Enable sccache for Servo CI

[luser]

I've been testing sccache with Servo lately, and it's working pretty well for caching crate compilation. Details are available on this users.rlo thread. I needed to make one change to an upstream crate, and I finally submitted a PR to update that dependency which has now been merged.

Unless large chunks of the dependency graph change frequently I would expect a pretty good cache hit rate, which should save a fair amount of time for CI builds. Additionally, once we get S3 buckets configured we can set [build] ccache = 'sccache' and use sccache to cache the C++ compilation from the build, which may produce better results than using ccache, since multiple builders can share the same cache.

cc/ @alexcrichton @edunham @larsbergstrom

[aneeshusa]

Using environment variables is easier than updating the .servobuild file for CI. Will installing sccache and setting RUSTC_WRAPPER=sccache work? Those would both happen in saltfs.

[luser]

That will make it work for caching rust compiles, but not C++ compiles.

[larsbergstrom]

OMG this is amazing - thank you so much for doing this, including the testing, @luser!

One question: do you know how well this handles invalidation when rustc or cargo are updated? I was chatting with @edunham and we were thinking about doing a "merge commit by merge commit" test of sccache as incremental builds for the last few months, catching a few updates to see.

@metajack We're gonna HAVE to try rolling this out to see if we can shave some time off of macOS builds :-)

[aneeshusa]

Are there pre-built releases of sccache available? That would be the easiest way to deploy onto our build machines.

[aneeshusa]

Also, it looks like the build.ccache setting in .servobuild is just used to set the CCACHE environment variable, so we can do that directly.

[luser]

One question: do you know how well this handles invalidation when rustc or cargo are updated?

sccache hashes the contents of all the sharedlibs under $sysroot/lib and uses that as an input to the hash used for the cache key, so it shouldn't be a problem.

[luser]

Are there pre-built releases of sccache available? That would be the easiest way to deploy onto our build machines.

@alexcrichton has binary builds for Rust CI, so you ought to be able to use those until I get actual binary releases setup in sccache CI.

[luser]

FWIW, I've been testing enabling sccache for Rust compilation in Firefox builds, and for our stylo builds (which build a big chunk of Servo code), it looks to save about 5 minutes per build.

[jdm]

This has been enabled.

[aneeshusa]

According to jdm who is in person with me right now, this is not actually fixed yet.

[luser]

FYI: I fixed my AppVeyor config, and the 0.2.6 release of sccache includes a Windows binary package: https://github.com/mozilla/sccache/releases/tag/0.2.6

[jdm]

This was fixed a while ago.