QA: Fix race condition in wallet_encryption test #16420

There is some imprecision probably in the internal HTTPRPCTimer class (haven't exactly figured out where). But we can't expect that waiting excatly 2 seconds right after calling walletpassphrase(2) will result in a locked wallet due to the nature how we internally handle threads/timers.

The wallet_encryption test fails regularely in CIs.

Here is a logged session:

 node0 2019-07-18T18:51:22.569739Z [] ThreadRPCServer method=walletpassphrase user=__cookie__ 
 node0 2019-07-18T18:51:22.628656Z [] queue run of timer lockwallet() in 2 seconds (using HTTP) 
 node0 2019-07-18T18:51:22.629002Z [] Received a POST request for / from 127.0.0.1:46898 
 node0 2019-07-18T18:51:22.629081Z [] ThreadRPCServer method=dumpprivkey user=__cookie__ 
 node0 2019-07-18T18:51:24.445620Z [] Flushing wallet.dat 
 node0 2019-07-18T18:51:24.451421Z [] Flushed wallet.dat 6ms 
 node0 2019-07-18T18:51:24.631703Z [] Received a POST request for / from 127.0.0.1:46898 
 node0 2019-07-18T18:51:24.631737Z [] ThreadRPCServer method=dumpprivkey user=__cookie__ 
 test  2019-07-18T18:51:24.632000Z TestFramework (ERROR): Assertion failed 
                                   Traceback (most recent call last):
                                     File "/home/ubuntu/src/test/functional/test_framework/test_framework.py", line 193, in main
                                       self.run_test()
                                     File "/home/ubuntu/src/test/functional/wallet_encryption.py", line 53, in run_test
                                       assert_raises_rpc_error(-13, "Please enter the wallet passphrase with walletpassphrase first", self.nodes[0].dumpprivkey, address)

ACK 024ecd7e01d436ad7de608e571a20c7284b3951b, simple fix, one second shouldn't hurt.

An alternative is to lower to 1 second the lock timeout in line 48:

self.nodes[0].walletpassphrase(passphrase, 1)

A more robust alternative is to:

• wait_until with a predicate that returns True when walletpassphrase raises an exception;
• and make wait_until return the elapsed time so that the test could assert it is greater than 2 seconds.

ACK 024ecd7e01d436ad7de608e571a20c7284b3951b

ACK 024ecd7e01d436ad7de608e571a20c7284b3951b