`spack install` terminal output handling in foreground/background (#15723)

Makes the following changes:

* (Fixes #15620) tty configuration was failing when stdout was 
  redirected. The implementation now creates a pseudo terminal for
  stdin and checks stdout properly, so redirections of stdin/out/err
  should be handled now.
* Handles terminal configuration when the Spack process moves between
  the foreground and background (possibly multiple times) during a
  build.
* Spack adjusts terminal settings to allow users to to enable/disable
  build process output to the terminal using a "v" toggle, abnormal
  exit cases (like CTRL-C) could leave the terminal in an unusable
  state. This is addressed here with a special-case handler which
  restores terminal settings.

Significantly extend testing of process output logger:

* New PseudoShell object for setting up a master and child process
  and configuring file descriptor inheritance between the two
* Tests for "v" verbosity toggle making use of the added PseudoShell
  object
* Added `uniq` function which takes a list of elements and replaces
  any consecutive sequence of duplicate elements with a single
  instance (e.g. "112211" -> "121")

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

7 files changed, 1155 insertions, 224 deletions

diff --git a/lib/spack/llnl/util/filesystem.py b/lib/spack/llnl/util/filesystem.py
index b4bb23d444..bee6e1e43c 100644
--- a/lib/spack/llnl/util/filesystem.py
+++ b/lib/spack/llnl/util/filesystem.py
@@ -624,9 +624,9 @@ def replace_directory_transaction(directory_name, tmp_root=None):
     # Check the input is indeed a directory with absolute path.
     # Raise before anything is done to avoid moving the wrong directory
     assert os.path.isdir(directory_name), \
-        '"directory_name" must be a valid directory'
+        'Invalid directory: ' + directory_name
     assert os.path.isabs(directory_name), \
-        '"directory_name" must contain an absolute path'
+        '"directory_name" must contain an absolute path: ' + directory_name
 
     directory_basename = os.path.basename(directory_name)
 
diff --git a/lib/spack/llnl/util/lang.py b/lib/spack/llnl/util/lang.py
index 55407992a1..aacef6d3db 100644
--- a/lib/spack/llnl/util/lang.py
+++ b/lib/spack/llnl/util/lang.py
@@ -619,3 +619,28 @@ def load_module_from_file(module_name, module_path):
         import imp
         module = imp.load_source(module_name, module_path)
     return module
+
+
+def uniq(sequence):
+    """Remove strings of duplicate elements from a list.
+
+    This works like the command-line ``uniq`` tool.  It filters strings
+    of duplicate elements in a list. Adjacent matching elements are
+    merged into the first occurrence.
+
+    For example::
+
+        uniq([1, 1, 1, 1, 2, 2, 2, 3, 3]) == [1, 2, 3]
+        uniq([1, 1, 1, 1, 2, 2, 2, 1, 1]) == [1, 2, 1]
+
+    """
+    if not sequence:
+        return []
+
+    uniq_list = [sequence[0]]
+    last = sequence[0]
+    for element in sequence[1:]:
+        if element != last:
+            uniq_list.append(element)
+            last = element
+    return uniq_list
diff --git a/lib/spack/llnl/util/tty/log.py b/lib/spack/llnl/util/tty/log.py
index e608992642..76e07c0e08 100644
--- a/lib/spack/llnl/util/tty/log.py
+++ b/lib/spack/llnl/util/tty/log.py
@@ -7,6 +7,8 @@
 """
 from __future__ import unicode_literals
 
+import atexit
+import errno
 import multiprocessing
 import os
 import re
@@ -25,6 +27,7 @@ try:
 except ImportError:
     termios = None
 
+
 # Use this to strip escape sequences
 _escape = re.compile(r'\x1b[^m]*m|\x1b\[?1034h')
 
@@ -38,17 +41,22 @@ control = re.compile('(\x11\n|\x13\n)')
 
 
 @contextmanager
-def background_safe():
-    signal.signal(signal.SIGTTOU, signal.SIG_IGN)
-    yield
-    signal.signal(signal.SIGTTOU, signal.SIG_DFL)
+def ignore_signal(signum):
+    """Context manager to temporarily ignore a signal."""
+    old_handler = signal.signal(signum, signal.SIG_IGN)
+    try:
+        yield
+    finally:
+        signal.signal(signum, old_handler)
 
 
-def _is_background_tty():
-    """Return True iff this process is backgrounded and stdout is a tty"""
-    if sys.stdout.isatty():
-        return os.getpgrp() != os.tcgetpgrp(sys.stdout.fileno())
-    return False  # not writing to tty, not background
+def _is_background_tty(stream):
+    """True if the stream is a tty and calling process is in the background.
+    """
+    return (
+        stream.isatty() and
+        os.getpgrp() != os.tcgetpgrp(stream.fileno())
+    )
 
 
 def _strip(line):
@@ -56,27 +64,80 @@ def _strip(line):
     return _escape.sub('', line)
 
 
-class _keyboard_input(object):
+class keyboard_input(object):
     """Context manager to disable line editing and echoing.
 
     Use this with ``sys.stdin`` for keyboard input, e.g.::
 
