Officially test proto-ed `stackscope` integration

By re-purposing our `pexpect`-based console matching with a new
`debugging/shield_hang_in_sub.py` example, this tests a few "hanging
actor" conditions more formally:

- that despite a hanging actor's task we can dump
  a `stackscope.extract()` tree on relay of `SIGUSR1`.
- the actor tree will terminate despite a shielded forever-sleep by our
  "T-800" zombie reaper machinery activating and hard killing the
  underlying subprocess.

Some test deats:
- simulates the expect actions of a real user by manually using
  `os.kill()` to send both signals to the actor-tree program.
- `pexpect`-matches against `log.devx()` emissions under normal
  `debug_mode == True` usage.
- ensure we get the actual "T-800 deployed" `log.error()` msg and
  that the actor tree eventually terminates!

Surrounding (re-org/impl/test-suite) changes:
- allow disabling usage via a `maybe_enable_greenback: bool` to
  `open_root_actor()` but enable by def.
- pretty up the actual `.devx()` content from `.devx._stackscope`
  including be extra pedantic about the conc-primitives for each signal
  event.
- try to avoid double handles of `SIGUSR1` even though it seems the
  original (what i thought was a) problem was actually just double
  logging in the handler..
  |_ avoid double applying the handler func via `signal.signal()`,
  |_ use a global to avoid double handle func calls and,
  |_ a `threading.RLock` around handling.
- move common fixtures and helper routines from `test_debugger` to
  `tests/devx/conftest.py` and import them for use in both test mods.

examples/debugging/shield_hang_in_sub.py

@@ -0,0 +1,81 @@
+'''
+Verify we can dump a `stackscope` tree on a hang.
+
+'''
+import os
+import signal
+
+import trio
+import tractor
+
+@tractor.context
+async def start_n_shield_hang(
+    ctx: tractor.Context,
+):
+    # actor: tractor.Actor = tractor.current_actor()
+
+    # sync to parent-side task
+    await ctx.started(os.getpid())
+
+    print('Entering shield sleep..')
+    with trio.CancelScope(shield=True):
+        await trio.sleep_forever()  # in subactor
+
+    # XXX NOTE ^^^ since this shields, we expect
+    # the zombie reaper (aka T800) to engage on
+    # SIGINT from the user and eventually hard-kill
+    # this subprocess!
+
+
+async def main(
+    from_test: bool = False,
+) -> None:
+
+    async with (
+        tractor.open_nursery(
+            debug_mode=True,
+            enable_stack_on_sig=True,
+            # maybe_enable_greenback=False,
+            loglevel='devx',
+        ) as an,
+    ):
+
+        ptl: tractor.Portal  = await an.start_actor(
+            'hanger',
+            enable_modules=[__name__],
+            debug_mode=True,
+        )
+        async with ptl.open_context(
+            start_n_shield_hang,
+        ) as (ctx, cpid):
+
+            _, proc, _ = an._children[ptl.chan.uid]
+            assert cpid == proc.pid
+
+            print(
+                'Yo my child hanging..?\n'
+                'Sending SIGUSR1 to see a tree-trace!\n'
+            )
+
+            # XXX simulate the wrapping test's "user actions"
+            # (i.e. if a human didn't run this manually but wants to
+            # know what they should do to reproduce test behaviour)
+            if from_test:
+                os.kill(
+                    cpid,
+                    signal.SIGUSR1,
+                )
+
+                # simulate user cancelling program
+                await trio.sleep(0.5)
+                os.kill(
+                    os.getpid(),
+                    signal.SIGINT,
+                )
+            else:
+                # actually let user send the ctl-c
+                await trio.sleep_forever()  # in root
+
+
+if __name__ == '__main__':
+    trio.run(main)

tests/devx/conftest.py

@@ -0,0 +1,167 @@
+'''
+`tractor.devx.*` tooling sub-pkg test space.
+
+'''
+from typing import (
+    Callable,
+)
+
+import pytest
+from pexpect.exceptions import (
+    TIMEOUT,
+)
+from tractor._testing import (
+    mk_cmd,
+)
+
+
+@pytest.fixture
+def spawn(
+    start_method,
+    testdir: pytest.Testdir,
+    reg_addr: tuple[str, int],
+
+) -> Callable[[str], None]:
+    '''
+    Use the `pexpect` module shipped via `testdir.spawn()` to
+    run an `./examples/..` script by name.
+
+    '''
+    if start_method != 'trio':
+        pytest.skip(
+            '`pexpect` based tests only supported on `trio` backend'
+        )
+
+    def _spawn(
+        cmd: str,
+        **mkcmd_kwargs,
+    ):
+        return testdir.spawn(
+            cmd=mk_cmd(
+                cmd,
+                **mkcmd_kwargs,
+            ),
+            expect_timeout=3,
+        )
+
+    # such that test-dep can pass input script name.
+    return _spawn
+
+
+@pytest.fixture(
+    params=[False, True],
+    ids='ctl-c={}'.format,
+)
+def ctlc(
+    request,
+    ci_env: bool,
+
+) -> bool:
+
+    use_ctlc = request.param
+
+    node = request.node
+    markers = node.own_markers
+    for mark in markers:
+        if mark.name == 'has_nested_actors':
+            pytest.skip(
+                f'Test {node} has nested actors and fails with Ctrl-C.\n'
+                f'The test can sometimes run fine locally but until'
+                ' we solve' 'this issue this CI test will be xfail:\n'
+                'https://github.com/goodboy/tractor/issues/320'
+            )
+
+    if use_ctlc:
+        # XXX: disable pygments highlighting for auto-tests
+        # since some envs (like actions CI) will struggle
+        # the the added color-char encoding..
+        from tractor.devx._debug import TractorConfig
+        TractorConfig.use_pygements = False
+
+    yield use_ctlc
+
+
+def expect(
+    child,
+
+    # normally a `pdb` prompt by default
+    patt: str,
+
+    **kwargs,
+
+) -> None:
+    '''
+    Expect wrapper that prints last seen console
+    data before failing.
+
+    '''
+    try:
+        child.expect(
+            patt,
+            **kwargs,
+        )
+    except TIMEOUT:
+        before = str(child.before.decode())
+        print(before)
+        raise
+
+
+def in_prompt_msg(
+    prompt: str,
+    parts: list[str],
+
+    pause_on_false: bool = False,
+    err_on_false: bool = False,
+    print_prompt_on_false: bool = True,
+
+) -> bool:
+    '''
+    Predicate check if (the prompt's) std-streams output has all
+    `str`-parts in it.
+
+    Can be used in test asserts for bulk matching expected
+    log/REPL output for a given `pdb` interact point.
+
+    '''
+    __tracebackhide__: bool = False
+
+    for part in parts:
+        if part not in prompt:
+            if pause_on_false:
+                import pdbp
+                pdbp.set_trace()
+
+            if print_prompt_on_false:
+                print(prompt)
+
+            if err_on_false:
+                raise ValueError(
+                    f'Could not find pattern: {part!r} in `before` output?'
+                )
+            return False
+
+    return True
+
+
+# TODO: todo support terminal color-chars stripping so we can match
+# against call stack frame output from the the 'll' command the like!
+# -[ ] SO answer for stipping ANSI codes: https://stackoverflow.com/a/14693789
+def assert_before(
+    child,
+    patts: list[str],
+
+    **kwargs,
+
+) -> None:
+    __tracebackhide__: bool = False
+
+    # as in before the prompt end
+    before: str = str(child.before.decode())
+    assert in_prompt_msg(
+        prompt=before,
+        parts=patts,
+
+        # since this is an "assert" helper ;)
+        err_on_false=True,
+        **kwargs
+    )

tests/devx/test_debugger.py

@@ -16,7 +16,6 @@ import platform
 import time
 
 import pytest
-import pexpect
 from pexpect.exceptions import (
     TIMEOUT,
     EOF,
@@ -27,12 +26,14 @@ from tractor.devx._debug import (
     _crash_msg,
     _repl_fail_msg,
 )
-from tractor._testing import (
-    mk_cmd,
-)
 from conftest import (
     _ci_env,
 )
+from .conftest import (
+    expect,
+    in_prompt_msg,
+    assert_before,
+)
 
 # TODO: The next great debugger audit could be done by you!
 # - recurrent entry to breakpoint() from single actor *after* and an
@@ -69,154 +70,9 @@ has_nested_actors = pytest.mark.has_nested_actors
 # )
 
 
-@pytest.fixture
-def spawn(
-    start_method,
-    testdir,
-    reg_addr,
-
-) -> 'pexpect.spawn':
-    '''
-    Use the `pexpect` module shipped via `testdir.spawn()` to
-    run an `./examples/..` script by name.
-
-    '''
-    if start_method != 'trio':
-        pytest.skip(
-            '`pexpect` based tests only supported on `trio` backend'
-        )
-
-    def _spawn(
-        cmd: str,
-        **mkcmd_kwargs,
-    ):
-        return testdir.spawn(
-            cmd=mk_cmd(
-                cmd,
-                **mkcmd_kwargs,
-            ),
-            expect_timeout=3,
-        )
-
-    # such that test-dep can pass input script name.
-    return _spawn
-
-
 PROMPT = r"\(Pdb\+\)"
 
 
-def expect(
-    child,
-
-    # prompt by default
-    patt: str = PROMPT,
-
-    **kwargs,
-
-) -> None:
-    '''
-    Expect wrapper that prints last seen console
-    data before failing.
-
-    '''
-    try:
-        child.expect(
-            patt,
-            **kwargs,
-        )
-    except TIMEOUT:
-        before = str(child.before.decode())
-        print(before)
-        raise
-
-
-def in_prompt_msg(
-    prompt: str,
-    parts: list[str],
-
-    pause_on_false: bool = False,
-    print_prompt_on_false: bool = True,
-
-) -> bool:
-    '''
-    Predicate check if (the prompt's) std-streams output has all
-    `str`-parts in it.
-
-    Can be used in test asserts for bulk matching expected
-    log/REPL output for a given `pdb` interact point.
-
-    '''
-    __tracebackhide__: bool = False
-
-    for part in parts:
-        if part not in prompt:
-            if pause_on_false:
-                import pdbp
-                pdbp.set_trace()
-
-            if print_prompt_on_false:
-                print(prompt)
-
-            return False
-
-    return True
-
-
-# TODO: todo support terminal color-chars stripping so we can match
-# against call stack frame output from the the 'll' command the like!
-# -[ ] SO answer for stipping ANSI codes: https://stackoverflow.com/a/14693789
-def assert_before(
-    child,
-    patts: list[str],
-
-    **kwargs,
-
-) -> None:
-    __tracebackhide__: bool = False
-
-    # as in before the prompt end
-    before: str = str(child.before.decode())
-    assert in_prompt_msg(
-        prompt=before,
-        parts=patts,
-
-        **kwargs
-    )
-
-
-@pytest.fixture(
-    params=[False, True],
-    ids='ctl-c={}'.format,
-)
-def ctlc(
-    request,
-    ci_env: bool,
-
-) -> bool:
-
-    use_ctlc = request.param
-
-    node = request.node
-    markers = node.own_markers
-    for mark in markers:
-        if mark.name == 'has_nested_actors':
-            pytest.skip(
-                f'Test {node} has nested actors and fails with Ctrl-C.\n'
-                f'The test can sometimes run fine locally but until'
-                ' we solve' 'this issue this CI test will be xfail:\n'
-                'https://github.com/goodboy/tractor/issues/320'
-            )
-
-    if use_ctlc:
-        # XXX: disable pygments highlighting for auto-tests
-        # since some envs (like actions CI) will struggle
-        # the the added color-char encoding..
-        from tractor.devx._debug import TractorConfig
-        TractorConfig.use_pygements = False
-
-    yield use_ctlc
-
-
 @pytest.mark.parametrize(
     'user_in_out',
     [
@@ -281,7 +137,7 @@ def test_root_actor_bp(spawn, user_in_out):
     child.expect('\r\n')
 
     # process should exit
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
     if expect_err_str is None:
         assert 'Error' not in str(child.before)
@@ -365,7 +221,7 @@ def test_root_actor_bp_forever(
 
     # quit out of the loop
     child.sendline('q')
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
 
 @pytest.mark.parametrize(
@@ -430,7 +286,7 @@ def test_subactor_error(
     child.expect('\r\n')
 
     # process should exit
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
 
 def test_subactor_breakpoint(
@@ -493,7 +349,7 @@ def test_subactor_breakpoint(
     child.sendline('c')
 
