Intermittent failure in feature_fee_estimation.py in sanity_check_rbf_estimates: est_feerate = node.estimatesmartfee(2)["feerate"] (KeyError: 'feerate') #30640

https://cirrus-ci.com/task/5165357323780096?logs=ci#L2840

 test  2024-08-09T22:10:43.283000Z TestFramework (ERROR): Key error 
                                   Traceback (most recent call last):
                                     File "/ci_container_base/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/test_framework/test_framework.py", line 132, in main
                                       self.run_test()
                                     File "/ci_container_base/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/feature_fee_estimation.py", line 468, in run_test
                                       self.sanity_check_rbf_estimates(self.confutxo + self.memutxo)
                                     File "/ci_container_base/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/feature_fee_estimation.py", line 302, in sanity_check_rbf_estimates
                                       est_feerate = node.estimatesmartfee(2)["feerate"]
                                   KeyError: 'feerate'

I read the log from https://api.cirrus-ci.com/v1/task/5165357323780096/logs/ci.log and tried to recreate this intermittent failure.

cmake -B build -DWITH_ZMQ=ON -DREDUCE_EXPORTS=ON -DCMAKE_C_FLAGS="-g0 -O2 -funsigned-char" -DCMAKE_CXX_FLAGS="-g0 -O2 -funsigned-char -DBOOST_MULTI_INDEX_ENABLE_SAFE_MODE"

I used the seed where the failure occurred was but it passes locally:

<details> <summary>logs</summary>

build/test/functional/feature_fee_estimation.py  --randomseed=7115946170308495203 --timeout-factor=40
2024-09-14T12:50:36.417000Z TestFramework (INFO): User supplied random seed 7115946170308495203
2024-09-14T12:50:36.417000Z TestFramework (INFO): PRNG seed is: 7115946170308495203
2024-09-14T12:50:36.417000Z TestFramework (INFO): Initializing test directory /var/folders/dj/d8p8jhd172n7wnq81ryfl6rc0000gn/T/bitcoin_func_test_hnob35wg
2024-09-14T12:50:36.455000Z TestFramework (INFO): This test is time-consuming, please be patient.
2024-09-14T12:50:36.455000Z TestFramework (INFO): Splitting inputs so we can generate transactions
2024-09-14T12:50:38.091000Z TestFramework (INFO): Finished splitting
2024-09-14T12:50:38.906000Z TestFramework (INFO): Testing estimates with single transactions.
2024-09-14T12:50:38.907000Z TestFramework (INFO): Will output estimates for 1/2/3/6/15/25 blocks
2024-09-14T12:50:38.907000Z TestFramework (INFO): Creating transactions and mining them with a block size that can't keep up
2024-09-14T12:50:42.144000Z TestFramework (INFO): Creating transactions and mining them at a block size that is just big enough
2024-09-14T12:50:47.583000Z TestFramework (INFO): Creating transactions and mining them with a block size that can't keep up
2024-09-14T12:50:51.960000Z TestFramework (INFO): Creating transactions and mining them at a block size that is just big enough
2024-09-14T12:50:57.870000Z TestFramework (INFO): Final estimates after emptying mempools
2024-09-14T12:50:57.891000Z TestFramework (INFO): Test fee_estimates.dat is flushed periodically
2024-09-14T12:50:58.739000Z TestFramework (INFO): Testing fee rate estimation after restarting the node with high `MempoolMinFee`
2024-09-14T12:50:59.555000Z TestFramework (INFO): Testing the `acceptstalefeeestimates` option
2024-09-14T12:50:59.920000Z TestFramework (INFO): Testing reading old fee_estimates.dat
2024-09-14T12:51:00.657000Z TestFramework (INFO): Restarting node with fresh estimation
2024-09-14T12:51:01.312000Z TestFramework (INFO): Testing estimates with RBF.
2024-09-14T12:51:03.329000Z TestFramework (INFO): Restarting node with fresh estimation
2024-09-14T12:51:03.868000Z TestFramework (INFO): Testing `estimatesmartfee` modes
2024-09-14T12:51:22.243000Z TestFramework (INFO): Testing that fee estimation is disabled in `blocksonly`
2024-09-14T12:51:22.720000Z TestFramework (INFO): Stopping nodes
2024-09-14T12:51:22.830000Z TestFramework (INFO): Cleaning up /var/folders/dj/d8p8jhd172n7wnq81ryfl6rc0000gn/T/bitcoin_func_test_hnob35wg on exit
2024-09-14T12:51:22.830000Z TestFramework (INFO): Tests successful

</details>

My initial guess was that the sync_mempools wait time in sanity_check_rbf_estimates was too short, and the miner node didn't finish syncing. But this was not the case because the test still passes after commenting the syncs out.