[Bug]: `Prelude.init: empty list` while shrinking

[rappie]

Describe the issue:

I'm shrinking about 10 optimization functions with seqLen 250 and 16 workers. Once all the workers are stopped, 1 shrink iteration is performed and then after a while I get this error.

Code example to reproduce the issue:

The repo is private 😞

Version:

Echidna 2.2.5 (built today from master)

Relevant log output:

[rappie]

Commenting out all optimization functions except 1 fixes this.

I'm guessing it might be some kind of timing issue, because with more optimization functions enabled Echidna as a whole is very slow (for example: scrolling)

[elopez]

Reproducer:

contract C {

function echidna_opt_foo() public returns (int256) {
  return 0;
}
}

% echidna a.sol --test-mode optimization --format text
[2024-10-17 11:06:34.64] Compiling a.sol... Done! (0.300388s)
Analyzing contract: a.sol:C
[2024-10-17 11:06:34.94] Running slither on a.sol... Done! (0.365533s)
[2024-10-17 11:06:35.31] [Worker 3] New maximum value of echidna_opt_foo: 0
[2024-10-17 11:06:35.31] [Worker 1] New maximum value of echidna_opt_foo: 0
[2024-10-17 11:06:35.31] [Worker 2] New maximum value of echidna_opt_foo: 0
[2024-10-17 11:06:35.31] [Worker 0] New maximum value of echidna_opt_foo: 0
[2024-10-17 11:06:35.31] [Worker 3] New coverage: 45 instr, 1 contracts, 1 seqs in corpus
[2024-10-17 11:06:35.31] [Worker 1] New coverage: 45 instr, 1 contracts, 2 seqs in corpus
[2024-10-17 11:06:35.32] [Worker 0] New coverage: 48 instr, 1 contracts, 3 seqs in corpus
[2024-10-17 11:06:35.63] [Worker 2] Crashed:

Prelude.init: empty list
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/List.hs:1644:3 in base:GHC.List
  errorEmptyList, called at libraries/base/GHC/List.hs:180:28 in base:GHC.List
  init, called at lib/Echidna/Shrink.hs:76:21 in echidna-2.2.5-Ku1bMKy38m038Xfqf0w2hr:Echidna.Shrink

Please report it to https://github.com/crytic/echidna/issues
[2024-10-17 11:06:35.63] [Worker 3] Test limit reached. Stopping.
[2024-10-17 11:06:35.63] [Worker 1] Test limit reached. Stopping.
[2024-10-17 11:06:35.63] [Worker 0] Test limit reached. Stopping.
[2024-10-17 11:06:35.63] [status] tests: 1/1, fuzzing: 50084/50000, values: [0], cov: 48, corpus: 3
echidna_opt_foo: max value: 0
Call sequence:
(no transactions)


Unique instructions: 48
Unique codehashes: 1
Corpus size: 3
Seed: 4148347590236040080

[GalloDaSballo]

Am running into the same issue, when this happens the rest of the shrinking seems to stop working

Commenting out these "empty list" sequences, seems to keep shrinking going (intended goal)

I believe there must be a bug