Crash on Pm_Write

[briansorahan]

i see this fairly regularly on linux (via the launchpad package) but haven't yet figured out how to reproduce

fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x2 addr=0x7ff50c021000 pc=0x7ff51ce5ff46]

runtime stack:
runtime.throw(0x631c1f, 0x2a)
        /usr/lib/go/src/runtime/panic.go:566 +0x95
runtime.sigpanic()
        /usr/lib/go/src/runtime/sigpanic_unix.go:12 +0x2cc

goroutine 35 [syscall, locked to thread]:
runtime.cgocall(0x5bd000, 0xc420216c78, 0x0)
        /usr/lib/go/src/runtime/cgocall.go:131 +0x110 fp=0xc420216c30 sp=0xc420216bf0
github.com/scgolang/beats/vendor/github.com/rakyll/portmidi._Cfunc_Pm_Write(0x7ff50c000bf0, 0xc42016e4a0, 0x1, 0x0)
        ??:0 +0x4d fp=0xc420216c78 sp=0xc420216c30
github.com/scgolang/beats/vendor/github.com/rakyll/portmidi.(*Stream).Write(0xc4201fc4d0, 0xc420216cf8, 0x1, 0x1, 0x0, 0x0)
        /home/brian/go/src/github.com/scgolang/beats/vendor/github.com/rakyll/portmidi/stream.go:121 +0x11d fp=0xc420216cc8 sp=0xc420216c78
github.com/scgolang/beats/vendor/github.com/rakyll/portmidi.(*Stream).WriteShort(0xc4201fc4d0, 0x90, 0x54, 0xc, 0x0, 0x0)
        /home/brian/go/src/github.com/scgolang/beats/vendor/github.com/rakyll/portmidi/stream.go:132 +0x9b fp=0xc420216d28 sp=0xc420216cc8

[kisielk]

Can you try running the code with theGODEBUG=cgocheck=2 environment variable set?

[briansorahan]

@kisielk Tried that and got the same error.

[kisielk]

Okay, but you didn't get any messages from the Go runtime? Also, which version of Go are you using?

[briansorahan]

go version go1.7.4 linux/amd64

Didn't notice any extra messages from the Go runtime.

[briansorahan]

Also here is pacman -Qi portmidi

Name            : portmidi
Version         : 217-5
Description     : Platform independent library for real-time MIDI input/output
Architecture    : x86_64
URL             : http://portmedia.sourceforge.net/
Licenses        : GPL
Groups          : None
Provides        : None
Depends On      : alsa-lib
Optional Deps   : java-runtime: for using pmdefaults
Required By     : None
Optional For    : None
Conflicts With  : None
Replaces        : None
Installed Size  : 154.00 KiB
Packager        : Eric Belanger <eric@archlinux.org>
Build Date      : Sat 26 Oct 2013 12:14:09 PM CDT
Install Date    : Sat 14 Jan 2017 04:26:42 PM CST
Install Reason  : Explicitly installed
Install Script  : No
Validated By    : Signature

[briansorahan]

Reviewing the cgo docs it says that the GODEBUG checks are sidestepped by code that uses package unsafe, and the code that is crashing in this case does use unsafe. I wonder if it's possible to not use unsafe?

[briansorahan]

Looks like we have to use unsafe.Pointer since PortMidiStream is void

[briansorahan]

FWIW I started playing with an alternative midi implementation https://github.com/scgolang/midi seems pretty stable on linux so far, haven't used it on mac much yet

[kisielk]

@briansorahan cool, I will check it out. One thing I'm struggling with in PortMIDI is to have an efficient way to have channels for incoming MIDI events implemented in an efficient way, preferably without polling. Seems like using ALSA might solve that.

[briansorahan]

@kisielk Writing that package has been interesting so far. The linux part took me maybe two hours and resulted in a beautiful tiny interface which basically just had Open, Read, Write, and Close. Read would block until I sent data from the device, which seemed just perfect to me since consumers of the package can easily type go to make it nonblocking.
Wrapping coremidi caused me to rewrite the linux code though because in order to get data from the device you register a callback.
So now I return a channel for reading in both the linux and darwin code.