-        with keyboard_input(sys.stdin):
-            r, w, x = select.select([sys.stdin], [], [])
-            # ... do something with keypresses ...
-
-    This disables canonical input so that keypresses are available on the
-    stream immediately. Typically standard input allows line editing,
-    which means keypresses won't be sent until the user hits return.
+        with keyboard_input(sys.stdin) as kb:
+            while True:
+                kb.check_fg_bg()
+                r, w, x = select.select([sys.stdin], [], [])
+                # ... do something with keypresses ...
+
+    The ``keyboard_input`` context manager disables canonical
+    (line-based) input and echoing, so that keypresses are available on
+    the stream immediately, and they are not printed to the
+    terminal. Typically, standard input is line-buffered, which means
+    keypresses won't be sent until the user hits return. In this mode, a
+    user can hit, e.g., 'v', and it will be read on the other end of the
+    pipe immediately but not printed.
+
+    The handler takes care to ensure that terminal changes only take
+    effect when the calling process is in the foreground. If the process
+    is backgrounded, canonical mode and echo are re-enabled. They are
+    disabled again when the calling process comes back to the foreground.
+
+    This context manager works through a single signal handler for
+    ``SIGTSTP``, along with a poolling routine called ``check_fg_bg()``.
+    Here are the relevant states, transitions, and POSIX signals::
+
+        [Running] -------- Ctrl-Z sends SIGTSTP ------------.
+        [ in FG ] <------- fg sends SIGCONT --------------. |
+           ^                                              | |
+           | fg (no signal)                               | |
+           |                                              | v
+        [Running] <------- bg sends SIGCONT ---------- [Stopped]
+        [ in BG ]                                      [ in BG ]
+
+    We handle all transitions exept for ``SIGTSTP`` generated by Ctrl-Z
+    by periodically calling ``check_fg_bg()``.  This routine notices if
+    we are in the background with canonical mode or echo disabled, or if
+    we are in the foreground without canonical disabled and echo enabled,
+    and it fixes the terminal settings in response.
+
+    ``check_fg_bg()`` works *except* for when the process is stopped with
+    ``SIGTSTP``.  We cannot rely on a periodic timer in this case, as it
+    may not rrun before the process stops.  We therefore restore terminal
+    settings in the ``SIGTSTP`` handler.
+
+    Additional notes:
+
+    * We mostly use polling here instead of a SIGARLM timer or a
+      thread. This is to avoid the complexities of many interrupts, which
+      seem to make system calls (like I/O) unreliable in older Python
+      versions (2.6 and 2.7).  See these issues for details:
+
+      1. https://www.python.org/dev/peps/pep-0475/
+      2. https://bugs.python.org/issue8354
+
+      There are essentially too many ways for asynchronous signals to go
+      wrong if we also have to support older Python versions, so we opt
+      not to use them.
+
+    * ``SIGSTOP`` can stop a process (in the foreground or background),
+      but it can't be caught. Because of this, we can't fix any terminal
+      settings on ``SIGSTOP``, and the terminal will be left with
+      ``ICANON`` and ``ECHO`` disabled until it is resumes execution.
+
+    * Technically, a process *could* be sent ``SIGTSTP`` while running in
+      the foreground, without the shell backgrounding that process. This
+      doesn't happen in practice, and we assume that ``SIGTSTP`` always
+      means that defaults should be restored.
+
+    * We rely on ``termios`` support.  Without it, or if the stream isn't
+      a TTY, ``keyboard_input`` has no effect.
 
-    It also disables echoing, so that keys pressed aren't printed to the
-    terminal.  So, the user can hit, e.g., 'v', and it's read on the
-    other end of the pipe immediately but not printed.
-
-    When the with block completes, prior TTY settings are restored.
-
-    Note: this depends on termios support.  If termios isn't available,
-    or if the stream isn't a TTY, this context manager has no effect.
     """
     def __init__(self, stream):
         """Create a context manager that will enable keyboard input on stream.
@@ -89,42 +150,97 @@ class _keyboard_input(object):
         """
         self.stream = stream
 
+    def _is_background(self):
+        """True iff calling process is in the background."""
+        return _is_background_tty(self.stream)
+
+    def _get_canon_echo_flags(self):
+        """Get current termios canonical and echo settings."""
+        cfg = termios.tcgetattr(self.stream)
+        return (
+            bool(cfg[3] & termios.ICANON),
+            bool(cfg[3] & termios.ECHO),
+        )
+
+    def _enable_keyboard_input(self):
+        """Disable canonical input and echoing on ``self.stream``."""
+        # "enable" input by disabling canonical mode and echo
+        new_cfg = termios.tcgetattr(self.stream)
+        new_cfg[3] &= ~termios.ICANON
+        new_cfg[3] &= ~termios.ECHO
+
+        # Apply new settings for terminal
+        with ignore_signal(signal.SIGTTOU):
+            termios.tcsetattr(self.stream, termios.TCSANOW, new_cfg)
+
+    def _restore_default_terminal_settings(self):
+        """Restore the original input configuration on ``self.stream``."""
+        # _restore_default_terminal_settings Can be called in foreground
+        # or background. When called in the background, tcsetattr triggers
+        # SIGTTOU, which we must ignore, or the process will be stopped.
+        with ignore_signal(signal.SIGTTOU):
+            termios.tcsetattr(self.stream, termios.TCSANOW, self.old_cfg)
+
+    def _tstp_handler(self, signum, frame):
+        self._restore_default_terminal_settings()
+        os.kill(os.getpid(), signal.SIGSTOP)
+
+    def check_fg_bg(self):
+        # old_cfg is set up in __enter__ and indicates that we have
+        # termios and a valid stream.
+        if not self.old_cfg:
+            return
+
+        # query terminal flags and fg/bg status
+        flags = self._get_canon_echo_flags()
+        bg = self._is_background()
+
+        # restore sanity if flags are amiss -- see diagram in class docs
+        if not bg and any(flags):    # fg, but input not enabled
+            self._enable_keyboard_input()
+        elif bg and not all(flags):  # bg, but input enabled
+            self._restore_default_terminal_settings()
+
     def __enter__(self):
-        """Enable immediate keypress input on stream.
+        """Enable immediate keypress input, while this process is foreground.
 
         If the stream is not a TTY or the system doesn't support termios,
         do nothing.
         """
         self.old_cfg = None
+        self.old_handlers = {}
 
         # Ignore all this if the input stream is not a tty.
         if not self.stream or not self.stream.isatty():
-            return
+            return self
 
-        # If this fails, self.old_cfg will remain None
-        if termios and not _is_background_tty():
-            # save old termios settings
-            old_cfg = termios.tcgetattr(self.stream)
+        if termios:
+            # save old termios settings to restore later
+            self.old_cfg = termios.tcgetattr(self.stream)
 
-            try:
-                # create new settings with canonical input and echo
-                # disabled, so keypresses are immediate & don't echo.
-                self.new_cfg = termios.tcgetattr(self.stream)
-                self.new_cfg[3] &= ~termios.ICANON
-                self.new_cfg[3] &= ~termios.ECHO
+            # Install a signal handler to disable/enable keyboard input
+            # when the process moves between foreground and background.
+            self.old_handlers[signal.SIGTSTP] = signal.signal(
+                signal.SIGTSTP, self._tstp_handler)
 
-                # Apply new settings for terminal
-                termios.tcsetattr(self.stream, termios.TCSADRAIN, self.new_cfg)
-                self.old_cfg = old_cfg
+            # add an atexit handler to ensure the terminal is restored
+            atexit.register(self._restore_default_terminal_settings)
 
-            except Exception:
-                pass  # some OS's do not support termios, so ignore
+            # enable keyboard input initially (if foreground)
+            if not self._is_background():
+                self._enable_keyboard_input()
+
+        return self
 
     def __exit__(self, exc_type, exception, traceback):
-        """If termios was avaialble, restore old settings."""
+        """If termios was available, restore old settings."""
         if self.old_cfg:
-            with background_safe():  # change it back even if backgrounded now
-                termios.tcsetattr(self.stream, termios.TCSADRAIN, self.old_cfg)
+            self._restore_default_terminal_settings()
+
+        # restore SIGSTP and SIGCONT handlers
+        if self.old_handlers:
+            for signum, old_handler in self.old_handlers.items():
+                signal.signal(signum, old_handler)
 
 
 class Unbuffered(object):
@@ -300,11 +416,11 @@ class log_output(object):
         self._saved_debug = tty._debug
 
