Invalid proof when generating proofs concurrently in Zeto test

[Chengxuan]

Example error log when running the test with 2 thread running in parallel:

  1) (Gas cost analysis) Zeto based fungible token with anonymity using nullifiers and encryption with KYC
       Transfer 20 tokens (half from deposit, half from mint) from single address to another
         Alice transfer 20 tokens to Bob in 10 txs:
     Error: VM Exception while processing transaction: reverted with reason string 'Invalid proof'
    at Zeto_AnonEncNullifierKyc.transfer (contracts/zeto_anon_enc_nullifier_kyc.sol:111)
    at <UnrecognizedContract>.<unknown> (0x2279b7a0a67db372996a5fab50d91eaa73d2ebe6)
    at EdrProviderWrapper.request (node_modules/hardhat/src/internal/hardhat-network/provider/provider.ts:431:41)
    at async HardhatEthersSigner.sendTransaction (node_modules/@nomicfoundation/hardhat-ethers/src/signers.ts:125:18)
    at async send (node_modules/ethers/src.ts/contract/contract.ts:313:20)
    at async Proxy.transfer (node_modules/ethers/src.ts/contract/contract.ts:352:16)
    at async sendTx (test/gas_cost/zeto_anon_enc_nullifier_kyc_cost_analysis.ts:557:16)
    at async doTransfer (test/gas_cost/zeto_anon_enc_nullifier_kyc_cost_analysis.ts:452:22)
    at async /Users/cxing/go/src/github.com/hyperledger-labs/zeto/solidity/test/gas_cost/zeto_anon_enc_nullifier_kyc_cost_analysis.ts:362:13
    at async Promise.all (index 6)
    at async Context.<anonymous> (test/gas_cost/zeto_anon_enc_nullifier_kyc_cost_analysis.ts:388:9)

[Chengxuan]

The issue is related to concurrently generating witnesses is not multi-threads safe, added a lock as a workaround https://github.com/hyperledger-labs/zeto/pull/79/files#diff-d9c87e662d919b1fda426b303d12170fd15f677870092c06e8c3062aa8eae21bR518 while working on a solution for this

[Chengxuan]

Closing this issue as it's being addressed by iden3/circom#299