Memory safety

[thinxer]

The garbage collector in newer Go (e.g. 1.5) got smarter. jerr sometimes gets collected and we crash.

Here:

go-libjpeg/jpeg/decompress.go

Line 77 in a79c3a8

var jerr C.struct_jpeg_error_mgr

and here:

go-libjpeg/jpeg/compress.go

Line 57 in a79c3a8

var jerr C.struct_jpeg_error_mgr

It is possible the source/dest managers will have similar issues. I've added some ugly code to keep jerr and mgr referenced and the crashes went away.

Some related issues:

• runtime: Will the variable pointer in Go side be invalid during calling C function due to garbage collection? golang/go#11345
• cmd/cgo: should make pointers passed to C functions escape golang/go#10303

[edvakf]

I see. go-thumber does it better (but uglier).

https://github.com/pixiv/go-thumber/blob/master/jpeg/jpeg_read.go#L155-L177

I'm trying to fix it cleanly, if I can. Any idea?

[edvakf]

#22 This probably solves.

[thinxer]

The mgr made by makeDestinationManager may still be garbage collected.

In the meanwhile, It is okay to have a Go struct allocated on the C heap. However, variables reference by that struct will not be scanned by Go GC. In this case, the r variable is not referenced in Go anymore. I'm not sure if Go will garbage collect it.

[edvakf]

Really? makeDestinationManager returns the mgr and accessed later by defer releaseDestinationManager(dstManager), so it doesn't look like a problem to me.

[thinxer]

@edvakf sorry I missed the commits following e801a2.

[edvakf]

Cool

[thinxer]

https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md

We cannot assign io.Reader/io.Writer to a C struct in Go 1.6.

[harukasan]

See #25.

[harukasan]

#25 is merged.