         # OS-level pipe for redirecting output to logger
-        self.read_fd, self.write_fd = os.pipe()
+        read_fd, write_fd = os.pipe()
 
         # Multiprocessing pipe for communication back from the daemon
         # Currently only used to save echo value between uses
-        self.parent, self.child = multiprocessing.Pipe()
+        self.parent_pipe, child_pipe = multiprocessing.Pipe()
 
         # Sets a daemon that writes to file what it reads from a pipe
         try:
@@ -315,10 +431,15 @@ class log_output(object):
                 input_stream = None  # just don't forward input if this fails
 
             self.process = multiprocessing.Process(
-                target=self._writer_daemon, args=(input_stream,))
+                target=_writer_daemon,
+                args=(
+                    input_stream, read_fd, write_fd, self.echo, self.log_file,
+                    child_pipe
+                )
+            )
             self.process.daemon = True  # must set before start()
             self.process.start()
-            os.close(self.read_fd)  # close in the parent process
+            os.close(read_fd)  # close in the parent process
 
         finally:
             if input_stream:
@@ -340,9 +461,9 @@ class log_output(object):
             self._saved_stderr = os.dup(sys.stderr.fileno())
 
             # redirect to the pipe we created above
-            os.dup2(self.write_fd, sys.stdout.fileno())
-            os.dup2(self.write_fd, sys.stderr.fileno())
-            os.close(self.write_fd)
+            os.dup2(write_fd, sys.stdout.fileno())
+            os.dup2(write_fd, sys.stderr.fileno())
+            os.close(write_fd)
 
         else:
             # Handle I/O the Python way. This won't redirect lower-level
@@ -355,7 +476,7 @@ class log_output(object):
             self._saved_stderr = sys.stderr
 
             # create a file object for the pipe; redirect to it.
-            pipe_fd_out = os.fdopen(self.write_fd, 'w')
+            pipe_fd_out = os.fdopen(write_fd, 'w')
             sys.stdout = pipe_fd_out
             sys.stderr = pipe_fd_out
 
@@ -394,14 +515,14 @@ class log_output(object):
 
         # print log contents in parent if needed.
         if self.write_log_in_parent:
-            string = self.parent.recv()
+            string = self.parent_pipe.recv()
             self.file_like.write(string)
 
         if self.close_log_in_parent:
             self.log_file.close()
 
         # recover and store echo settings from the child before it dies
-        self.echo = self.parent.recv()
+        self.echo = self.parent_pipe.recv()
 
         # join the daemon process. The daemon will quit automatically
         # when the write pipe is closed; we just wait for it here.
@@ -426,90 +547,166 @@ class log_output(object):
         # exactly before and after the text we want to echo.
         sys.stdout.write(xon)
         sys.stdout.flush()
-        yield
-        sys.stdout.write(xoff)
-        sys.stdout.flush()
+        try:
+            yield
+        finally:
+            sys.stdout.write(xoff)
+            sys.stdout.flush()
+
+
+def _writer_daemon(stdin, read_fd, write_fd, echo, log_file, control_pipe):
+    """Daemon used by ``log_output`` to write to a log file and to ``stdout``.
+
+    The daemon receives output from the parent process and writes it both
+    to a log and, optionally, to ``stdout``.  The relationship looks like
+    this::
+
+        Terminal
+           |
+           |          +-------------------------+
+           |          | Parent Process          |
+           +--------> |   with log_output():    |
+           | stdin    |     ...                 |
+           |          +-------------------------+
+           |            ^             | write_fd (parent's redirected stdout)
+           |            | control     |
+           |            | pipe        |
+           |            |             v read_fd
+           |          +-------------------------+   stdout
+           |          | Writer daemon           |------------>
+           +--------> |   read from read_fd     |   log_file
+             stdin    |   write to out and log  |------------>
+                      +-------------------------+
+
+    Within the ``log_output`` handler, the parent's output is redirected
+    to a pipe from which the daemon reads.  The daemon writes each line
+    from the pipe to a log file and (optionally) to ``stdout``.  The user
+    can hit ``v`` to toggle output on ``stdout``.
+
+    In addition to the input and output file descriptors, the daemon
+    interacts with the parent via ``control_pipe``.  It reports whether
+    ``stdout`` was enabled or disabled when it finished and, if the
+    ``log_file`` is a ``StringIO`` object, then the daemon also sends the
+    logged output back to the parent as a string, to be written to the
+    ``StringIO`` in the parent. This is mainly for testing.
+
+    Arguments:
+        stdin (stream): input from the terminal
+        read_fd (int): pipe for reading from parent's redirected stdout
+        write_fd (int): parent's end of the pipe will write to (will be
+            immediately closed by the writer daemon)
+        echo (bool): initial echo setting -- controlled by user and
+            preserved across multiple writer daemons
+        log_file (file-like): file to log all output
+        control_pipe (Pipe): multiprocessing pipe on which to send control
+            information to the parent
 
-    def _writer_daemon(self, stdin):
-        """Daemon that writes output to the log file and stdout."""
-        # Use line buffering (3rd param = 1) since Python 3 has a bug
-        # that prevents unbuffered text I/O.
-        in_pipe = os.fdopen(self.read_fd, 'r', 1)
-        os.close(self.write_fd)
-
-        echo = self.echo        # initial echo setting, user-controllable
-        force_echo = False      # parent can force echo for certain output
-
-        # list of streams to select from
-        istreams = [in_pipe, stdin] if stdin else [in_pipe]
-
-        log_file = self.log_file
-
-        def handle_write(force_echo):
-            # Handle output from the with block process.
-            # If we arrive here it means that in_pipe was
-            # ready for reading : it should never happen that
-            # line is false-ish
-            line = in_pipe.readline()
-            if not line:
-                return (True, force_echo)  # break while loop
-
-            # find control characters and strip them.
-            controls = control.findall(line)
-            line = re.sub(control, '', line)
-
-            # Echo to stdout if requested or forced
-            if echo or force_echo:
-                try:
-                    if termios:
-                        conf = termios.tcgetattr(sys.stdout)
-                        tostop = conf[3] & termios.TOSTOP
-                    else:
-                        tostop = True
-                except Exception:
-                    tostop = True
-                if not (tostop and _is_background_tty()):
-                    sys.stdout.write(line)
-                    sys.stdout.flush()
-
-            # Stripped output to log file.
-            log_file.write(_strip(line))
-            log_file.flush()
-
-            if xon in controls:
-                force_echo = True
-            if xoff in controls:
-                force_echo = False
-            return (False, force_echo)
+    """
+    # Use line buffering (3rd param = 1) since Python 3 has a bug
+    # that prevents unbuffered text I/O.
+    in_pipe = os.fdopen(read_fd, 'r', 1)
+    os.close(write_fd)
 
