backtraces broken on the Android bot

[thestinger]

PIE is now required by Android (#17437) but it breaks backtrace support on the Android bot. It may just need to be updated to the current NDK / Android versions.

[tamird]

I've locally enabled PIE for android and tested this with the latest NDK (r10d) generated for android-21 and an android emulator running the latest API (android-22), and it still fails with:

test [run-pass] run-pass/backtrace.rs ... FAILED

failures:

---- [run-pass] run-pass/backtrace.rs stdout ----

error: test run failed!
status: exit code: 101
command: x86_64-apple-darwin/test/run-pass/backtrace.stage2-arm-linux-androideabi
stdout:
------------------------------------------

------------------------------------------
stderr:
------------------------------------------
thread '<main>' panicked at 'bad output: thread '<main>' panicked at 'explicit panic', /Users/tamird/src/rust/src/test/run-pass/backtrace.rs:23
stack backtrace:
   1: 0xb6c53da7 - <unknown>
   2: 0xb6c5ea8b - <unknown>
   3: 0xb6c0df37 - <unknown>
   4: 0xb6f87fb7 - <unknown>
   5: 0xb6f87d73 - <unknown>
   6: 0xb6f8b913 - <unknown>
   7: 0xb6d0dfe3 - <unknown>
   8: 0xb6d0dfbb - <unknown>
   9: 0xb6d0dfbb - <unknown>
  10: 0xb6d0dfbb - <unknown>
  11: 0xb6d0dfbb - <unknown>
  12: 0xb6d0dfbb - <unknown>
  13: 0xb6d0dfbb - <unknown>
  14: 0xb6d0dfbb - <unknown>
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  17: 0xb6d0dfbb - <unknown>
  18: 0xb6d0dfbb - <unknown>
  19: 0xb6d0dfbb - <unknown>
  20: 0xb6d0dfbb - <unknown>
  21: 0xb6d0dfbb - <unknown>
  22: 0xb6d0dfbb - <unknown>
  23: 0xb6d0dfbb - <unknown>
  24: 0xb6d0dfbb - <unknown>
  25: 0xb6d0dfbb - <unknown>
  26: 0xb6d0dfbb - <unknown>
  27: 0xb6d0dfbb - <unknown>
  28: 0xb6d0dfbb - <unknown>
  29: 0xb6d0dfbb - <unknown>
  30: 0xb6d0dfbb - <unknown>
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  34: 0xb6d0dfbb - <unknown>
  35: 0xb6d0dfbb - <unknown>
  36: 0xb6d0dfbb - <unknown>
  37: 0xb6d0dfbb - <unknown>
  38: 0xb6d0dfbb - <unknown>
  39: 0xb6d0dfbb - <unknown>
  40: 0xb6d0dfbb - <unknown>
  41: 0xb6d0dfbb - <unknown>
  42: 0xb6d0dfbb - <unknown>
  43: 0xb6d0dfbb - <unknown>
  44: 0xb6d0dfbb - <unknown>
  45: 0xb6d0dfbb - <unknown>
  46: 0xb6d0dfbb - <unknown>
  47: 0xb6d0dfbb - <unknown>
  48: 0xb6d0dfbb - <unknown>
  49: 0xb6d0dfbb - <unknown>
  50: 0xb6d0dfbb - <unknown>
  51: 0xb6d0dfbb - <unknown>
  52: 0xb6d0dfbb - <unknown>
  53: 0xb6d0dfbb - <unknown>
  54: 0xb6d0dfbb - <unknown>
  55: 0xb6d0dfbb - <unknown>
  56: 0xb6d0dfbb - <unknown>
  57: 0xb6d0dfbb - <unknown>
  58: 0xb6d0dfbb - <unknown>
  59: 0xb6d0dfbb - <unknown>
  60: 0xb6d0dfbb - <unknown>
  61: 0xb6d0dfbb - <unknown>
  62: 0xb6d0dfbb - <unknown>
  63: 0xb6d0dfbb - <unknown>
  64: 0xb6d0dfbb - <unknown>
  65: 0xb6d0dfbb - <unknown>
  66: 0xb6d0dfbb - <unknown>
  67: 0xb6d0dfbb - <unknown>
  68: 0xb6d0dfbb - <unknown>
  69: 0xb6d0dfbb - <unknown>
  70: 0xb6d0dfbb - <unknown>
  71: 0xb6d0dfbb - <unknown>
  72: 0xb6d0dfbb - <unknown>
  73: 0xb6d0dfbb - <unknown>
  74: 0xb6d0dfbb - <unknown>
  75: 0xb6d0dfbb - <unknown>
  76: 0xb6d0dfbb - <unknown>
  77: 0xb6d0dfbb - <unknown>
  78: 0xb6d0dfbb - <unknown>
  79: 0xb6d0dfbb - <unknown>
  80: 0xb6d0dfbb - <unknown>
  81: 0xb6d0dfbb - <unknown>
  82: 0xb6d0dfbb - <unknown>
  83: 0xb6d0dfbb - <unknown>
  84: 0xb6d0dfbb - <unknown>
  85: 0xb6d0dfbb - <unknown>
  86: 0xb6d0dfbb - <unknown>
  87: 0xb6d0dfbb - <unknown>
  88: 0xb6d0dfbb - <unknown>
  89: 0xb6d0dfbb - <unknown>
  90: 0xb6d0dfbb - <unknown>
  91: 0xb6d0dfbb - <unknown>
  92: 0xb6d0dfbb - <unknown>
  93: 0xb6d0dfbb - <unknown>
  94: 0xb6d0dfbb - <unknown>
  95: 0xb6d0dfbb - <unknown>
  96: 0xb6d0dfbb - <unknown>
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  98: 0xb6d0dfbb - <unknown>
  99: 0xb6d0dfbb - <unknown>
  100: 0xb6d0dfbb - <unknown>
 ... <frames omitted>
', /Users/tamird/src/rust/src/test/run-pass/backtrace.rs:54

