Extend `._taskc.maybe_raise_from_masking_exc()`

To handle captured non-egs (when the now optional `tn` isn't provided)
as well as yield up a `BoxedMaybeException` which contains any detected
and un-masked `exc_ctx` as its `.value`.

Also add some additional tooling,
- a `raise_unmasked: bool` toggle for when the caller just wants to
  report the masked exc and not raise-it-in-place of the masker.
- `extra_note: str` which by default is tuned to the default
  `unmask_from = (trio.Cancelled,)` but which can be used to deliver
  custom exception msg content.
- `always_warn_on: tuple[BaseException]` which will always emit
  a warning log of what would have been the raised-in-place-of
  `ctx_exc`'s msg for special cases where you want to report
  a masking case that might not be otherwise noticed by the runtime
  (cough like a `Cancelled` masking another `Cancelled) but which
  you'd still like to warn the caller about.
- factor out the masked-`ext_ctx` predicate logic into
  a `find_masked_excs()` and also use it for non-eg cases.

Still maybe todo?
- rewrapping multiple masked sub-excs in an eg back into an eg? left in
  #TODOs and a pause-point where applicable.

tractor/trionics/_taskc.py

@@ -22,50 +22,161 @@ from __future__ import annotations
 from contextlib import (
     asynccontextmanager as acm,
 )
+from typing import TYPE_CHECKING
 
 import trio
-# from trio import TaskStatus
+from tractor.log import get_logger
+
+log = get_logger(__name__)
+
+
+if TYPE_CHECKING:
+    from tractor.devx.debug import BoxedMaybeException
+
+
+def find_masked_excs(
+    maybe_masker: BaseException,
+    unmask_from: set[BaseException],
+) -> BaseException|None:
+    ''''
+    Deliver any `maybe_masker.__context__` provided
+    it a declared masking exc-type entry in `unmask_from`.
+
+    '''
+    if (
+        type(maybe_masker) in unmask_from
+        and
+        (exc_ctx := maybe_masker.__context__)
+
+        # TODO? what about any cases where
+        # they could be the same type but not same instance?
+        # |_i.e. a cancel masking a cancel ??
+        # or (
+        #     exc_ctx is not maybe_masker
+        # )
+    ):
+        return exc_ctx
+
+    return None
 
 
 @acm
 async def maybe_raise_from_masking_exc(
-    tn: trio.Nursery,
-    unmask_from: BaseException|None = trio.Cancelled
+    tn: trio.Nursery|None = None,
+    unmask_from: (
+        BaseException|
+        tuple[BaseException]
+    ) = (trio.Cancelled,),
 
-    # TODO, maybe offer a collection?
-    # unmask_from: set[BaseException] = {
-    #     trio.Cancelled,
-    # },
-):
-    if not unmask_from:
-        yield
+    raise_unmasked: bool = True,
+    extra_note: str = (
+        'This can occurr when,\n'
+        ' - a `trio.Nursery` scope embeds a `finally:`-block '
+        'which executes a checkpoint!'
+        #
+        # ^TODO? other cases?
+    ),
+
+    always_warn_on: tuple[BaseException] = (
+        trio.Cancelled,
+    ),
+    # ^XXX, special case(s) where we warn-log bc likely
+    # there will be no operational diff since the exc
+    # is always expected to be consumed.
+) -> BoxedMaybeException:
+    '''
+    Maybe un-mask and re-raise exception(s) suppressed by a known
+    error-used-as-signal type (cough namely `trio.Cancelled`).
+
+    Though this unmasker targets cancelleds, it can be used more
+    generally to capture and unwrap masked excs detected as
+    `.__context__` values which were suppressed by any error type
+    passed in `unmask_from`.
+
+    -------------
+    STILL-TODO ??
+    -------------
+    -[ ] support for egs which have multiple masked entries in
+        `maybe_eg.exceptions`, in which case we should unmask the
+        individual sub-excs but maintain the eg-parent's form right?
+
+    '''
+    from tractor.devx.debug import (
+        BoxedMaybeException,
+        pause,
+    )
+    boxed_maybe_exc = BoxedMaybeException(
+        raise_on_exit=raise_unmasked,
+    )
+    matching: list[BaseException]|None = None
+    maybe_eg: ExceptionGroup|None
+    maybe_eg: ExceptionGroup|None
+
+    if tn:
+        try:  # handle egs
+            yield boxed_maybe_exc
             return
-
-    try:
-        yield
-    except* unmask_from as be_eg:
-
-        # TODO, if we offer `unmask_from: set`
-        # for masker_exc_type in unmask_from:
-
-        matches, rest = be_eg.split(unmask_from)
+        except* unmask_from as _maybe_eg:
+            maybe_eg = _maybe_eg
+            matches: ExceptionGroup
+            matches, _ = maybe_eg.split(
+                unmask_from
+            )
             if not matches:
                 raise
 
-        for exc_match in be_eg.exceptions:
-            if (
-                (exc_ctx := exc_match.__context__)
-                and
-                type(exc_ctx) not in {
-                    # trio.Cancelled,  # always by default?
-                    unmask_from,
-                }
-            ):
-                exc_ctx.add_note(
-                    f'\n'
-                    f'WARNING: the above error was masked by a {unmask_from!r} !?!\n'
-                    f'Are you always cancelling? Say from a `finally:` ?\n\n'
+            matching: list[BaseException] = matches.exceptions
+    else:
+        try:  # handle non-egs
+            yield boxed_maybe_exc
+            return
+        except unmask_from as _maybe_exc:
+            maybe_exc = _maybe_exc
+            matching: list[BaseException] = [
+                maybe_exc
+            ]
 
-                    f'{tn!r}'
+        # XXX, only unmask-ed for debuggin!
+        # TODO, remove eventually..
+        except BaseException as _berr:
+            berr = _berr
+            await pause(shield=True)
+            raise berr
+
+    if matching is None:
+        raise
+
+    masked: list[tuple[BaseException, BaseException]] = []
+    for exc_match in matching:
+
+        if exc_ctx := find_masked_excs(
+            maybe_masker=exc_match,
+            unmask_from={unmask_from},
+        ):
+            masked.append((exc_ctx, exc_match))
+            boxed_maybe_exc.value = exc_match
+            note: str = (
+                f'\n'
+                f'^^WARNING^^ the above {exc_ctx!r} was masked by a {unmask_from!r}\n'
             )
+            if extra_note:
+                note += (
+                    f'\n'
+                    f'{extra_note}\n'
+                )
+            exc_ctx.add_note(note)
+
+            if type(exc_match) in always_warn_on:
+                log.warning(note)
+
+            # await tractor.pause(shield=True)
+            if raise_unmasked:
+
+                if len(masked) < 2:
                     raise exc_ctx from exc_match
+                else:
+                    # ?TODO, see above but, possibly unmasking sub-exc
+                    # entries if there are > 1
+                    await pause(shield=True)
+    else:
+        raise