Add `ShmArray.ustruct()`: return an unstructured array copy

We return a copy (since since a view doesn't seem to work..) of the
(field filtered) shm array contents which is the same index-length as
the source data.

Further, fence off the resource tracker disable-hack into a helper
routine.

piker/data/_sharedmem.py

@@ -22,7 +22,6 @@ from __future__ import annotations
 from sys import byteorder
 from typing import Optional
 from multiprocessing.shared_memory import SharedMemory, _USE_POSIX
-from multiprocessing import resource_tracker as mantracker
 
 if _USE_POSIX:
     from _posixshmem import shm_unlink
@@ -30,6 +29,7 @@ if _USE_POSIX:
 import tractor
 import numpy as np
 from pydantic import BaseModel
+from numpy.lib import recfunctions as rfn
 
 from ..log import get_logger
 from ._source import base_iohlc_dtype
@@ -46,6 +46,10 @@ _default_size = 10 * _secs_in_day
 _rt_buffer_start = int(9*_secs_in_day)
 
 
+def cuckoff_mantracker():
+
+    from multiprocessing import resource_tracker as mantracker
+
     # Tell the "resource tracker" thing to fuck off.
     class ManTracker(mantracker.ResourceTracker):
         def register(self, name, rtype):
@@ -68,6 +72,9 @@ mantracker.unregister = mantracker._resource_tracker.unregister
     mantracker.getfd = mantracker._resource_tracker.getfd
 
 
+cuckoff_mantracker()
+
+
 class SharedInt:
     """Wrapper around a single entry shared memory array which
     holds an ``int`` value used as an index counter.
@@ -191,7 +198,11 @@ class ShmArray:
         self._post_init: bool = False
 
         # pushing data does not write the index (aka primary key)
+        dtype = shmarr.dtype
+        if dtype.fields:
             self._write_fields = list(shmarr.dtype.fields.keys())[1:]
+        else:
+            self._write_fields = None
 
     # TODO: ringbuf api?
 
@@ -237,6 +248,48 @@ class ShmArray:
 
         return a
 
+    def ustruct(
+        self,
+        fields: Optional[list[str]] = None,
+
+        # type that all field values will be cast to
+        # in the returned view.
+        common_dtype: np.dtype = np.float,
+
+    ) -> np.ndarray:
+
+        array = self._array
+
+        if fields:
+            selection = array[fields]
+            fcount = len(fields)
+        else:
+            selection = array
+            fcount = len(array.dtype.fields)
+
+        # XXX: manual ``.view()`` attempt that also doesn't work.
+        # uview = selection.view(
+        #     dtype='<f16',
+        # ).reshape(-1, 4, order='A')
+
+        # assert len(selection) == len(uview)
+
+        u = rfn.structured_to_unstructured(
+            selection,
+            # dtype=float,
+            copy=True,
+        )
+
+        # unstruct = np.ndarray(u.shape, dtype=a.dtype, buffer=shm.buf)
+        # array[:] = a[:]
+        return u
+        # return ShmArray(
+        #     shmarr=u,
+        #     first=self._first,
+        #     last=self._last,
+        #     shm=self._shm
+        # )
+
     def last(
         self,
         length: int = 1,
@@ -386,7 +439,11 @@ def open_shm_array(
         create=True,
         size=a.nbytes
     )
-    array = np.ndarray(a.shape, dtype=a.dtype, buffer=shm.buf)
+    array = np.ndarray(
+        a.shape,
+        dtype=a.dtype,
+        buffer=shm.buf
+    )
     array[:] = a[:]
     array.setflags(write=int(not readonly))