Contracts: migration v15 acc dec_consumers check

[pgherveou]

Current migration step could produce error logs
Logic error: Unexpected underflow in reducing consumer

Tested with

 RUST_LOG="runtime::contracts=info,try-runtime::cli=debug" try-polkadot-parachain try-runtime \
 --runtime target/release/wbuild/contracts-rococo-runtime/contracts_rococo_runtime.wasm \
 --chain contracts-rococo \
 on-runtime-upgrade snap --snapshot-path ./contracts-rococo-9420@latest.snap

Patch generated with

RUST_LOG="try-runtime::cli=debug" try-polkadot-parachain try-runtime \
 --runtime existing \
 --chain contracts-rococo \
create-snapshot \
--uri wss://rococo-contracts-rpc.polkadot.io:443

[pgherveou]

@juangirini any idea why the consumer refcount could be 0 pre-migration in some cases?

[juangirini]

Please see paritytech/substrate#14589 (comment), we don't need such a check. If we can't decrease the consumers then there is an error and it is ok to print it. Anyway the flow continues and the rest executes correctly, the only thing that it is done with the conditional is to hide the error.

@juangirini any idea why the consumer refcount could be 0 pre-migration in some cases?

The error was introduced on v10 when the deposit account was created without incrementing the consumers. As the deposits accounts don't exist anymore it is ok not to fix that v10 bug as is not relevant anymore.

We should close this PR

[pgherveou]

Thanks Juan, maybe we should add a comment on the code to explain what you just said. Afk but will update later