DM-33207: improve reading historical data for an indexed SAL component read with index=0 (meaning "all instances")

doc/version_history.rst

@@ -11,6 +11,11 @@ v6.9.0
 * Use the new `parse_idl_file` and `make_dds_topic_class` functions in ADLink's ``ddsutil.py``, instead of our versions.
   This change requires ts-dds version 6.9 (community) or 6.10 (licensed) build 18.
 * Remove deprecated support for environment variable ``LSST_DDS_DOMAIN``.
+* `Remote` and `SalInfo`: improve retrieval of historical data in one special case:
+  reading an indexed SAL component using index=0 in the `Remote` (meaning "read data from all indices").
+  Formerly there would be only 1 sample of historical data: the most recent sample output with any index.
+  Now retrieve the most recent sample _for each index_, in the order received.
+
 
 Requirements:
 

python/lsst/ts/salobj/sal_info.py

@@ -682,21 +682,34 @@ async def start(self) -> None:
                                 "giving up"
                             ) from e
                     self.log.debug(f"Read {len(data_list)} history items for {reader}")
-                    sd_list = [
+                    # All historical data for the specified index
+                    full_sd_list = [
                         self._sample_to_data(sd, si)
                         for sd, si in data_list
                         if si.valid_data
                     ]
-                    if len(sd_list) < len(data_list):
-                        ninvalid = len(data_list) - len(sd_list)
+                    if len(full_sd_list) < len(data_list):
+                        ninvalid = len(data_list) - len(full_sd_list)
                         self.log.warning(
                             f"Read {ninvalid} invalid late-joiner items from {reader}. "
                             "The invalid items were safely skipped, but please examine "
                             "the code in SalInfo.start to see if it needs an update "
                             "for changes to OpenSplice dds."
                         )
                     if reader.max_history > 0:
-                        sd_list = sd_list[-reader.max_history :]
+                        if self.index == 0 and self.indexed:
+                            # Get the most recent sample for each index
+                            index_field = f"{self.name}ID"
+                            data_dict = {
+                                getattr(data, index_field): data
+                                for data in full_sd_list
+                            }
+                            sd_list: typing.Collection[
+                                type_hints.BaseDdsDataType
+                            ] = data_dict.values()
+                        else:
+                            # Get the max_history most recent samples
+                            sd_list = full_sd_list[-reader.max_history :]
                         if sd_list:
                             reader._queue_data(sd_list, loop=None)
             self._read_loop_task = asyncio.create_task(self._read_loop(loop=loop))

python/lsst/ts/salobj/topics/read_topic.py

@@ -191,8 +191,15 @@ class ReadTopic(BaseTopic):
     max_history : `int`
         Maximum number of historical items to read:
 
-        * 0 is required for commands, events, and the ackcmd topic
-        * 1 is recommended for telemetry
+        * 0 is required for commands, events, and the ackcmd topic.
+        * 1 is recommended for telemetry. For an indexed component
+          it is possible for data from one index to push data for another
+          index off the DDS queue, so historical data is not guaranteed.
+        * For the special case of reading an indexed SAL component
+          with index=0 (read all indices) the only allowed values are 0 or 1.
+          If 1 then retrieve the most recent sample for each index
+          that is still in the read queue, in the order received.
+          max_history > 1 is forbidden, because it is difficult to implement.
     queue_len : `int`, optional
         The maximum number of messages that can be read and not dealt with
         by a callback function or `next` before older messages will be dropped.
@@ -212,6 +219,10 @@ class ReadTopic(BaseTopic):
         If queue_len < MIN_QUEUE_LEN.
     ValueError
         If max_history > queue_len.
+    ValueError
+        If for an indexed component if index=0 and max_history > 1.
+        Reading more than one historical sample per index is more trouble
+        than it is worth.
     UserWarning
         If max_history > DDS history queue depth or DDS durability service
         history depth for this topic.
@@ -283,6 +294,11 @@ def __init__(
             raise ValueError(f"max_history={max_history} must be >= 0")
         if max_history > 0 and self.volatile:
             raise ValueError(f"max_history={max_history} must be 0 for volatile topics")
+        if salinfo.indexed and salinfo.index == 0 and max_history > 1:
+            raise ValueError(
+                f"max_history={max_history} must be 0 or 1 "
+                "for an indexed component read with index=0"
+            )
         if queue_len <= MIN_QUEUE_LEN:
             raise ValueError(
                 f"queue_len={queue_len} must be >= MIN_QUEUE_LEN={MIN_QUEUE_LEN}"
@@ -711,14 +727,14 @@ async def _run_callback(self, data: type_hints.BaseDdsDataType) -> None:
 
     def _queue_data(
         self,
-        data_list: typing.Sequence[type_hints.BaseDdsDataType],
+        data_list: typing.Collection[type_hints.BaseDdsDataType],
         loop: typing.Optional[asyncio.AbstractEventLoop],
     ) -> None:
         """Queue multiple one or more messages.
 
         Parameters
         ----------
-        data_list : `list` [dds_messages]
+        data_list : typing.Collection[type_hints.BaseDdsDataType]
             DDS messages to be queueued.
         loop : `asyncio.AbstractEventLoop` or `None`
             Foreground asyncio loop.

tests/test_topics.py

@@ -1248,6 +1248,18 @@ async def test_read_topic_constructor_errors_and_warnings(self) -> None:
                         queue_len=warn_max_history + MIN_QUEUE_LEN,
                     )
 
+            # max_history can only be 0 or 1 with index=0
+            # for an indexed component
+            salinfo0 = salobj.SalInfo(domain=domain, name="Test", index=0)
+            for bad_max_history in (-1, 2, 3, 10):
+                with pytest.raises(ValueError):
+                    salobj.topics.ReadTopic(
+                        salinfo=salinfo0,
+                        name="scalars",
+                        sal_prefix="logevent_",
+                        max_history=bad_max_history,
+                    )
+
     async def test_asynchronous_event_callback(self) -> None:
         async with self.make_csc(initial_state=salobj.State.ENABLED):
             cmd_scalars_data = self.csc.make_random_cmd_scalars()
@@ -1381,7 +1393,7 @@ async def test_partitions(self) -> None:
             assert read_codes2 == expected_codes2
 
     async def test_sal_index(self) -> None:
-        """Test separation of data using SAL index.
+        """Test separation of data using SAL index, including historical data.
 
         Readers with index=0 should see data from all writers of that topic,
         regardless of index.
@@ -1397,7 +1409,7 @@ async def test_sal_index(self) -> None:
             writer2 = salobj.topics.ControllerEvent(salinfo=salinfo2, name="errorCode")
 
             # write late joiner data (before we have readers);
-            # only the last value should be seen
+            # only the last value for each index should be seen
             for i in (3, 4, 5):
                 writer0.set_put(errorCode=i)
                 await asyncio.sleep(0.01)
@@ -1430,7 +1442,7 @@ async def test_sal_index(self) -> None:
             read_codes0 = []
             read_codes1 = []
             read_codes2 = []
-            expected_codes0 = [25, 6, 16, 26, 7, 17, 27, 8, 18, 28]
+            expected_codes0 = [5, 15, 25, 6, 16, 26, 7, 17, 27, 8, 18, 28]
             expected_codes1 = [15, 16, 17, 18]
             expected_codes2 = [25, 26, 27, 28]
             try: