Bug fix: go_objects should be able to implement interfaces

[atishnazir]

There was a tiny typo that meant class data was not attached to the interface callback objects.

When constructing Go Objects that implement one or more interfaces this would result in a nil pointer exception within goInterfaceInit.

Discovered whilst using go-gst:

  gst.RegisterElement(
    plugin,
    elementName,
    gst.RankNone,
    &httpSrc,
    base.ExtendsBaseSrc,
    gst.InterfaceURIHandler,
  )

[atishnazir]

I think this PR needs something extra, works fine through a debugger but at full speed there's a hiccough:

=== RUN   TestGstreamer
Running Suite: Pipelines/Gstreamer Suite
========================================
Random Seed: 1629269532
Will run 2 of 32 specs

time="2021-08-18T07:52:12+01:00" level=info msg="GStreamer MediaBackend"
SSSSSSSSSSSSSSSSSfatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x80 addr=0x0 pc=0x7f0246add747]

runtime stack:
runtime.throw(0x920ebf, 0x2a)
	/home/na/sdk/go1.16.4/src/runtime/panic.go:1117 +0x72
runtime.sigpanic()
	/home/na/sdk/go1.16.4/src/runtime/signal_unix.go:718 +0x2e5

goroutine 6 [syscall, locked to thread]:
runtime.cgocall(0x7ff350, 0xc0001ea918, 0xcdce00)
	/home/na/sdk/go1.16.4/src/runtime/cgocall.go:154 +0x5b fp=0xc0001ea8e8 sp=0xc0001ea8b0 pc=0x448d9b
github.com/tinyzimmer/go-gst/gst._Cfunc_gst_element_register(0x7f0200173c40, 0x7f0200000ec0, 0x0, 0x7f02001b9a10, 0x0)
	_cgo_gotypes.go:7322 +0x48 fp=0xc0001ea918 sp=0xc0001ea8e8 pc=0x5f7c28
github.com/tinyzimmer/go-gst/gst.RegisterElement.func1(0xc0003ae050, 0x910e93, 0xc, 0x0, 0x99b360, 0xc00039e078, 0x99f970, 0xcab900, 0xc0001eaa60, 0x1, ...)
	/home/na/go/pkg/mod/github.com/cognitive-i/go-gst@v0.2.30-goobject-bugfix/gst/gst_element.go:103 +0x145 fp=0xc0001ea990 sp=0xc0001ea918 pc=0x6127c5
github.com/tinyzimmer/go-gst/gst.RegisterElement(0xc0003ae050, 0x910e93, 0xc, 0x0, 0x99b360, 0xc00039e078, 0x99f970, 0xcab900, 0xc0001eaa60, 0x1, ...)
	/home/na/go/pkg/mod/github.com/cognitive-i/go-gst@v0.2.30-goobject-bugfix/gst/gst_element.go:103 +0xae fp=0xc0001eaa00 sp=0xc0001ea990 pc=0x602b8e
eng.atxnetworks.com/vada/ronin/pipelines/gstreamer.httpFactoryFn(0xc0003ae050, 0xc0003ae050)
	/home/na/workspace/redacted/gstHttpSource.go:18 +0xca fp=0xc0001eaa80 sp=0xc0001eaa00 pc=0x78726a
github.com/tinyzimmer/go-gst/gst.goPluginInit(0x7f0200173c40, 0x7f02001b9870, 0x0)
	/home/na/go/pkg/mod/github.com/cognitive-i/go-gst@v0.2.30-goobject-bugfix/gst/cgo_exports.go:239 +0x142 fp=0xc0001eaae0 sp=0xc0001eaa80 pc=0x5ffbe2
_cgoexp_9fe65b6dd3a1_goPluginInit(0x7f0207ffeb10)
	_cgo_gotypes.go:17429 +0x37 fp=0xc0001eab08 sp=0xc0001eaae0 pc=0x61b497
runtime.cgocallbackg1(0x61b460, 0x7f0207ffeb10, 0x0)
	/home/na/sdk/go1.16.4/src/runtime/cgocall.go:292 +0x18c fp=0xc0001eaba8 sp=0xc0001eab08 pc=0x44914c
runtime.cgocallbackg(0x61b460, 0x7f0207ffeb10, 0x0)
	/home/na/sdk/go1.16.4/src/runtime/cgocall.go:228 +0xda fp=0xc0001eac18 sp=0xc0001eaba8 pc=0x448f1a
runtime.cgocallback(0x448dbf, 0x803cf0, 0xc0001eacb0)
	/home/na/sdk/go1.16.4/src/runtime/asm_amd64.s:788 +0xa9 fp=0xc0001eac40 sp=0xc0001eac18 pc=0x4b74e9
runtime.asmcgocall(0x803cf0, 0xc0001eacb0)
	/home/na/sdk/go1.16.4/src/runtime/asm_amd64.s:652 +0x42 fp=0xc0001eac48 sp=0xc0001eac40 pc=0x4b73c2
runtime.cgocall(0x803cf0, 0xc0001eacb0, 0x8114f0)
	/home/na/sdk/go1.16.4/src/runtime/cgocall.go:164 +0x7f fp=0xc0001eac80 sp=0xc0001eac48 pc=0x448dbf
github.com/tinyzimmer/go-gst/gst._Cfunc_gst_plugin_register_static_full(0x1200000001, 0x7f0200000ee0, 0x7f02001a6c00, 0x803a40, 0x7f0200000cf0, 0x7f0200000d10, 0x7f0200000d30, 0x7f02001a2850, 0x7f02001b9100, 0x7f02001b9870, ...)
	_cgo_gotypes.go:11765 +0x4b fp=0xc0001eacb0 sp=0xc0001eac80 pc=0x5fa96b
github.com/tinyzimmer/go-gst/gst.RegisterPlugin.func2(0xc0001eae28, 0x7f0200000ee0, 0x7f02001a6c00, 0x7f0200000cf0, 0x7f0200000d10, 0x7f0200000d30, 0x7f02001a2850, 0x7f02001b9100, 0x7f02001b9870, 0xc0001ead90)
	/home/na/go/pkg/mod/github.com/cognitive-i/go-gst@v0.2.30-goobject-bugfix/gst/gst_plugin.go:150 +0xf3 fp=0xc0001ead30 sp=0xc0001eacb0 pc=0x6199b3
github.com/tinyzimmer/go-gst/gst.RegisterPlugin(0xc0001eae28, 0x930f48, 0xc0003c0000)

[tinyzimmer]

Hey sorry I've had a busy month and just seeing this for the first time. That functionality is very experimental and definitely will have bugs like this. I can take a look at what you've proposed so far at some point and try to help it along.

[atishnazir]

No worries, I've not a had a chance to tinker recently as was on holiday. WIth a view to easily reproduce the issue I've crafted a static plugin example here: https://github.com/cognitive-i/go-gst/commits/bugfix/plugins-with-interfaces

In the process I added NewElementFromUri functionality.

Of course now that I've created a standalone pseudo test case, the stray is no longer reproducing. Will bash it a little more.

[RSWilli]

@atishnazir please move this PR to https://github.com/go-gst/go-glib (where future development of the bindings will take place)

[atishnazir]

@RSWilli I haven't looked this one in a while as I was never satisfied I fully understood the issue. Will have a gander over next few days and either close/transfer as appropriate. @tinyzimmer, thanks for your work and @RSWilli thanks for picking up!

[RSWilli]

@atishnazir FYI the original author is still on board, but is currently very busy

we are using the new org to create a bigger community around this project