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
@@ -46,26 +46,33 @@ _default_size = 10 * _secs_in_day
 _rt_buffer_start = int(9*_secs_in_day)
 
 
-# Tell the "resource tracker" thing to fuck off.
-class ManTracker(mantracker.ResourceTracker):
-    def register(self, name, rtype):
-        pass
+def cuckoff_mantracker():
 
-    def unregister(self, name, rtype):
-        pass
+    from multiprocessing import resource_tracker as mantracker
 
-    def ensure_running(self):
-        pass
+    # Tell the "resource tracker" thing to fuck off.
+    class ManTracker(mantracker.ResourceTracker):
+        def register(self, name, rtype):
+            pass
+
+        def unregister(self, name, rtype):
+            pass
+
+        def ensure_running(self):
+            pass
 
 
-# "know your land and know your prey"
-# https://www.dailymotion.com/video/x6ozzco
-mantracker._resource_tracker = ManTracker()
-mantracker.register = mantracker._resource_tracker.register
-mantracker.ensure_running = mantracker._resource_tracker.ensure_running
-ensure_running = mantracker._resource_tracker.ensure_running
-mantracker.unregister = mantracker._resource_tracker.unregister
-mantracker.getfd = mantracker._resource_tracker.getfd
+    # "know your land and know your prey"
+    # https://www.dailymotion.com/video/x6ozzco
+    mantracker._resource_tracker = ManTracker()
+    mantracker.register = mantracker._resource_tracker.register
+    mantracker.ensure_running = mantracker._resource_tracker.ensure_running
+    ensure_running = mantracker._resource_tracker.ensure_running
+    mantracker.unregister = mantracker._resource_tracker.unregister
+    mantracker.getfd = mantracker._resource_tracker.getfd
+
+
+cuckoff_mantracker()
 
 
 class SharedInt:
@@ -191,7 +198,11 @@ class ShmArray:
         self._post_init: bool = False
 
         # pushing data does not write the index (aka primary key)
-        self._write_fields = list(shmarr.dtype.fields.keys())[1:]
+        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))