blockservice & exchange & bitswap: add non variadic NotifyNewBlock

[Jorropo]

Variadicts in go are just syntactic sugar around passing a slice, that means all go memory reachability rules apply, this force the compiler to heap allocate the variadic slice for virtual call, because the implementation is allowed to leak the slice (and go's interprocedural optimisations do not cover virtuals).

Passing a block without variadic will pass the itab either on the stack or decomposed through registers. Skipping having to allocate a slice.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.25%. Comparing base (171b0b7) to head (73226d5).

Files with missing lines	Patch %	Lines
blockservice/blockservice.go	50.00%	1 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #242      +/-   ##
==========================================
+ Coverage   60.18%   60.25%   +0.07%     
==========================================
  Files         241      241              
  Lines       30707    30722      +15     
==========================================
+ Hits        18482    18513      +31     
+ Misses      10574    10561      -13     
+ Partials     1651     1648       -3     

Files with missing lines	Coverage Δ
bitswap/bitswap.go	69.23% <100.00%> (+1.78%)	⬆️
bitswap/client/client.go	88.34% <100.00%> (+0.22%)	⬆️
bitswap/server/server.go	66.57% <100.00%> (+1.93%)	⬆️
exchange/offline/offline.go	88.88% <100.00%> (+1.01%)	⬆️
blockservice/blockservice.go	78.00% <50.00%> (-0.23%)	⬇️

... and 8 files with indirect coverage changes

[aschmahmann]

Any idea how widespread the dependency on this interface (and therefore the result of the breakage) is and the benefits of implementing this? For example, does this show up at all in profiles as a meaningful thing?

[Jorropo]

I think everyone relies either on the offline exchange (in order to wrap a blockstore) or bitswap. I don't belive many implementations exists, even in outside codebases.

Even if someone else implement this, just forwarding the call to NotifyNewBlocks with a slice of 1, this is legal and easy to write.
This just aims to get over golang's lacking interprocedural virtual optimisations, it does not change any feature.

Most of our other APIs have both single and multiple versions for this reason (Put vs PutMany, GetBlock vs GetBlocks, ...).

[BigLep]

@Jorropo : I don't see an issue linked to this PR. In the absence of this, can you share more on what's motivating this PR?

[Jorropo]

@BigLep the PR and Commit bodies:

Variadicts in go are just syntactic sugar around passing a slice, that means all go memory reachability rules apply, this force the compiler to heap allocate the variadic slice for virtual call, because the implementation is allowed to leak the slice (and go's interprocedural optimisations do not cover virtuals).
Passing a block without variadic will pass the itab either on the stack or decomposed through registers. Skipping having to allocate a slice.

I can add the explanation about keeping it in-line with our other exchange APIs.

---

This seems like a good usecase to try the idea @guseggert is cooking (some automated code migrator), that would add the NotifyNewBlock method (which redirects to NotifyNewBlocks) automatically if you run it.

[hacdias]

I don't have strong opinions on this. It seems a relatively small change that can bring memory benefits - although arguably low. If others agree, I'm fine to have it merged.

FYSA: I rebased the PR and added the updated exchange for the gateway examples.

[BigLep]

2023-04-06 conversation: will defer for now. Want to have clarity on how we handle breaking changes in Boxo.

[gammazero]

This is ready to merge. Let's decide if we want this or not.