Using tramp, I had a remote cpp file open in a window.

I pressed "C-c c", which invokes a function in my .emacs called
compile-hereish, which (in this case) invokes the compile command with
the remote directory containing the remote cpp.

Compilation begins as expected.

The cpp file contains a number of syntax errors. There is what I understand
to be a longstanding Tramp bug such that, if a lot of outputs (syntax
errors) are sent at high speed to the compile window, Tramp hangs.

In emacs 26.2, Ctrl-G (usually ctrl-g three times) would interrupt the hung
Tramp window, and indeed cause the errors to be displayed in the window as
best as Tramp is able.

In emacs 27.1, Ctrl-G does nothing in this "tramp-hung-while-compiling"
situation. I also tried ctrl-c ctrl-c, but that also does nothing. It
appears that the only way to kill the hung Tramp compile is to force-quit
emacs as a whole at the OS level.

Machine running emacs: MacBook Pro (2015), macos Big Sur v11.1. I am using
emacs 27.1 installed via brew-cask on macos.
On the Macbook: ssh -V
    OpenSSH_8.1p1, LibreSSL 2.7.3

Remote host for compile: Ubuntu Linux 20.04.1 LTS (GNU/Linux
5.4.0-54-generic x86_64)
On remote host: ssh -V
  OpenSSH_8.2p1 Ubuntu-4ubuntu0.1, OpenSSL 1.1.1f  31 Mar 2020
Also on remote host: gcc --version
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

If I revert back to emacs 26.2 on the Macbook, ctrl-G once again works as
expected in the event of a Tramp hang.

I am attaching to the remote host using Tramp-ssh mode, but I also tried
Tramp-scp and saw the same behaviour. My .emacs includes:
(setq tramp-default-method "ssh")
(setq tramp-chunksize 500)

The following cpp file (on Ubuntu machine) is sufficient to generate a
large number of syntax errors and hang Tramp, blocking ctrl-G (crashing
emacs) in emacs 27.1 for me.
// test.cpp
  DummyClass(
  DummyClass(
  DummyClass(
  DummyClass(
  DummyClass(
  DummyClass(
  DummyClass(
  DummyClass(
  DummyClass(
  DummyClass(
int validstuff();
int validstuff()
{
    return(0);
}
// end test.cpp

Thanks as always.
D.

Following was generated via: M-x report-emacs-bug

In GNU Emacs 27.1 (build 1, x86_64-apple-darwin18.7.0, NS appkit-1671.60
Version 10.14.6 (Build 18G95))
 of 2020-08-11 built on builder10-14.porkrind.org
Windowing system distributor 'Apple', version 10.3.2022
System Description:  macOS 11.1

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Package cl is deprecated
Mark set

Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
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Configured features:
NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES
THREADS JSON PDUMPER

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  global-undo-tree-mode: t
  undo-tree-mode: t
  global-auto-complete-mode: t
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  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
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  auto-compression-mode: t
  buffer-read-only: t
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Load-path shadows:
/Users/dgreatwood/.emacs.d/lisp/linum hides
/Applications/Emacs.app/Contents/Resources/lisp/linum

Features:
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Memory information:
((conses 16 70462 8546)
 (symbols 48 8556 1)
 (strings 32 22334 1877)
 (string-bytes 1 713421)
 (vectors 16 12259)
 (vector-slots 8 156248 17226)
 (floats 8 39 36)
 (intervals 56 248 0)
 (buffers 1000 14))