Define and pass a default `Flume._readonly: bool`

Allows opening with `.from_msg(readonly=False)` for write permissions
making underlyig shm arrays readonly. Also, make sure to pop the
`ShmArray` field entries prior to msg-ization, not sure how that worked
with the `Feed.flumes` equivalent..but?

piker/data/flows.py

@@ -44,13 +44,13 @@ if TYPE_CHECKING:
 
 class Flume(Struct):
     '''
-    Composite reference type which points to all the addressing handles
+    Composite reference type which points to all the addressing
-    and other meta-data necessary for the read, measure and management
+    handles and other meta-data necessary for the read, measure and
-    of a set of real-time updated data flows.
+    management of a set of real-time updated data flows.
 
     Can be thought of as a "flow descriptor" or "flow frame" which
-    describes the high level properties of a set of data flows that can
+    describes the high level properties of a set of data flows that
-    be used seamlessly across process-memory boundaries.
+    can be used seamlessly across process-memory boundaries.
 
     Each instance's sub-components normally includes:
      - a msg oriented quote stream provided via an IPC transport
@@ -73,6 +73,7 @@ class Flume(Struct):
     # private shm refs loaded dynamically from tokens
     _hist_shm: ShmArray | None = None
     _rt_shm: ShmArray | None = None
+    _readonly: bool = True
 
     stream: tractor.MsgStream | None = None
     izero_hist: int = 0
@@ -89,7 +90,7 @@ class Flume(Struct):
         if self._rt_shm is None:
             self._rt_shm = attach_shm_array(
                 token=self._rt_shm_token,
-                readonly=True,
+                readonly=self._readonly,
             )
 
         return self._rt_shm
@@ -102,12 +103,10 @@ class Flume(Struct):
                 'No shm token has been set for the history buffer?'
             )
 
-        if (
+        if self._hist_shm is None:
-            self._hist_shm is None
-        ):
             self._hist_shm = attach_shm_array(
                 token=self._hist_shm_token,
-                readonly=True,
+                readonly=self._readonly,
             )
 
         return self._hist_shm
@@ -126,10 +125,10 @@ class Flume(Struct):
         period and ratio between them.
 
         '''
-        times = self.hist_shm.array['time']
+        times: np.ndarray = self.hist_shm.array['time']
-        end = pendulum.from_timestamp(times[-1])
+        end: float | int = pendulum.from_timestamp(times[-1])
-        start = pendulum.from_timestamp(times[times != times[-1]][-1])
+        start: float | int = pendulum.from_timestamp(times[times != times[-1]][-1])
-        hist_step_size_s = (end - start).seconds
+        hist_step_size_s: float = (end - start).seconds
 
         times = self.rt_shm.array['time']
         end = pendulum.from_timestamp(times[-1])
@@ -149,17 +148,25 @@ class Flume(Struct):
         msg = self.to_dict()
         msg['mkt'] = self.mkt.to_dict()
 
-        # can't serialize the stream or feed objects, it's expected
+        # NOTE: pop all un-msg-serializable fields:
-        # you'll have a ref to it since this msg should be rxed on
+        # - `tractor.MsgStream`
-        # a stream on whatever far end IPC..
+        # - `Feed`
+        # - `Shmarray`
+        # it's expected the `.from_msg()` on the other side
+        # will get instead some kind of msg-compat version
+        # that it can load.
         msg.pop('stream')
         msg.pop('feed')
+        msg.pop('_rt_shm')
+        msg.pop('_hist_shm')
+
         return msg
 
     @classmethod
     def from_msg(
         cls,
         msg: dict,
+        readonly: bool = True,
 
     ) -> dict:
         '''
@@ -170,7 +177,11 @@ class Flume(Struct):
         mkt_msg = msg.pop('mkt')
         from ..accounting import MktPair  # cycle otherwise..
         mkt = MktPair.from_msg(mkt_msg)
-        return cls(mkt=mkt, **msg)
+        msg |= {'_readonly': readonly}
+        return cls(
+            mkt=mkt,
+            **msg,
+        )
 
     def get_index(
         self,