-        try:
-            with _keyboard_input(stdin):
-                while True:
-                    # No need to set any timeout for select.select
-                    # Wait until a key press or an event on in_pipe.
-                    rlist, _, _ = select.select(istreams, [], [])
-                    # Allow user to toggle echo with 'v' key.
-                    # Currently ignores other chars.
-                    # only read stdin if we're in the foreground
-                    if stdin in rlist and not _is_background_tty():
-                        if stdin.read(1) == 'v':
-                            echo = not echo
-
-                    if in_pipe in rlist:
-                        br, fe = handle_write(force_echo)
-                        force_echo = fe
-                        if br:
-                            break
-
-        except BaseException:
-            tty.error("Exception occurred in writer daemon!")
-            traceback.print_exc()
+    # list of streams to select from
+    istreams = [in_pipe, stdin] if stdin else [in_pipe]
+    force_echo = False      # parent can force echo for certain output
+
+    try:
+        with keyboard_input(stdin) as kb:
+            while True:
+                # fix the terminal settings if we recently came to
+                # the foreground
+                kb.check_fg_bg()
+
+                # wait for input from any stream. use a coarse timeout to
+                # allow other checks while we wait for input
+                rlist, _, _ = _retry(select.select)(istreams, [], [], 1e-1)
+
+                # Allow user to toggle echo with 'v' key.
+                # Currently ignores other chars.
+                # only read stdin if we're in the foreground
+                if stdin in rlist and not _is_background_tty(stdin):
+                    # it's possible to be backgrounded between the above
+                    # check and the read, so we ignore SIGTTIN here.
+                    with ignore_signal(signal.SIGTTIN):
+                        try:
+                            if stdin.read(1) == 'v':
+                                echo = not echo
+                        except IOError as e:
+                            # If SIGTTIN is ignored, the system gives EIO
+                            # to let the caller know the read failed b/c it
+                            # was in the bg. Ignore that too.
+                            if e.errno != errno.EIO:
+                                raise
+
+                if in_pipe in rlist:
+                    # Handle output from the calling process.
+                    line = _retry(in_pipe.readline)()
+                    if not line:
+                        break
+
+                    # find control characters and strip them.
+                    controls = control.findall(line)
+                    line = control.sub('', line)
+
+                    # Echo to stdout if requested or forced.
+                    if echo or force_echo:
+                        sys.stdout.write(line)
+                        sys.stdout.flush()
+
+                    # Stripped output to log file.
+                    log_file.write(_strip(line))
+                    log_file.flush()
+
+                    if xon in controls:
+                        force_echo = True
+                    if xoff in controls:
+                        force_echo = False
+
+    except BaseException:
+        tty.error("Exception occurred in writer daemon!")
+        traceback.print_exc()
+
+    finally:
+        # send written data back to parent if we used a StringIO
+        if isinstance(log_file, StringIO):
+            control_pipe.send(log_file.getvalue())
+        log_file.close()
+
+    # send echo value back to the parent so it can be preserved.
+    control_pipe.send(echo)
 
-        finally:
-            # send written data back to parent if we used a StringIO
-            if self.write_log_in_parent:
-                self.child.send(log_file.getvalue())
-            log_file.close()
 
-        # send echo value back to the parent so it can be preserved.
-        self.child.send(echo)
+def _retry(function):
+    """Retry a call if errors indicating an interrupted system call occur.
+
+    Interrupted system calls return -1 and set ``errno`` to ``EINTR`` if
+    certain flags are not set.  Newer Pythons automatically retry them,
+    but older Pythons do not, so we need to retry the calls.
+
+    This function converts a call like this:
+
+        syscall(args)
+
+    and makes it retry by wrapping the function like this:
+
+        _retry(syscall)(args)
+
+    This is a private function because EINTR is unfortunately raised in
+    different ways from different functions, and we only handle the ones
+    relevant for this file.
+
+    """
+    def wrapped(*args, **kwargs):
+        while True:
+            try:
+                return function(*args, **kwargs)
+            except IOError as e:
+                if e.errno == errno.EINTR:
+                    continue
+                raise
+            except select.error as e:
+                if e.args[0] == errno.EINTR:
+                    continue
+                raise
+    return wrapped
diff --git a/lib/spack/llnl/util/tty/pty.py b/lib/spack/llnl/util/tty/pty.py
new file mode 100644
index 0000000000..ef5d40ea57
--- /dev/null
+++ b/lib/spack/llnl/util/tty/pty.py
@@ -0,0 +1,344 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+"""The pty module handles pseudo-terminals.
+
+Currently, the infrastructure here is only used to test llnl.util.tty.log.
+
+If this is used outside a testing environment, we will want to reconsider
+things like timeouts in ``ProcessController.wait()``, which are set to
+get tests done quickly, not to avoid high CPU usage.
+
+"""
+from __future__ import print_function
+
+import os
+import signal
+import multiprocessing
+import re
+import sys
+import termios
+import time
+import traceback
+
+import llnl.util.tty.log as log
+
+from spack.util.executable import which
+
+
+class ProcessController(object):
+    """Wrapper around some fundamental process control operations.
+
+    This allows one process to drive another similar to the way a shell
+    would, by sending signals and I/O.
+
+    """
+    def __init__(self, pid, master_fd,
+                 timeout=1, sleep_time=1e-1, debug=False):
+        """Create a controller to manipulate the process with id ``pid``
+
+        Args:
+            pid (int): id of process to control
+            master_fd (int): master file descriptor attached to pid's stdin
+            timeout (int): time in seconds for wait operations to time out
+                (default 1 second)
+            sleep_time (int): time to sleep after signals, to control the
+                signal rate of the controller (default 1e-1)
+            debug (bool): whether ``horizontal_line()`` and ``status()`` should
+                produce output when called (default False)
+
+        ``sleep_time`` allows the caller to insert delays after calls
+        that signal or modify the controlled process. Python behaves very
+        poorly if signals arrive too fast, and drowning a Python process
+        with a Python handler with signals can kill the process and hang
+        our tests, so we throttle this a closer-to-interactive rate.
+
+        """
+        self.pid = pid
+        self.pgid = os.getpgid(pid)
+        self.master_fd = master_fd
+        self.timeout = timeout
+        self.sleep_time = sleep_time
+        self.debug = debug
+
+        # we need the ps command to wait for process statuses
+        self.ps = which("ps", required=True)
+
+    def get_canon_echo_attrs(self):
+        """Get echo and canon attributes of the terminal of master_fd."""
+        cfg = termios.tcgetattr(self.master_fd)
+        return (
+            bool(cfg[3] & termios.ICANON),
+            bool(cfg[3] & termios.ECHO),
+        )
+
+    def horizontal_line(self, name):
+        """Labled horizontal line for debugging."""
+        if self.debug:
+            sys.stderr.write(
+                "------------------------------------------- %s\n" % name
+            )
+
+    def status(self):
+        """Print debug message with status info for the child."""
+        if self.debug:
+            canon, echo = self.get_canon_echo_attrs()
+            sys.stderr.write("canon: %s, echo: %s\n" % (
+                "on" if canon else "off",
+                "on" if echo else "off",
+            ))
+            sys.stderr.write("input: %s\n" % self.input_on())
+            sys.stderr.write("bg: %s\n" % self.background())
+            sys.stderr.write("\n")
+
+    def input_on(self):
+        """True if keyboard input is enabled on the master_fd pty."""
+        return self.get_canon_echo_attrs() == (False, False)
+
+    def background(self):
+        """True if pgid is in a background pgroup of master_fd's terminal."""
+        return self.pgid != os.tcgetpgrp(self.master_fd)
+
+    def tstp(self):
+        """Send SIGTSTP to the controlled process."""
+        self.horizontal_line("tstp")
+        os.killpg(self.pgid, signal.SIGTSTP)
+        time.sleep(self.sleep_time)
+
+    def cont(self):
+        self.horizontal_line("cont")
+        os.killpg(self.pgid, signal.SIGCONT)
+        time.sleep(self.sleep_time)
+
+    def fg(self):
+        self.horizontal_line("fg")
+        with log.ignore_signal(signal.SIGTTOU):
+            os.tcsetpgrp(self.master_fd, os.getpgid(self.pid))
+        time.sleep(self.sleep_time)
+
+    def bg(self):
+        self.horizontal_line("bg")
+        with log.ignore_signal(signal.SIGTTOU):
+            os.tcsetpgrp(self.master_fd, os.getpgrp())
+        time.sleep(self.sleep_time)
+
+    def write(self, byte_string):
+        self.horizontal_line("write '%s'" % byte_string.decode("utf-8"))
+        os.write(self.master_fd, byte_string)
+
+    def wait(self, condition):
+        start = time.time()
+        while (((time.time() - start) < self.timeout) and not condition()):
+            time.sleep(1e-2)
+        assert condition()
+
+    def wait_enabled(self):
+        self.wait(lambda: self.input_on() and not self.background())
+
+    def wait_disabled(self):
+        self.wait(lambda: not self.input_on() and self.background())
+
+    def wait_disabled_fg(self):
+        self.wait(lambda: not self.input_on() and not self.background())
+
+    def proc_status(self):
+        status = self.ps("-p", str(self.pid), "-o", "stat", output=str)
+        status = re.split(r"\s+", status.strip(), re.M)
+        return status[1]
+
+    def wait_stopped(self):
+        self.wait(lambda: "T" in self.proc_status())
+
+    def wait_running(self):
+        self.wait(lambda: "T" not in self.proc_status())
+
+
+class PseudoShell(object):
+    """Sets up master and child processes with a PTY.
+
+    You can create a ``PseudoShell`` if you want to test how some
+    function responds to terminal input.  This is a pseudo-shell from a
+    job control perspective; ``master_function`` and ``child_function``
+    are set up with a pseudoterminal (pty) so that the master can drive
+    the child through process control signals and I/O.
+
+    The two functions should have signatures like this::
+
+        def master_function(proc, ctl, **kwargs)
+        def child_function(**kwargs)
+
+    ``master_function`` is spawned in its own process and passed three
+    arguments:
+
+    proc
+        the ``multiprocessing.Process`` object representing the child