     # process should exit
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
     before = str(child.before.decode())
     assert in_prompt_msg(
@@ -636,7 +492,7 @@ def test_multi_subactors(
 
     # process should exit
     child.sendline('c')
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
     # repeat of previous multierror for final output
     assert_before(child, [
@@ -776,7 +632,7 @@ def test_multi_daemon_subactors(
     )
 
     child.sendline('c')
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
 
 @has_nested_actors
@@ -852,7 +708,7 @@ def test_multi_subactors_root_errors(
     ])
 
     child.sendline('c')
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
     assert_before(child, [
         # "Attaching to pdb in crashed actor: ('root'",
@@ -982,7 +838,7 @@ def test_root_nursery_cancels_before_child_releases_tty_lock(
 
     for i in range(3):
         try:
-            child.expect(pexpect.EOF, timeout=0.5)
+            child.expect(EOF, timeout=0.5)
             break
         except TIMEOUT:
             child.sendline('c')
@@ -1024,7 +880,7 @@ def test_root_cancels_child_context_during_startup(
         do_ctlc(child)
 
     child.sendline('c')
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
 
 def test_different_debug_mode_per_actor(
@@ -1045,7 +901,7 @@ def test_different_debug_mode_per_actor(
         do_ctlc(child)
 
     child.sendline('c')
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
     before = str(child.before.decode())
 
@@ -1196,7 +1052,7 @@ def test_pause_from_sync(
             )
 
     child.sendline('c')
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
 
 def test_post_mortem_api(
@@ -1301,7 +1157,7 @@ def test_post_mortem_api(
     # )
 
     child.sendline('c')
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
 
 def test_shield_pause(
@@ -1376,7 +1232,7 @@ def test_shield_pause(
         ]
     )
     child.sendline('c')
-    child.expect(pexpect.EOF)
+    child.expect(EOF)
 
 
 # TODO: better error for "non-ideal" usage from the root actor.

tests/devx/test_tooling.py

@@ -0,0 +1,120 @@
+'''
+That "native" runtime-hackin toolset better be dang useful!
+
+Verify the funtion of a variety of "developer-experience" tools we
+offer from the `.devx` sub-pkg:
+
+- use of the lovely `stackscope` for dumping actor `trio`-task trees
+  during operation and hangs.
+
+TODO:
+- demonstration of `CallerInfo` call stack frame filtering such that
+  for logging and REPL purposes a user sees exactly the layers needed
+  when debugging a problem inside the stack vs. in their app.
+
+'''
+import os
+import signal
+
+from .conftest import (
+    expect,
+    assert_before,
+    # in_prompt_msg,
+)
+
+
+def test_shield_pause(
+    spawn,
+):
+    '''
+    Verify the `tractor.pause()/.post_mortem()` API works inside an
+    already cancelled `trio.CancelScope` and that you can step to the
+    next checkpoint wherein the cancelled will get raised.
+
+    '''
+    child = spawn(
+        'shield_hang_in_sub'
+    )
+    expect(
+        child,
+        'Yo my child hanging..?',
+    )
+    assert_before(
+        child,
+        [
+            'Entering shield sleep..',
+            'Enabling trace-trees on `SIGUSR1` since `stackscope` is installed @',
+        ]
+    )
+
+    print(
+        'Sending SIGUSR1 to see a tree-trace!',
+    )
+    os.kill(
+        child.pid,
+        signal.SIGUSR1,
+    )
+    expect(
+        child,
+        # end-of-tree delimiter
+        "------ \('root', ",
+    )
+
+    assert_before(
+        child,
+        [
+            'Trying to dump `stackscope` tree..',
+            'Dumping `stackscope` tree for actor',
+            "('root'",  # uid line
+
+            # parent block point (non-shielded)
+            'await trio.sleep_forever()  # in root',
+        ]
+    )
+
+    # expect(
+    #     child,
+    #     # relay to the sub should be reported
+    #     'Relaying `SIGUSR1`[10] to sub-actor',
+    # )
+
+    expect(
+        child,
+        # end-of-tree delimiter
+        "------ \('hanger', ",
+    )
+    assert_before(
+        child,
+        [
+            # relay to the sub should be reported
+            'Relaying `SIGUSR1`[10] to sub-actor',
+
+            "('hanger'",  # uid line
+
+            # hanger LOC where it's shield-halted
+            'await trio.sleep_forever()  # in subactor',
+        ]
+    )
+    # breakpoint()
+
+    # simulate the user sending a ctl-c to the hanging program.
+    # this should result in the terminator kicking in since
+    # the sub is shield blocking and can't respond to SIGINT.
+    os.kill(
+        child.pid,
+        signal.SIGINT,
+    )
+    expect(
+        child,
+        'Shutting down actor runtime',
+        timeout=6,
+    )
+    assert_before(
+        child,
+        [
+            'raise KeyboardInterrupt',
+            # 'Shutting down actor runtime',
+            '#T-800 deployed to collect zombie B0',
+            "'--uid', \"('hanger',",
+        ]
+    )

