Integrate with simple Heartbeats

[connorimes]

This PR adds Heartbeats capability to servo. Heartbeats are used for detailed performance and power/energy profiling. We will add the power/energy readings in the future.

New dependencies are introduced which need in-depth reviews. I'm the only one who has had eyes on any of this, and I have limited resources for testing cross-platform compatibility.

• https://github.com/libheartbeats/heartbeats-simple - provides native C libraries from a shared code base:
  • hbs[-static] - performance monitoring
  • hbs-acc[-static] - performance with accuracy monitoring
  • hbs-pow[-static] - performance with power/energy monitoring (the one we're using)
  • hbs-acc-pow[-static] - performance with accuracy and power/energy monitoring
• https://github.com/connorimes/heartbeats-simple-sys provides rust wrappers for the native C libraries above - one crate for each + a common crate. These link with the *-static versions of the heartbeats libraries.
• https://github.com/connorimes/heartbeats-simple-rust provides rust abstractions over the -sys crates above - one crate for each.

The new heartbeats module in the profile crate looks for environment variables telling it to use heartbeats for each ProfilerCategory and where to put log files. (Of course, if somebody knows how to iterate over the enum instead of hardcoding each one, that would be fantastic.) If the environment variables aren't set for particular categories, heartbeats aren't created or used.

An interface change is made in the profile_traits crate to pass both the start and end time in a ProfilerMsg instead of just the elapsed time. Later we will add energy readings as well.

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[connorimes]

I expect this review to be an iterative process, so I will continue with bug fixes and improvements until you ask for a code freeze, at which point I'll squash.

[larsbergstrom]

Does this work on other platforms? I'm especially concerned about 32-bit Android. There are instructions to build at: https://github.com/servo/servo/wiki/Building-for-Android

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Reviewed 7 of 8 files at r1, 1 of 1 files at r2, 6 of 6 files at r3.
Review status: all files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

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^{components/profile/heartbeats.rs, line 16 [r1] (raw file):}
Can you use the lazy_static support instead? http://kimundi.github.io/lazy-static.rs/lazy_static/index.html I believe it would remove the unsafe code in init by allowing you to move the HashMap creation to the lazy static and just add the items in init

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Comments from the review on Reviewable.io

[bors-servo]

☔ The latest upstream changes (presumably #7243) made this pull request unmergeable. Please resolve the merge conflicts.

[connorimes]

Review status: 3 of 8 files reviewed at latest revision, all discussions resolved, all commit checks successful.

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^{components/profile/heartbeats.rs, line 16 [r1] (raw file):}
lazy_static requires Sync trait, but the HashMap values are of Heartbeat type which wraps some FFI types which are not Sync.
Do you have a good way around this?
FYI, I think we still need a cleanup function to force the Heartbeats to drop and flush any remaining log data to files. Otherwise, I believe static data does not drop when the application finishes, but please correct me if I'm wrong.

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Comments from the review on Reviewable.io

[connorimes]

I've tested this locally with an android toolchain and it compiles. I haven't tried to run it in an emulator though.
There aren't any runtime dependencies for the heartbeats native libraries. The only dependency that appears in heartbeats-simple is librt in example code which obviously we don't use. Hopefully this means the libraries should should just work assuming the build environment is setup properly.

[larsbergstrom]

Reviewed 5 of 5 files at r4.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from the review on Reviewable.io

[larsbergstrom]

@bors-servo r+

[bors-servo]

📌 Commit 595a9d7 has been approved by larsbergstrom

[jdm]

@bors-servo: r=larsbergstrom

[bors-servo]

📌 Commit 054cbf2 has been approved by larsbergstrom

[bors-servo]

⌛ Testing commit 054cbf2 with merge d89e4f7...

[bors-servo]

☀️ Test successful - android, gonk, linux1, linux2, mac1, mac2, mac3