+    ctl
+        a ``ProcessController`` object tied to the child
+    kwargs
+        keyword arguments passed from ``PseudoShell.start()``.
+
+    ``child_function`` is only passed ``kwargs`` delegated from
+    ``PseudoShell.start()``.
+
+    The ``ctl.master_fd`` will have its ``master_fd`` connected to
+    ``sys.stdin`` in the child process. Both processes will share the
+    same ``sys.stdout`` and ``sys.stderr`` as the process instantiating
+    ``PseudoShell``.
+
+    Here are the relationships between processes created::
+
+        ._________________________________________________________.
+        | Child Process                                           | pid     2
+        | - runs child_function                                   | pgroup  2
+        |_________________________________________________________| session 1
+            ^
+            | create process with master_fd connected to stdin
+            | stdout, stderr are the same as caller
+        ._________________________________________________________.
+        | Master Process                                          | pid     1
+        | - runs master_function                                  | pgroup  1
+        | - uses ProcessController and master_fd to control child | session 1
+        |_________________________________________________________|
+            ^
+            | create process
+            | stdin, stdout, stderr are the same as caller
+        ._________________________________________________________.
+        | Caller                                                  |  pid     0
+        | - Constructs, starts, joins PseudoShell                 |  pgroup  0
+        | - provides master_function, child_function              |  session 0
+        |_________________________________________________________|
+
+    """
+    def __init__(self, master_function, child_function):
+        self.proc = None
+        self.master_function = master_function
+        self.child_function = child_function
+
+        # these can be optionally set to change defaults
+        self.controller_timeout = 1
+        self.sleep_time = 0
+
+    def start(self, **kwargs):
+        """Start the master and child processes.
+
+        Arguments:
+            kwargs (dict): arbitrary keyword arguments that will be
+                passed to master and child functions
+
+        The master process will create the child, then call
+        ``master_function``.  The child process will call
+        ``child_function``.
+
+        """
+        self.proc = multiprocessing.Process(
+            target=PseudoShell._set_up_and_run_master_function,
+            args=(self.master_function, self.child_function,
+                  self.controller_timeout, self.sleep_time),
+            kwargs=kwargs,
+        )
+        self.proc.start()
+
+    def join(self):
+        """Wait for the child process to finish, and return its exit code."""
+        self.proc.join()
+        return self.proc.exitcode
+
+    @staticmethod
+    def _set_up_and_run_child_function(
+            tty_name, stdout_fd, stderr_fd, ready, child_function, **kwargs):
+        """Child process wrapper for PseudoShell.
+
+        Handles the mechanics of setting up a PTY, then calls
+        ``child_function``.
+
+        """
+        # new process group, like a command or pipeline launched by a shell
+        os.setpgrp()
+
+        # take controlling terminal and set up pty IO
+        stdin_fd = os.open(tty_name, os.O_RDWR)
+        os.dup2(stdin_fd, sys.stdin.fileno())
+        os.dup2(stdout_fd, sys.stdout.fileno())
+        os.dup2(stderr_fd, sys.stderr.fileno())
+        os.close(stdin_fd)
+
+        if kwargs.get("debug"):
+            sys.stderr.write(
+                "child: stdin.isatty(): %s\n" % sys.stdin.isatty())
+
+        # tell the parent that we're really running
+        if kwargs.get("debug"):
+            sys.stderr.write("child: ready!\n")
+        ready.value = True
+
+        try:
+            child_function(**kwargs)
+        except BaseException:
+            traceback.print_exc()
+
+    @staticmethod
+    def _set_up_and_run_master_function(
+            master_function, child_function, controller_timeout, sleep_time,
+            **kwargs):
+        """Set up a pty, spawn a child process, and execute master_function.
+
+        Handles the mechanics of setting up a PTY, then calls
+        ``master_function``.
+
+        """
+        os.setsid()   # new session; this process is the controller
+
+        master_fd, child_fd = os.openpty()
+        pty_name = os.ttyname(child_fd)
+
+        # take controlling terminal
+        pty_fd = os.open(pty_name, os.O_RDWR)
+        os.close(pty_fd)
+
+        ready = multiprocessing.Value('i', False)
+        child_process = multiprocessing.Process(
+            target=PseudoShell._set_up_and_run_child_function,
+            args=(pty_name, sys.stdout.fileno(), sys.stderr.fileno(),
+                  ready, child_function),
+            kwargs=kwargs,
+        )
+        child_process.start()
+
+        # wait for subprocess to be running and connected.
+        while not ready.value:
+            time.sleep(1e-5)
+            pass
+
+        if kwargs.get("debug"):
+            sys.stderr.write("pid:        %d\n" % os.getpid())
+            sys.stderr.write("pgid:       %d\n" % os.getpgrp())
+            sys.stderr.write("sid:        %d\n" % os.getsid(0))
+            sys.stderr.write("tcgetpgrp:  %d\n" % os.tcgetpgrp(master_fd))
+            sys.stderr.write("\n")
+
+            child_pgid = os.getpgid(child_process.pid)
+            sys.stderr.write("child pid:  %d\n" % child_process.pid)
+            sys.stderr.write("child pgid: %d\n" % child_pgid)
+            sys.stderr.write("child sid:  %d\n" % os.getsid(child_process.pid))
+            sys.stderr.write("\n")
+            sys.stderr.flush()
+        # set up master to ignore SIGTSTP, like a shell
+        signal.signal(signal.SIGTSTP, signal.SIG_IGN)
+
+        # call the master function once the child is ready
+        try:
+            controller = ProcessController(
+                child_process.pid, master_fd, debug=kwargs.get("debug"))
+            controller.timeout = controller_timeout
+            controller.sleep_time = sleep_time
+            error = master_function(child_process, controller, **kwargs)
+        except BaseException:
+            error = 1
+            traceback.print_exc()
+
+        child_process.join()
+
+        # return whether either the parent or child failed
+        return error or child_process.exitcode
diff --git a/lib/spack/spack/test/llnl/util/lang.py b/lib/spack/spack/test/llnl/util/lang.py
index b285d8d0b2..83342dd6ed 100644
--- a/lib/spack/spack/test/llnl/util/lang.py
+++ b/lib/spack/spack/test/llnl/util/lang.py
@@ -130,3 +130,10 @@ def test_load_modules_from_file(module_path):
     foo = llnl.util.lang.load_module_from_file('foo', module_path)
     assert foo.value == 1
     assert foo.path == os.path.join('/usr', 'bin')
+
+
+def test_uniq():
+    assert [1, 2, 3] == llnl.util.lang.uniq([1, 2, 3])
+    assert [1, 2, 3] == llnl.util.lang.uniq([1, 1, 1, 1, 2, 2, 2, 3, 3])
+    assert [1, 2, 1] == llnl.util.lang.uniq([1, 1, 1, 1, 2, 2, 2, 1, 1])
+    assert [] == llnl.util.lang.uniq([])
diff --git a/lib/spack/spack/test/llnl/util/log.py b/lib/spack/spack/test/llnl/util/log.py
deleted file mode 100644
index 0c879c5a6e..0000000000
