Fix IndexError: list index out of range in resampler (#1117)

When awaiting the resamplers to finish resampling, we need to make a
copy of the resamplers dict, because new resamplers could be added or
removed while we are awaiting, which would cause the results to be out
of sync.

Here is a traceback of an instance of this bug:

```
2024-11-25 12:45:26,052,52 WARNING  [_broadcast.py:416] Broadcast receiver [Broadcast:GrpcStreamBroadcaster-raw-component-data-2006:_Receiver] is full. Oldest message was dropped.
2024-11-25 12:45:26,052,52 ERROR    [_resampling.py:195] The resample() function got an unexpected error, restarting...
Traceback (most recent call last):
  File "/home/marenz/frequenz/sdk/src/frequenz/sdk/microgrid/_resampling.py", line 179, in _log_resampling_task_error
    resampling_task.result()
  File "/home/marenz/frequenz/sdk/src/frequenz/sdk/timeseries/_resampling.py", line 509, in resample
    {
  File "/home/marenz/frequenz/sdk/src/frequenz/sdk/timeseries/_resampling.py", line 514, in <dictcomp>
    if isinstance(results[i], (Exception, asyncio.CancelledError))
                  ~~~~~~~^^^
IndexError: list index out of range
```

Author: llucax

RELEASE_NOTES.md

@@ -1,6 +1,17 @@
 # Frequenz Python SDK Release Notes
 
+## Summary
+
+<!-- Here goes a general summary of what this release is about -->
+
+## Upgrading
+
+<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+
+## New Features
+
+<!-- Here goes the main new features and examples or instructions on how to use them -->
+
 ## Bug Fixes
 
-* Fix bug with `LoggingConfigUpdater` not updating root logger level.
-* The `frequenz-quantities` dependency requirement was widened to allow any v1.x version (it was pinned to `1.0.0rc3` before).
+- Fix a bug in the resampler that could end up with an *IndexError: list index out of range* exception when a new resampler was added while awaiting the existing resampler to finish resampling.

src/frequenz/sdk/timeseries/_resampling.py

@@ -14,7 +14,6 @@
 from collections.abc import AsyncIterator, Callable, Coroutine, Sequence
 from dataclasses import dataclass
 from datetime import datetime, timedelta, timezone
-from typing import cast
 
 from frequenz.channels.timer import Timer, TriggerAllMissed, _to_microseconds
 from frequenz.quantities import Quantity
@@ -495,25 +494,24 @@ async def resample(self, *, one_shot: bool = False) -> None:
                     self._config.resampling_period,
                 )
 
+            # We need to make a copy here because we need to match the results to the
+            # current resamplers, and since we await here, new resamplers could be added
+            # or removed from the dict while we awaiting the resampling, which would
+            # cause the results to be out of sync.
+            resampler_sources = list(self._resamplers)
             results = await asyncio.gather(
                 *[r.resample(self._window_end) for r in self._resamplers.values()],
                 return_exceptions=True,
             )
 
             self._window_end += self._config.resampling_period
-            # We need the cast because mypy is not able to infer that this can only
-            # contain Exception | CancelledError because of the condition in the list
-            # comprehension below.
-            exceptions = cast(
-                dict[Source, Exception | asyncio.CancelledError],
-                {
-                    source: results[i]
-                    for i, source in enumerate(self._resamplers)
-                    # CancelledError inherits from BaseException, but we don't want
-                    # to catch *all* BaseExceptions here.
-                    if isinstance(results[i], (Exception, asyncio.CancelledError))
-                },
-            )
+            exceptions = {
+                source: result
+                for source, result in zip(resampler_sources, results)
+                # CancelledError inherits from BaseException, but we don't want
+                # to catch *all* BaseExceptions here.
+                if isinstance(result, (Exception, asyncio.CancelledError))
+            }
             if exceptions:
                 raise ResamplingError(exceptions)
             if one_shot: