Should support alternative implementations of zlib

[joshtriplett]

libz-sys should have a feature to use an alternative zlib implementation, such as zlib-ng. That would reduce the problem such alternative implementations currently have, of potentially ending up with two conflicting versions of zlib in the same binary. (This could still happen if linking to a system library that links to zlib, though.)

[brainstorm]

Indeed, would you accept repointing of the submodule to this implementation instead?:

https://blog.cloudflare.com/cloudflare-fights-cancer/
https://github.com/cloudflare/zlib

I think this would speed up a few crates that use libz-sys significantly "for free" (see benchmark plots on the blogpost).

[brainstorm]

For now I'll try to use this crate: https://crates.io/crates/cloudflare-zlib-sys

[joshtriplett]

@brainstorm Not "repointing", no; cloudflare's zlib has portability issues. It also isn't the highest performance; see GitoxideLabs/gitoxide#1 (comment) for some informal benchmarks.

[brainstorm]

Super interesting, thanks for sharing the benchmarks! I'll try to carve some time to give zlib-ng a go (according to 754ca14) ... I hope it's not x86_64-unknown-linux-musl problematic, I've not seen the target in appveyor.yml. Thanks for your reply and insight in any case!

[joshtriplett]

@brainstorm It should work fine on musl. Please give it a try and let me know if it works.

[joshtriplett]

@brainstorm Currently fixing some issues; new version shortly, which I've now confirmed works on musl (and which will test musl in CI).

[joshtriplett]

Done and released in libz-sys 1.1.0.

[brainstorm]

@brainstorm It should work fine on musl. Please give it a try and let me know if it works.

Getting an interesting segfault on our CI correction: just an assert failure about some boundary assumptions made on bgzf.c:461 from htslib so I'll have to look closer at zlib-ng and that assert:

https://github.com/rust-bio/rust-htslib/pull/237/checks?check_run_id=1024717376#step:7:1962

Will investigate on rust-bio/rust-htslib's PR.

[joshtriplett]

@brainstorm Ah, interesting. Yeah, that assert isn't valid with an alternate zlib implementation; compressBound is allowed to return a different value. It's a worst-case bound, not a normal case.