--- a/lib/spack/spack/test/llnl/util/log.py
+++ /dev/null
@@ -1,84 +0,0 @@
-# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
-# Spack Project Developers. See the top-level COPYRIGHT file for details.
-#
-# SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-from __future__ import print_function
-import pytest
-
-from llnl.util.tty.log import log_output
-from spack.util.executable import which
-
-
-def test_log_python_output_with_python_stream(capsys, tmpdir):
-    # pytest's DontReadFromInput object does not like what we do here, so
-    # disable capsys or things hang.
-    with tmpdir.as_cwd():
-        with capsys.disabled():
-            with log_output('foo.txt'):
-                print('logged')
-
-        with open('foo.txt') as f:
-            assert f.read() == 'logged\n'
-
-        assert capsys.readouterr() == ('', '')
-
-
-def test_log_python_output_with_fd_stream(capfd, tmpdir):
-    with tmpdir.as_cwd():
-        with log_output('foo.txt'):
-            print('logged')
-
-        with open('foo.txt') as f:
-            assert f.read() == 'logged\n'
-
-        # Coverage is cluttering stderr during tests
-        assert capfd.readouterr()[0] == ''
-
-
-def test_log_python_output_and_echo_output(capfd, tmpdir):
-    with tmpdir.as_cwd():
-        with log_output('foo.txt') as logger:
-            with logger.force_echo():
-                print('echo')
-            print('logged')
-
-        # Coverage is cluttering stderr during tests
-        assert capfd.readouterr()[0] == 'echo\n'
-
-        with open('foo.txt') as f:
-            assert f.read() == 'echo\nlogged\n'
-
-
-@pytest.mark.skipif(not which('echo'), reason="needs echo command")
-def test_log_subproc_output(capsys, tmpdir):
-    echo = which('echo')
-
-    # pytest seems to interfere here, so we need to use capsys.disabled()
-    # TODO: figure out why this is and whether it means we're doing
-    # sometihng wrong with OUR redirects.  Seems like it should work even
-    # with capsys enabled.
-    with tmpdir.as_cwd():
-        with capsys.disabled():
-            with log_output('foo.txt'):
-                echo('logged')
-
-        with open('foo.txt') as f:
-            assert f.read() == 'logged\n'
-
-
-@pytest.mark.skipif(not which('echo'), reason="needs echo command")
-def test_log_subproc_and_echo_output(capfd, tmpdir):
-    echo = which('echo')
-
-    with tmpdir.as_cwd():
-        with log_output('foo.txt') as logger:
-            with logger.force_echo():
-                echo('echo')
-            print('logged')
-
-        # Coverage is cluttering stderr during tests
-        assert capfd.readouterr()[0] == 'echo\n'
-
-        with open('foo.txt') as f:
-            assert f.read() == 'logged\n'
diff --git a/lib/spack/spack/test/llnl/util/tty/log.py b/lib/spack/spack/test/llnl/util/tty/log.py
new file mode 100644
index 0000000000..f23f663713
--- /dev/null
+++ b/lib/spack/spack/test/llnl/util/tty/log.py
@@ -0,0 +1,442 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from __future__ import print_function
+import contextlib
+import multiprocessing
+import os
+import signal
+import sys
+import time
+
+try:
+    import termios
+except ImportError:
+    termios = None
+
+import pytest
+
+import llnl.util.tty.log
+from llnl.util.lang import uniq
+from llnl.util.tty.log import log_output
+from llnl.util.tty.pty import PseudoShell
+
+from spack.util.executable import which
+
+
+@contextlib.contextmanager
+def nullcontext():
+    yield
+
+
+def test_log_python_output_with_echo(capfd, tmpdir):
+    with tmpdir.as_cwd():
+        with log_output('foo.txt', echo=True):
+            print('logged')
+
+        # foo.txt has output
+        with open('foo.txt') as f:
+            assert f.read() == 'logged\n'
+
+        # output is also echoed.
+        assert capfd.readouterr()[0] == 'logged\n'
+
+
+def test_log_python_output_without_echo(capfd, tmpdir):
+    with tmpdir.as_cwd():
+        with log_output('foo.txt'):
+            print('logged')
+
+        # foo.txt has output
+        with open('foo.txt') as f:
+            assert f.read() == 'logged\n'
+
+        # nothing on stdout or stderr
+        assert capfd.readouterr()[0] == ''
+
+
+def test_log_python_output_and_echo_output(capfd, tmpdir):
+    with tmpdir.as_cwd():
+        # echo two lines
+        with log_output('foo.txt') as logger:
+            with logger.force_echo():
+                print('force echo')
+            print('logged')
+
+        # log file contains everything
+        with open('foo.txt') as f:
+            assert f.read() == 'force echo\nlogged\n'
+
+        # only force-echo'd stuff is in output
+        assert capfd.readouterr()[0] == 'force echo\n'
+
+
+@pytest.mark.skipif(not which('echo'), reason="needs echo command")
+def test_log_subproc_and_echo_output_no_capfd(capfd, tmpdir):
+    echo = which('echo')
+
+    # this is split into two tests because capfd interferes with the
+    # output logged to file when using a subprocess.  We test the file
+    # here, and echoing in test_log_subproc_and_echo_output_capfd below.
+    with capfd.disabled():
+        with tmpdir.as_cwd():
+            with log_output('foo.txt') as logger:
+                with logger.force_echo():
+                    echo('echo')
+                print('logged')
+
+            with open('foo.txt') as f:
+                assert f.read() == 'echo\nlogged\n'
+
+
+@pytest.mark.skipif(not which('echo'), reason="needs echo command")
+def test_log_subproc_and_echo_output_capfd(capfd, tmpdir):
+    echo = which('echo')
+
+    # This tests *only* what is echoed when using a subprocess, as capfd
+    # interferes with the logged data. See
+    # test_log_subproc_and_echo_output_no_capfd for tests on the logfile.
+    with tmpdir.as_cwd():
+        with log_output('foo.txt') as logger:
+            with logger.force_echo():
+                echo('echo')
+            print('logged')
+
+        assert capfd.readouterr()[0] == "echo\n"
+
+
+#
+# Tests below use a pseudoterminal to test llnl.util.tty.log
+#
+def simple_logger(**kwargs):
+    """Mock logger (child) process for testing log.keyboard_input."""
+    def handler(signum, frame):
+        running[0] = False
+    signal.signal(signal.SIGUSR1, handler)
+
+    log_path = kwargs["log_path"]
+    running = [True]
+    with log_output(log_path):
+        while running[0]:
+            print("line")
+            time.sleep(1e-3)
+
+
+def mock_shell_fg(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.fg()
+    ctl.status()
+    ctl.wait_enabled()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_fg_no_termios(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.fg()
+    ctl.status()
+    ctl.wait_disabled_fg()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_bg(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.bg()
+    ctl.status()
+    ctl.wait_disabled()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_tstp_cont(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.tstp()
+    ctl.wait_stopped()
+
+    ctl.cont()
+    ctl.wait_running()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_tstp_tstp_cont(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.tstp()
+    ctl.wait_stopped()
+
+    ctl.tstp()
+    ctl.wait_stopped()
+
+    ctl.cont()
+    ctl.wait_running()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_tstp_tstp_cont_cont(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.tstp()
+    ctl.wait_stopped()
+
+    ctl.tstp()
