fix: don't sync block filter hashes if finalized checkpoint is too less than the last proved header

[yangby-cryptape]

Why < self.check_point_interval * 2?

ckb-light-client/src/protocols/light_client/peers.rs

Lines 539 to 541 in fc45e35

	fn if_require_next_check_point(&self, last_proved_number: BlockNumber) -> bool {
	self.number_of_next_check_point() + self.check_point_interval * 2 <= last_proved_number
	}

Because light client don't finalize the latest checkpoint, light client will keep a checkpoint as a safe distance, so it doesn't have to rollback such many blocks even there is a fork. (N.B. Light client won't handle a long fork.)

• If the last number is $N$, denote check_point_interval as $I$, light client will use the $(| \frac{N-I+1}{I}| - 1)\mathrm{-th}$ checkpoint as finalized checkpoint if there are more than $\frac{1}{2}\mathrm{MAX\_OUTBOUND\_PEERS}$ peers have this checkpoint.
• Before the finalized checkpoint (include), light client will only sync all checkpoints.
• After the finalized checkpoint, light client will sync all block filter hashes.

So, as an approximated value, I use < self.check_point_interval * 2 here.

Ref: the 2nd issue in #168 (comment).

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (fc45e35) 71.41% compared to head (82a46db) 71.39%.

Files	Patch %	Lines
src/protocols/light_client/peers.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #169      +/-   ##
===========================================
- Coverage    71.41%   71.39%   -0.02%     
===========================================
  Files           25       25              
  Lines         6478     6479       +1     
===========================================
  Hits          4626     4626              
- Misses        1852     1853       +1     

Flag	Coverage Δ
unittests	71.39% <50.00%> (-0.02%)	⬇️

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