Fix out of bounds error in the resampler

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.

Signed-off-by: Leandro Lucarella <[email protected]>

RELEASE_NOTES.md

@@ -14,4 +14,4 @@
 
 ## Bug Fixes
 
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+- 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

@@ -495,8 +495,13 @@ 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.
+            resamplers = self._resamplers.copy()
             results = await asyncio.gather(
-                *[r.resample(self._window_end) for r in self._resamplers.values()],
+                *[r.resample(self._window_end) for r in resamplers.values()],
                 return_exceptions=True,
             )
 
@@ -508,7 +513,7 @@ async def resample(self, *, one_shot: bool = False) -> None:
                 dict[Source, Exception | asyncio.CancelledError],
                 {
                     source: results[i]
-                    for i, source in enumerate(self._resamplers)
+                    for i, source in enumerate(resamplers)
                     # CancelledError inherits from BaseException, but we don't want
                     # to catch *all* BaseExceptions here.
                     if isinstance(results[i], (Exception, asyncio.CancelledError))