+    ctl.wait_stopped()
+
+    ctl.cont()
+    ctl.wait_running()
+
+    ctl.cont()
+    ctl.wait_running()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_bg_fg(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.bg()
+    ctl.status()
+    ctl.wait_disabled()
+
+    ctl.fg()
+    ctl.status()
+    ctl.wait_enabled()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_bg_fg_no_termios(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.bg()
+    ctl.status()
+    ctl.wait_disabled()
+
+    ctl.fg()
+    ctl.status()
+    ctl.wait_disabled_fg()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_fg_bg(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.fg()
+    ctl.status()
+    ctl.wait_enabled()
+
+    ctl.bg()
+    ctl.status()
+    ctl.wait_disabled()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_fg_bg_no_termios(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background."""
+    ctl.fg()
+    ctl.status()
+    ctl.wait_disabled_fg()
+
+    ctl.bg()
+    ctl.status()
+    ctl.wait_disabled()
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+@contextlib.contextmanager
+def no_termios():
+    saved = llnl.util.tty.log.termios
+    llnl.util.tty.log.termios = None
+    try:
+        yield
+    finally:
+        llnl.util.tty.log.termios = saved
+
+
+@pytest.mark.skipif(not which("ps"), reason="requires ps utility")
+@pytest.mark.skipif(not termios, reason="requires termios support")
+@pytest.mark.parametrize('test_fn,termios_on_or_off', [
+    # tests with termios
+    (mock_shell_fg, nullcontext),
+    (mock_shell_bg, nullcontext),
+    (mock_shell_bg_fg, nullcontext),
+    (mock_shell_fg_bg, nullcontext),
+    (mock_shell_tstp_cont, nullcontext),
+    (mock_shell_tstp_tstp_cont, nullcontext),
+    (mock_shell_tstp_tstp_cont_cont, nullcontext),
+    # tests without termios
+    (mock_shell_fg_no_termios, no_termios),
+    (mock_shell_bg, no_termios),
+    (mock_shell_bg_fg_no_termios, no_termios),
+    (mock_shell_fg_bg_no_termios, no_termios),
+    (mock_shell_tstp_cont, no_termios),
+    (mock_shell_tstp_tstp_cont, no_termios),
+    (mock_shell_tstp_tstp_cont_cont, no_termios),
+])
+def test_foreground_background(test_fn, termios_on_or_off, tmpdir):
+    """Functional tests for foregrounding and backgrounding a logged process.
+
+    This ensures that things like SIGTTOU are not raised and that
+    terminal settings are corrected on foreground/background and on
+    process stop and start.
+
+    """
+    shell = PseudoShell(test_fn, simple_logger)
+    log_path = str(tmpdir.join("log.txt"))
+
+    # run the shell test
+    with termios_on_or_off():
+        shell.start(log_path=log_path, debug=True)
+    exitcode = shell.join()
+
+    # processes completed successfully
+    assert exitcode == 0
+
+    # assert log was created
+    assert os.path.exists(log_path)
+
+
+def synchronized_logger(**kwargs):
+    """Mock logger (child) process for testing log.keyboard_input.
+
+    This logger synchronizes with the parent process to test that 'v' can
+    toggle output.  It is used in ``test_foreground_background_output`` below.
+
+    """
+    def handler(signum, frame):
+        running[0] = False
+    signal.signal(signal.SIGUSR1, handler)
+
+    log_path = kwargs["log_path"]
+    write_lock = kwargs["write_lock"]
+    v_lock = kwargs["v_lock"]
+
+    running = [True]
+    sys.stderr.write(os.getcwd() + "\n")
+    with log_output(log_path) as logger:
+        with logger.force_echo():
+            print("forced output")
+
+        while running[0]:
+            with write_lock:
+                if v_lock.acquire(False):  # non-blocking acquire
+                    print("off")
+                    v_lock.release()
+                else:
+                    print("on")       # lock held; v is toggled on
+            time.sleep(1e-2)
+
+
+def mock_shell_v_v(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background_output."""
+    write_lock = kwargs["write_lock"]
+    v_lock = kwargs["v_lock"]
+
+    ctl.fg()
+    ctl.wait_enabled()
+    time.sleep(.1)
+
+    write_lock.acquire()  # suspend writing
+    v_lock.acquire()      # enable v lock
+    ctl.write(b'v')       # toggle v on stdin
+    time.sleep(.1)
+    write_lock.release()  # resume writing
+
+    time.sleep(.1)
+
+    write_lock.acquire()  # suspend writing
+    ctl.write(b'v')       # toggle v on stdin
+    time.sleep(.1)
+    v_lock.release()      # disable v lock
+    write_lock.release()  # resume writing
+    time.sleep(.1)
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+def mock_shell_v_v_no_termios(proc, ctl, **kwargs):
+    """PseudoShell master function for test_foreground_background_output."""
+    write_lock = kwargs["write_lock"]
+    v_lock = kwargs["v_lock"]
+
+    ctl.fg()
+    ctl.wait_disabled_fg()
+    time.sleep(.1)
+
+    write_lock.acquire()  # suspend writing
+    v_lock.acquire()      # enable v lock
+    ctl.write(b'v\n')     # toggle v on stdin
+    time.sleep(.1)
+    write_lock.release()  # resume writing
+
+    time.sleep(.1)
+
+    write_lock.acquire()  # suspend writing
+    ctl.write(b'v\n')     # toggle v on stdin
+    time.sleep(.1)
+    v_lock.release()      # disable v lock
+    write_lock.release()  # resume writing
+    time.sleep(.1)
+
+    os.kill(proc.pid, signal.SIGUSR1)
+
+
+@pytest.mark.skipif(not which("ps"), reason="requires ps utility")
+@pytest.mark.skipif(not termios, reason="requires termios support")
+@pytest.mark.parametrize('test_fn,termios_on_or_off', [
+    (mock_shell_v_v, nullcontext),
+    (mock_shell_v_v_no_termios, no_termios),
+])
+def test_foreground_background_output(
+        test_fn, capfd, termios_on_or_off, tmpdir):
+    """Tests hitting 'v' toggles output, and that force_echo works."""
+    shell = PseudoShell(test_fn, synchronized_logger)
+    log_path = str(tmpdir.join("log.txt"))
+
+    # Locks for synchronizing with child
+    write_lock = multiprocessing.Lock()  # must be held by child to write
+    v_lock = multiprocessing.Lock()  # held while master is in v mode
+
+    with termios_on_or_off():
+        shell.start(
+            write_lock=write_lock,
+            v_lock=v_lock,
+            debug=True,
+            log_path=log_path
+        )
+
+    exitcode = shell.join()
+    out, err = capfd.readouterr()
+    print(err)  # will be shown if something goes wrong
+    print(out)
+
+    # processes completed successfully
+    assert exitcode == 0
+
+    # split output into lines
+    output = out.strip().split("\n")
+
+    # also get lines of log file
+    assert os.path.exists(log_path)
+    with open(log_path) as log:
+        log = log.read().strip().split("\n")
+
+    # Master and child process coordinate with locks such that the child
+    # writes "off" when echo is off, and "on" when echo is on.  The
+    # output should contain mostly "on" lines, but may contain an "off"
+    # or two. This is because the master toggles echo by sending "v" on
+    # stdin to the child, but this is not synchronized with our locks.
+    # It's good enough for a test, though.  We allow at most 2 "off"'s in
+    # the output to account for the race.
+    assert (
+        ['forced output', 'on'] == uniq(output) or
+        output.count("off") <= 2  # if master_fd is a bit slow
+    )
+
+    # log should be off for a while, then on, then off
+    assert (
+        ['forced output', 'off', 'on', 'off'] == uniq(log) and
+        log.count("off") > 2  # ensure some "off" lines were omitted
+    )