Uncatchable panic with no backtrace

[jdm]

STR:

1. ./mach run https://samuknet.github.io/test_cases/iframeCSSRotate/ --debug
2. b rust_panic
3. run
4. then wait 30s, then press escape
5. in the terminal:

ERROR:servo: called `Result::unwrap()` on an `Err` value: Error { repr: Custom(Custom { kind: NotFound, error: StringError("Bogus destination port.") }) }

with no backtrace, and the breakpoint doesn't get hit before servo exits.

[jdm]

RUST_LOG=constellation::constellation showed me:

    DEBUG:constellation::constellation: constellation exiting
    DEBUG:constellation::constellation: Pipeline PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(0) } exited.
    DEBUG:constellation::constellation: constellation got key event message
    DEBUG:constellation::constellation: constellation got key event message
    DEBUG:constellation::constellation: constellation got key event message
    DEBUG:constellation::constellation: Pipeline PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(0) } exited.
    DEBUG:constellation::constellation: Exiting image cache.
    DEBUG:constellation::constellation: Exiting core resource threads.
    DEBUG:constellation::constellation: Exiting storage resource threads.
    DEBUG:constellation::constellation: Exiting file manager resource threads.
    DEBUG:constellation::constellation: Exiting bluetooth thread.
    DEBUG:constellation::constellation: Exiting service worker manager thread.
    DEBUG:constellation::constellation: Exiting font cache thread.
    DEBUG:constellation::constellation: Asking compositor to complete shutdown.
    DEBUG:constellation::constellation: Sending log entry Warn("Panic hook called."). 

Panics that occur after the constellation disappears are now more challenging to diagnose. I'm also concerned that rust_panic isn't being hit.

[jdm]

cc @asajeffrey

[asajeffrey]

@jdm: we could log the backtrace when the panic happens, rather than expecting the constellation to do it, we just need to make sure that the panic isn't printed twice.

[emilio]

FWIW, That's the kind of problem I tried to solve with #12591, though arguably something that works out of the box would be a much better solution.

[asajeffrey]

IRC chat with @jdm: http://logs.glob.uno/?c=mozilla%23servo&s=29+Jul+2016&e=29+Jul+2016#c491961

Summary: println! the backtrace if RUST_BACKTRACE is set.