b18 python.exe`tok_get_normal_mode [inlined] tok_nextc(tok=0x000000013f80a800) at lexer.c:75:14 [opt]
    frame #6: 0x00000001001b0af4 python.exe`tok_get_normal_mode(tok=0x000000013f80a800, current_tok=<unavailable>, token=<unavailable>) at lexer.c:521:17 [opt]
    frame #7: 0x00000001001b0668 python.exe`_PyTokenizer_Get [inlined] tok_get(tok=0x000000013f80a800, token=0x000000016fc7aba0) at lexer.c:0 [opt]
    frame #8: 0x00000001001afea0 python.exe`_PyTokenizer_Get(tok=0x000000013f80a800, token=0x000000016fc7aba0) at lexer.c:1626:18 [opt]
    frame #9: 0x0000000100184ffc python.exe`_PyPegen_fill_token(p=0x00000001009a7310) at pegen.c:0 [opt]
    frame #10: 0x000000010018b888 python.exe`_PyPegen_parse [inlined] statement_newline_rule(p=0x00000001009a7310) at parser.c:1347:31 [opt]
    frame #11: 0x000000010018b870 python.exe`_PyPegen_parse [inlined] interactive_rule(p=0x00000001009a7310) at parser.c:1049:18 [opt]
    frame #12: 0x000000010018b870 python.exe`_PyPegen_parse(p=0x00000001009a7310) at parser.c:38326:18 [opt]
    frame #13: 0x000000010018611c python.exe`_PyPegen_run_parser(p=0x00000001009a7310) at pegen.c:942:17 [opt]
    frame #14: 0x0000000100186500 python.exe`_PyPegen_run_parser_from_file_pointer(fp=<unavailable>, start_rule=<unavailable>, filename_ob=0x0000000100afcc30, enc=<unavailable>, ps1=<unavailable>, ps2="... ", flags=0x000000016fc7ae80, errcode=0x000000016fc7ad2c, interactive_src=0x000000016fc7ad20, arena=0x0000000100823d90) at pegen.c:1020:14 [opt]
    frame #15: 0x00000001001af9b8 python.exe`_PyParser_InteractiveASTFromFile(fp=<unavailable>, filename_ob=<unavailable>, enc=<unavailable>, mode=<unavailable>, ps1=<unavailable>, ps2=<unavailable>, flags=<unavailable>, errcode=<unavailable>, interactive_src=<no summary available>, arena=<no summary available>) at peg_api.c:38:12 [opt] [artificial]
    frame #16: 0x000000010038b310 python.exe`PyRun_InteractiveOneObjectEx [inlined] pyrun_one_parse_ast(fp=0x0000000200bf3c08, filename=0x0000000100afcc30, flags=0x000000016fc7ae80, arena=0x0000000100823d90, pmod=<unavailable>, interactive_src=0x000000016fc7ad20) at pythonrun.c:269:13 [opt]
    frame #17: 0x000000010038b134 python.exe`PyRun_InteractiveOneObjectEx(fp=0x0000000200bf3c08, filename=0x0000000100afcc30, flags=0x000000016fc7ae80) at pythonrun.c:300:21 [opt]
    frame #18: 0x000000010038aa28 python.exe`_PyRun_InteractiveLoopObject(fp=0x0000000200bf3c08, filename=0x0000000100afcc30, flags=0x000000016fc7ae80) at pythonrun.c:153:15 [opt]
    frame #19: 0x000000010038a780 python.exe`_PyRun_AnyFileObject(fp=0x0000000200bf3c08, filename=0x0000000100afcc30, closeit=0, flags=0x000000016fc7ae80) at pythonrun.c:75:15 [opt]
    frame #20: 0x000000010038b080 python.exe`PyRun_AnyFileExFlags(fp=0x0000000200bf3c08, filename=<unavailable>, closeit=0, flags=0x000000016fc7ae80) at pythonrun.c:102:15 [opt]
    frame #21: 0x00000001003b37a8 python.exe`pymain_run_stdin(config=0x00000001005ffca0) at main.c:570:19 [opt]
    frame #22: 0x00000001003b2d94 python.exe`Py_RunMain [inlined] pymain_run_python(exitcode=0x000000016fc7aee4) at main.c:697:21 [opt]
    frame #23: 0x00000001003b2954 python.exe`Py_RunMain at main.c:775:5 [opt]
    frame #24: 0x00000001003b3000 python.exe`pymain_main(args=0x000000016fc7b1e0) at main.c:805:12 [opt]
    frame #25: 0x00000001003b30a0 python.exe`Py_BytesMain(argc=<unavailable>, argv=<unavailable>) at main.c:829:12 [opt]
    frame #26: 0x0000000196efc274 dyld`start + 2840

• can check the frame1 frame #1: 0x0000000100a20b2c readline.cpython-315-darwin.socall_readline [inlined] readline_until_enter_or_signal(prompt=, signal=) at readline.c:1477:25 [opt]`
• but where is the readline in REPL?
• after some check its from site.py
• can we drop the import readline in site.py and avoid the hang? yes...it can

so the fix can be something like in site.py

    # Don't import readline if PYTHON_BASIC_REPL is set (gh-139027)
    if not PYTHON_BASIC_REPL:
        try:
            try:
                import readline
            except ImportError:
                readline = None
            else:
                import rlcompleter  # noqa: F401
        except ImportError:
            return

• but it will hang if use sand-box in Basic REPL then import readline
• try to find the real root cause
• its tcsetattr can not read the attr in sandbox for mac
• but why its about readline?

the process:
import readline
→ Module initialization
→ PyOS_ReadlineFunctionPointer = call_readline
→ setup_readline()
→ rl_initialize()
→ Internally calls tcsetattr() to set raw mode
→ tcsetattr() fails but readline doesn't check for the error

so when you try to go into this BASIC REPL

the process
→ call_readline()
→ readline_until_enter_or_signal()
→ Waits in a loop: while (completed_input_string == not_done_reading)
→ select() waits for data on stdin
→ But the terminal is still in canonical mode (because tcsetattr failed)
→ readline expects single-character input from raw mode
→ Mode mismatch → Deadlock??

the fix:

better in c level and can protect the future issue maybe?

diff --git a/Modules/readline.c b/Modules/readline.c
index 630a687999..06134231d3 100644
--- a/Modules/readline.c
+++ b/Modules/readline.c
@@ -17,6 +17,8 @@
 #include <signal.h>               // SIGWINCH
 #include <stdlib.h>               // free()
 #include <string.h>               // strdup()
+#include <termios.h>              // termios, tcgetattr(), tcsetattr()
+#include <unistd.h>               // isatty(), STDIN_FILENO
 #ifdef HAVE_SYS_SELECT_H
 #  include <sys/select.h>         // select()
 #endif
@@ -1645,6 +1647,23 @@ PyInit_readline(void)
     if (mod_state == NULL){
         goto error;
     }
+
+    /* gh-139027: Test if we can modify terminal settings.
+     * If tcsetattr fails (e.g., in sandboxed environments),
+     * don't initialize readline to avoid hangs.
+     */
+    if (isatty(STDIN_FILENO)) {
+        struct termios original_tty;
+        if (tcgetattr(STDIN_FILENO, &original_tty) == 0) {
+            struct termios test_tty = original_tty;
+            if (tcsetattr(STDIN_FILENO, TCSANOW, &test_tty) != 0) {
+                PyErr_SetString(PyExc_OSError,
+                    "termios failure (Operation not permitted)");
+                goto error;
+            }
+        }
+    }
+
     PyOS_ReadlineFunctionPointer = call_readline;
     if (setup_readline(mod_state) < 0) {
         PyErr_NoMemory();

after the fix