Segfault on Linux when opening a terminal; stack overflow in pty_utils.isPasswordMode #780

Full transparency: I do not know much Zig, most of the tracing on this came from AI, but I have reproduced and verified the issue/fix herein by hand.

M-x ghostel segfaults Emacs on first render on Linux (aarch64, glibc 2.42,
Emacs 30.2, ghostel 0.60.0) with both the prebuilt release binary and
with a source build (zig 0.17.0, ReleaseFast), and with emacs -Q -nw. It does
not happen in --batch, or with a Debug build.

Cause: isPasswordMode in src/pty_utils.zig declares var tio: sys.termios. On
Linux, std.posix.system.termios is the kernel struct (36 bytes: 4 flag words,
line, 19 control chars). It then calls glibc's tcgetattr, which writes glibc's
struct termios (60 bytes: NCCS=32 plus speeds). That overflows the stack by 24
bytes on every call. ReleaseFast overwrites the saved return address (gdb showed a
jump to 0x960000000000). A ReleaseSafe build with symbols trips the stack
protector:

  #0 compiler_rt.ssp.__chk_fail
  #1 pty_utils.isPasswordMode
  #2 GhostelTerm.isPasswordMode  (GhostelTerm.zig:335)

PosixPtyProcess.zig is unaffected because it uses the C-imported termios.

I think something like the following should fix this (tested locally, working):

   --- a/src/pty_utils.zig
   +++ b/src/pty_utils.zig
   @@ -34,8 +34,15 @@
        if (sys.errno(fd) != .SUCCESS) return false;
        defer _ = sys.close(fd);

   -    var tio: sys.termios = undefined;
   -    const result = sys.tcgetattr(fd, &tio);
   +    var buf: extern struct {
   +        tio: sys.termios,
   +        _pad: [64]u8 = undefined,
   +    } = undefined;
   +    const result = sys.tcgetattr(fd, &buf.tio);
        if (sys.errno(result) != .SUCCESS) return false;
   -    return tio.lflag.ICANON and !tio.lflag.ECHO;
   +    return buf.tio.lflag.ICANON and !buf.tio.lflag.ECHO;
    }