Fuchsia

From mseasborn@:

When profiling the root job,

The profiler sometimes works, but more often fails with this error:

$ ffx profiler attach --job-ids 1033
Press <enter> to stop profiling...

BUG: An internal command error occurred.
Error: A FIDL client's channel to the protocol fuchsia.cpu.profiler.Session was closed: BAD_STATE, reason: Proxy shut down (result: Ok(()))
    1.  BAD_STATE
More information may be available in ffx host logs in directory:
    /usr/local/google/home/mseaborn/.local/share/Fuchsia/ffx/cache/logs

I found that "fx log" shows errors like these from the profiler service:

[14112.824388][profiler] INFO: [kernel_sampler.cc(59)] Attaching to thread: 1839243419
[14112.824394][profiler] INFO: [kernel_sampler.cc(59)] Attaching to thread: 262192575
[14112.824396][profiler] INFO: [kernel_sampler.cc(59)] Attaching to thread: 2212537503
[14112.824399][profiler] INFO: [kernel_sampler.cc(59)] Attaching to thread: 1864404271
[14112.824401][profiler] INFO: [kernel_sampler.cc(59)] Attaching to thread: 1505798619
[14112.824406][profiler] INFO: [kernel_sampler.cc(59)] Attaching to thread: 2513473259
[14122.694785][profiler] ERROR: [src/performance/experimental/profiler/sampler.cc(150)] Failed to add process to session: 3972628: -26 (ZX_ERR_ALREADY_EXISTS)
[14122.694862][profiler] INFO: [kernel_sampler.cc(59)] Attaching to thread: 965775347
[14122.694886][profiler] ERROR: [src/performance/experimental/profiler/kernel_sampler.cc(253)] Failed to add thread to session: 3972630: -25 (ZX_ERR_NOT_FOUND)
[14125.746143][profiler] ERROR: [src/performance/experimental/profiler/kernel_sampler.cc(261)] Failed to remove exited thread: 3972630: -25 (ZX_ERR_NOT_FOUND)
[14135.750626][profiler] ERROR: [src/performance/experimental/profiler/sampler.cc(150)] Failed to add process to session: 3974534: -26 (ZX_ERR_ALREADY_EXISTS)
[14135.750704][profiler] INFO: [kernel_sampler.cc(59)] Attaching to thread: 2102439667
[14135.750715][profiler] ERROR: [src/performance/experimental/profiler/kernel_sampler.cc(253)] Failed to add thread to session: 3974536: -25 (ZX_ERR_NOT_FOUND)

I debugged this and found that GetProcessModules() is failing. gmtr@ confirmed that this is not expected to work: The profiler does not work on processes that exit before profiling is stopped, because the profiler will be unable to get information about loaded modules for those processes. Furthermore, when this happens, it stops the profiler from producing a profile.

Intercepting processes before they exit is hard, but at the minimum, if we miss one, it shouldn't stop us from producing a profile, even if it does have missing symbolization info.