------------------------------------------

thread '[run-pass] run-pass/backtrace.rs' panicked at 'explicit panic', /Users/tamird/src/rust/src/compiletest/runtest.rs:1525

[steveklabnik]

Triage: we're in the process of moving all of the bots to Travis/AppVeyor, so I'm not sure if this 1. is still true 2. will persist there.

[roblabla]

So after a bit of investigation, I found that android internally uses libunwind to handle its unwinding. It might be a good idea to try having rust use that ? If it's a good idea, I might start implementing it.

[DanAlbert]

It's an option. We only used libunwind for a handful of releases. We've started writing our own unwinder since none of the off the shelf options were reliable or performant enough for our needs. That's getting close to ready, and we will be including it in the NDK.

[roblabla]

@DanAlbert Oh, OK. So, if I make FFI bindings to libunwinderstack (when it's in the NDK) and use them in libstd, would I need to statically link to it ?

[DanAlbert]

Either statically linked or included in the APK. The former is probably simpler.

[roblabla]

Right. Thanks for the insights :D. I'll try figuring out how libunwinderstack works and hopefully draft a PR up over the next few days.

EDIT: Just found out libunwinderstack is written in CPP. Hopefully that won't hinder me too much, but it is going to be a bit more painful that I had anticipated at first.

[roblabla]

So after some unfruitful attempts at making bindgen and libunwinderstack work together (I couldn't get bindgen to generate constructors for unwinderstack::Maps), I decided to investigate other options.

In doing so, I found out that backtrace-rs has no trouble getting a correct stacktrace. Furthermore, I figured that rust's tracing part seemed correct (when using RUST_BACKTRACE=full and comparing, the stack depth and pc seem correct), so the printing module seems to be at fault.

I'm going to be looking into the differences between backtrace-rs and the built-in backtracing of rust. I already have a few ideas.