tractor/_root.py

@@ -80,7 +80,7 @@ async def open_root_actor(
 
     # enables the multi-process debugger support
     debug_mode: bool = False,
-    maybe_enable_greenback: bool = False,  # `.pause_from_sync()/breakpoint()` support
+    maybe_enable_greenback: bool = True,  # `.pause_from_sync()/breakpoint()` support
     enable_stack_on_sig: bool = False,
 
     # internal logging
@@ -233,14 +233,8 @@ async def open_root_actor(
         and
         enable_stack_on_sig
     ):
-        try:
+        from .devx._stackscope import enable_stack_on_sig
-            logger.info('Enabling `stackscope` traces on SIGUSR1')
+        enable_stack_on_sig()
-            from .devx import enable_stack_on_sig
-            enable_stack_on_sig()
-        except ImportError:
-            logger.warning(
-                '`stackscope` not installed for use in debug mode!'
-            )
 
     # closed into below ping task-func
     ponged_addrs: list[tuple[str, int]] = []

tractor/devx/_stackscope.py

@@ -24,13 +24,24 @@ disjoint, parallel executing tasks in separate actors.
 
 '''
 from __future__ import annotations
+# from functools import partial
+from threading import (
+    current_thread,
+    Thread,
+    RLock,
+)
 import multiprocessing as mp
 from signal import (
     signal,
+    getsignal,
     SIGUSR1,
 )
-import traceback
+# import traceback
-from typing import TYPE_CHECKING
+from types import ModuleType
+from typing import (
+    Callable,
+    TYPE_CHECKING,
+)
 
 import trio
 from tractor import (
@@ -51,26 +62,45 @@ if TYPE_CHECKING:
 
 @trio.lowlevel.disable_ki_protection
 def dump_task_tree() -> None:
-    import stackscope
+    '''
-    from tractor.log import get_console_log
+    Do a classic `stackscope.extract()` task-tree dump to console at
+    `.devx()` level.
 
+    '''
+    import stackscope
     tree_str: str = str(
         stackscope.extract(
             trio.lowlevel.current_root_task(),
             recurse_child_tasks=True
         )
     )
-    log = get_console_log(
-        name=__name__,
-        level='cancel',
-    )
     actor: Actor = _state.current_actor()
+    thr: Thread = current_thread()
     log.devx(
         f'Dumping `stackscope` tree for actor\n'
-        f'{actor.name}: {actor}\n'
+        f'{actor.uid}:\n'
-        f' |_{mp.current_process()}\n\n'
+        f'|_{mp.current_process()}\n'
+        f'  |_{thr}\n'
+        f'    |_{actor}\n\n'
+
+        # start-of-trace-tree delimiter (mostly for testing)
+        '------ - ------\n'
+        '\n'
+        +
         f'{tree_str}\n'
+        +
+        # end-of-trace-tree delimiter (mostly for testing)
+        f'\n'
+        f'------ {actor.uid!r} ------\n'
     )
+    # TODO: can remove this right?
+    # -[ ] was original code from author
+    #
+    # print(
+    #     'DUMPING FROM PRINT\n'
+    #     +
+    #     content
+    # )
     # import logging
     # try:
     #     with open("/dev/tty", "w") as tty:
@@ -80,58 +110,130 @@ def dump_task_tree() -> None:
     #         "task_tree"
     #     ).exception("Error printing task tree")
 
