Set ChainService process_block channel size to zero

[eval-exec]

What problem does this PR solve?

On the develop branch, the ChainService's process_block channel has a capacity of DEFAULT_CHANNEL_SIZE(32), but it does not provide any benefit for ckb-sync or ckb-chain: https://github.com/nervosnetwork/ckb/blob/149b5c53e2c6e089d3e4ff6013d1992b56f3b4c6/chain/src/chain.rs#L225

In a scenario where ckb-sync sends a block to ChainService through ChainController::process_block, and at the same time, ChainService receives a Ctrl-C exit signal. Then ChainService's process_block_sender have one ProcessBlockRequest in its buffer:

https://github.com/nervosnetwork/ckb/blob/149b5c53e2c6e089d3e4ff6013d1992b56f3b4c6/chain/src/chain.rs#L41

When self.process_block_sender and self.stop_receiver are ready at the same time, ChainService's select! may choose to execute self.stop_receiver: https://github.com/nervosnetwork/ckb/blob/149b5c53e2c6e089d3e4ff6013d1992b56f3b4c6/chain/src/chain.rs#L237-L272

then the process_block_receiver will be dropped.

However, the process_block_sender is held by the Synchronizer's ChainController, and there is one ProcessBlockRequest in process_block_sender's buffer, causing the ChainController::internal_block_process to block indefinitely.

In that case, Synchronizer::received will block indefinitely at this point: https://github.com/nervosnetwork/ckb/blob/149b5c53e2c6e089d3e4ff6013d1992b56f3b4c6/sync/src/synchronizer/mod.rs#L784

What is changed and how it works?

Related changes

• Set ChainService's process_block channel capacity size to 0.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code ci-runs-only: [ quick_checks,linters ]

Side effects

• None

Release note

Title Only: Include only the PR title in the release note.

[doitian]

Will it have any negative impact?

[eval-exec]

Will it have any negative impact?

There should be no negative impact.

[driftluo]

ref：https://github.com/crossbeam-rs/crossbeam/issues/1102 this is a bug for crossbeam channel