x/exp/shiny: darwin signal handling SIGSEGV

[jnjackins]

go version devel +57b1e55 Sat Jan 9 03:30:40 2016 +0000 darwin/amd64

I'm building a graphical program using x/exp/shiny, and recently I started getting this error whenever a mouse event is generated:

Segmentation fault: 11

I'm on a Mac, and using the "report to apple" dialogue that pops up, I also get the following (full output attached):

...
Crashed Thread:        0  Dispatch queue: com.apple.main-thread
Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       KERN_PROTECTION_FAILURE at 0x00007fff5f3ffff8
...
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   edit                            0x0000000004040a25 runtime.sighandler + 725 (signal_amd64x.go:129)
...

I'm not sure if this is a bug in the darwin code for shiny, a new bug introduced with the recent changes to signal handling, or an older bug that I've somehow tickled.

I can reproduce this consistently by running e.g. golang.org/x/exp/shiny/example/tile, and clicking. The error message varies between "signal: segmentation fault" and "Segmentation fault: 11".

Running with -race does not report any data races.

full crash report

[bradfitz]

/cc @nigeltao @crawshaw @ianlancetaylor

[jnjackins]

The crash was introduced by this commit: golang/exp@bb37e7d (found using git bisect). I can also reproduce the crashes with older versions of Go, so I think the bug probably lies somewhere in that change.

[jnjackins]

I mailed https://go-review.googlesource.com/#/c/18424/.

[gopherbot]

CL https://golang.org/cl/18424 mentions this issue.