+_handler_lock = RLock()
+_tree_dumped: bool = False
 
-def signal_handler(
+
+def dump_tree_on_sig(
     sig: int,
     frame: object,
 
     relay_to_subs: bool = True,
 
 ) -> None:
-    try:
+    global _tree_dumped, _handler_lock
-        trio.lowlevel.current_trio_token(
+    with _handler_lock:
-        ).run_sync_soon(dump_task_tree)
+        if _tree_dumped:
-    except RuntimeError:
+            log.warning(
-        # not in async context -- print a normal traceback
+                'Already dumped for this actor...??'
-        traceback.print_stack()
+            )
+            return
+
+        _tree_dumped = True
+
+        # actor: Actor = _state.current_actor()
+        log.devx(
+            'Trying to dump `stackscope` tree..\n'
+        )
+        try:
+            dump_task_tree()
+            # await actor._service_n.start_soon(
+            #     partial(
+            #         trio.to_thread.run_sync,
+            #         dump_task_tree,
+            #     )
+            # )
+            # trio.lowlevel.current_trio_token().run_sync_soon(
+            #     dump_task_tree
+            # )
+
+        except RuntimeError:
+            log.exception(
+                'Failed to dump `stackscope` tree..\n'
+            )
+            # not in async context -- print a normal traceback
+            # traceback.print_stack()
+            raise
+
+        except BaseException:
+            log.exception(
+                'Failed to dump `stackscope` tree..\n'
+            )
+            raise
+
+        log.devx(
+            'Supposedly we dumped just fine..?'
+        )
 
     if not relay_to_subs:
         return
 
     an: ActorNursery
     for an in _state.current_actor()._actoruid2nursery.values():
-
         subproc: ProcessType
         subactor: Actor
         for subactor, subproc, _ in an._children.values():
-            log.devx(
+            log.warning(
                 f'Relaying `SIGUSR1`[{sig}] to sub-actor\n'
                 f'{subactor}\n'
                 f' |_{subproc}\n'
             )
 
-            if isinstance(subproc, trio.Process):
+            # bc of course stdlib can't have a std API.. XD
-                subproc.send_signal(sig)
+            match subproc:
+                case trio.Process():
+                    subproc.send_signal(sig)
 
-            elif isinstance(subproc, mp.Process):
+                case mp.Process():
-                subproc._send_signal(sig)
+                    subproc._send_signal(sig)
 
 
 def enable_stack_on_sig(
-    sig: int = SIGUSR1
+    sig: int = SIGUSR1,
-) -> None:
+) -> ModuleType:
     '''
     Enable `stackscope` tracing on reception of a signal; by
     default this is SIGUSR1.
 
+    HOT TIP: a task/ctx-tree dump can be triggered from a shell with
+    fancy cmds.
+
+    For ex. from `bash` using `pgrep` and cmd-sustitution
+    (https://www.gnu.org/software/bash/manual/bash.html#Command-Substitution)
+    you could use:
+
+    >> kill -SIGUSR1 $(pgrep -f '<cmd>')
+
+    Or with with `xonsh` (which has diff capture-from-subproc syntax)
+
+    >> kill -SIGUSR1 @$(pgrep -f '<cmd>')
+
     '''
+    try:
+        import stackscope
+    except ImportError:
+        log.warning(
+            '`stackscope` not installed for use in debug mode!'
+        )
+        return None
+
+    handler: Callable|int = getsignal(sig)
+    if handler is dump_tree_on_sig:
+        log.devx(
+            'A `SIGUSR1` handler already exists?\n'
+            f'|_ {handler!r}\n'
+        )
+        return
+
     signal(
         sig,
-        signal_handler,
+        dump_tree_on_sig,
     )
-    # NOTE: not the above can be triggered from
+    log.devx(
-    # a (xonsh) shell using:
+        'Enabling trace-trees on `SIGUSR1` '
-    # kill -SIGUSR1 @$(pgrep -f '<cmd>')
+        'since `stackscope` is installed @ \n'
-    #
+        f'{stackscope!r}\n\n'
-    # for example if you were looking to trace a `pytest` run
+        f'With `SIGUSR1` handler\n'
-    # kill -SIGUSR1 @$(pgrep -f 'pytest')
+        f'|_{dump_tree_on_sig}\n'
+    )
+    return stackscope