if an index error occurs while fetching a node, return master node

[zakaf]

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Description of change

Please provide a description of the change here.

When an AWS Elasticache shard is added to an existing cluster, below exception occurs in redis-py.

Traceback (most recent call last):
  ...
  File "/usr/lib/python3.8-pyston2.3/site-packages/redis/cluster.py", line 876, in _determine_nodes
    node = self.nodes_manager.get_node_from_slot(
  File "/usr/lib/python3.8-pyston2.3/site-packages/redis/cluster.py", line 1382, in get_node_from_slot
    return self.slots_cache[slot][node_idx]
IndexError: list index out of range

The above exception seems to occur, when slots_cache changes dynamically, while get_node_from_slot is being executed.
In order to avoid the above exception, return the primary node, when an IndexError occurs

[codecov-commenter]

Codecov Report

Base: 92.28% // Head: 92.25% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (01c3251) compared to base (428d609).
Patch coverage: 50.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2575      +/-   ##
==========================================
- Coverage   92.28%   92.25%   -0.04%     
==========================================
  Files         115      115              
  Lines       29660    29663       +3     
==========================================
- Hits        27373    27365       -8     
- Misses       2287     2298      +11     

Impacted Files	Coverage Δ
redis/cluster.py	90.22% <50.00%> (-0.21%)	⬇️
tests/test_asyncio/test_search.py	98.29% <0.00%> (-0.35%)	⬇️
redis/asyncio/cluster.py	91.68% <0.00%> (-0.33%)	⬇️
tests/test_asyncio/test_pubsub.py	99.37% <0.00%> (-0.16%)	⬇️
tests/test_asyncio/test_cluster.py	97.48% <0.00%> (-0.13%)	⬇️
tests/test_cluster.py	96.75% <0.00%> (-0.11%)	⬇️

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[zakaf]

@dvora-h @chayim if you guys have spare time, could you take a look at this PR?

[github-actions]

This pull request is marked stale. It will be closed in 30 days if it is not updated.

[gordo-veda]

This is still a problem... the code around here should be hardened such that we throw a proper error to handle resharding, or in this case, trying another node seems harmless (since the node will point you elsewhere if it doesn't have your slot).

Is it safe to assume the zeroth node is the primary? (maybe not) but finding any working node that can point you to the right slot is i likely good enough.