gh-157538: Read getpass() input in non-canonical mode

[nouraellm]

unix_getpass() disables ECHO but leaves the tty in canonical mode and calls readline(). A long pasted secret with no newline (a JWT is the usual case) can fill the kernel input queue (MAX_INPUT is 1024 on Darwin). Terminal.app writes the paste synchronously, so the UI wedges until the process is killed.

Always switch the tty to non-canonical mode (VMIN=1, VTIME=0) and read byte-by-byte. Reuse the existing line editor so ERASE / KILL / INTR / EOF keep working when echo is off; only the echo_char drawing is skipped. readline() remains the fallback when the tty cannot be controlled.

Fixes #157538

• Issue: In mac, terminal freezes when taking JWT token as getpass input #157538

[nouraellm]

@picnixz feedback plz on why?

[picnixz]

1. Someone already suggested to work on that. It is rude tonjust open a pr without asking.
2. this needs more investigation and I am not entirely sure it is a specific issue as the issue already exists in 3.13. We should look at what changed there first. Line editor a new featurenin 3.15 iirc (or maybe 3.14).
3. There are unrelated changes

[nouraellm]

@picnixz

1. I started working on it before someone suggested taking it :)
2. It's rude to make assumptions and close a PR without feedback
3. Could've been better if you requested clarifications or the option to peer-fix the issue :)

[picnixz]

You should have posted on the issue first then, to mention that you worked on it. At least looking at the thread before. And I am still unsure about this fix. Switching to noncanonical had issues in the past (and thus I also wonder whether the problem is this specific change). So first we should diagnose the issue.

[nouraellm]

@picnixz well this is some useful feedback. Thanks, I'll leave it to Adarsh-Me. Have a great day!

[nouraellm]

@picnixz As previously mentioned in the issue I was able to reproduce, so we move past that.

As per your request I ran an even deeper investigation into the history of the module not just a repro of the hang, so here's what we are working with:

• Unix getpass has only cleared ECHO for a long time
• Canonical mode and ISIG were left on so the kernel would keep doing line editing (ERASE/KILL/WERASE) and job-control signals, so clearing ISIG as well (ECHO|ISIG) is what made Ctrl+C / Ctrl+Z dead and that was reverted in bpo-11236 also hg 154b323e0e7f (ECHO|ISIG → ECHO only)
• That is why the default path is still tcsetattr(ECHO off) + readline() please take a look at the current unix_getpass()
• Leaving canonical mode is also why echo_char hurt us and that's because 3.14 dropped ICANON so stars could appear per key (#77065 and #130496)
• Kernel shortcuts then stopped working and Ctrl+U ended up in the password #138577
• That limitation was documented first #138677 and 3.15 added _PasswordLineEditor and cleared IEXTEN so userspace owns editing #141597 which was noted in getpass docs
• In mac, terminal freezes when taking JWT token as getpass input #157538 is not that regression and it is reported on 3.13 on the old hidden-input path: echo_char is None, ICANON still set, readline() waiting for \n
• Darwin’s MAX_INPUT is 1024 termios(3) and Darwin tty
• A pasted JWT is longer and has no newline yet so the kernel queue fills
• Terminal.app writes the paste synchronously and the UI wedges
• Linux usually has a much larger queue which is why this looks Mac-only

The constraint from this whole history is the following:

• Do not drop ICANON unless userspace replaces the driver
• We already have that replacement for echo_char
• Reuse it when echo is off (draw nothing), set VMIN=1/VTIME=0, keep ISIG on, copy the cc array so the saved attrs are not mutated, and leave readline() as the fallback when tcsetattr fails
• Also keep the post-restore stream.flush() from bpo-7208.

Thus, in my opinion this does not need a new PR because this one is that exact change and strictly not a new tty design or a Darwin #ifdef.

Please let me know what you think and whether you'd like me to adjust the PR text or tests to spell the history out if useful.