fix: prevent actions on closed C graphviz context

[edamato]

Calling any of the gvc Context dependent functions post gvc.Close() causes a go panic. When using a deferred Close() it's quite easy to trigger this without realising it.

This also seems to be very similar to the trace on: #20

The solution here is not very graceful, but the context seems to be a passthrough to the C objects so it didn't seem very easy to create a reliable way to query if the context was active or not.

fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x2 addr=0x21baa83 pc=0x21baa83]

runtime stack:
runtime.throw(0x68b73f, 0x2a)
        /home/goprojects/go/src/runtime/panic.go:1116 +0x72 fp=0x7ffd0d88eac8 sp=0x7ffd0d88ea98 pc=0x44d1b2
runtime.sigpanic()
        /home/goprojects/go/src/runtime/signal_unix.go:726 +0x4ac fp=0x7ffd0d88eaf8 sp=0x7ffd0d88eac8 pc=0x46124c

goroutine 1 [syscall]:
runtime.cgocall(0x53b8d0, 0xc000153a98, 0x0)
        /home/goprojects/go/src/runtime/cgocall.go:133 +0x5b fp=0xc000153a68 sp=0xc000153a30 pc=0x41cf9b
github.com/goccy/go-graphviz/internal/ccall._Cfunc_gvLayout(0x21b6f70, 0x21b8810, 0x21d5de0, 0x0)
        _cgo_gotypes.go:2381 +0x4d fp=0xc000153a98 sp=0xc000153a68 pc=0x4d14ed
github.com/goccy/go-graphviz/internal/ccall.GvLayout.func1(0xc000b13678, 0xc0001300e0, 0x683ec2, 0x5, 0x53aef0)
        /home/goprojects/go/src/github.com/goccy/go-graphviz/internal/ccall/gvc.go:736 +0xe5 fp=0xc000153ae0 sp=0xc000153a98 pc=0x4d4985
github.com/goccy/go-graphviz/internal/ccall.GvLayout(0xc000b13678, 0xc0001300e0, 0x683ec2, 0x5, 0x21b6f70, 0xc000000000)
        /home/goprojects/go/src/github.com/goccy/go-graphviz/internal/ccall/gvc.go:736 +0x49 fp=0xc000153b18 sp=0xc000153ae0 pc=0x4d3ee9
github.com/goccy/go-graphviz/gvc.(*Context).Layout(...)
        /home/goprojects/go/src/github.com/goccy/go-graphviz/gvc/gvc.go:29
github.com/goccy/go-graphviz.(*Graphviz).RenderFilename(0xc000114150, 0xc0001300e8, 0x683c56, 0x3, 0x68ce05, 0x46, 0x0, 0x0)
        /home/goprojects/go/src/github.com/goccy/go-graphviz/graphviz.go:115 +0x74 fp=0xc000153b90 sp=0xc000153b18 pc=0x533194
github.com/goprojects/ggraph.(*Graph).RenderPNG(0xc000153e60, 0x68ce05, 0x46, 0x0, 0x0)
        /home/goprojects/go/src/github.com/goprojects/ggraph/ggraph.go:430 +0x1e5 fp=0xc000153c30 sp=0xc000153b90 pc=0x537b25
main.main()
        /home/goprojects/go/src/github.com/goprojects/go.toys/graph_exercises/main.go:130 +0xb0f fp=0xc000153f88 sp=0xc000153c30 pc=0x538f2f
runtime.main()
        /home/goprojects/go/src/runtime/proc.go:204 +0x209 fp=0xc000153fe0 sp=0xc000153f88 pc=0x44f9a9
runtime.goexit()
        /home/goprojects/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc000153fe8 sp=0xc000153fe0 pc=0x47e401

goroutine 2 [force gc (idle)]:
runtime.gopark(0x68d798, 0x80f590, 0x1411, 0x1)
        /home/goprojects/go/src/runtime/proc.go:306 +0xe5 fp=0xc000032fb0 sp=0xc000032f90 pc=0x44fda5
runtime.goparkunlock(...)
        /home/goprojects/go/src/runtime/proc.go:312
runtime.forcegchelper()
        /home/goprojects/go/src/runtime/proc.go:255 +0xc5 fp=0xc000032fe0 sp=0xc000032fb0 pc=0x44fc45
runtime.goexit()
        /home/goprojects/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc000032fe8 sp=0xc000032fe0 pc=0x47e401
created by runtime.init.6
        /home/goprojects/go/src/runtime/proc.go:243 +0x35

goroutine 3 [GC sweep wait]:
runtime.gopark(0x68d798, 0x80f6a0, 0x140c, 0x1)
        /home/goprojects/go/src/runtime/proc.go:306 +0xe5 fp=0xc0000337a8 sp=0xc000033788 pc=0x44fda5
runtime.goparkunlock(...)
        /home/goprojects/go/src/runtime/proc.go:312
runtime.bgsweep(0xc00001e1c0)
        /home/goprojects/go/src/runtime/mgcsweep.go:182 +0x13b fp=0xc0000337d8 sp=0xc0000337a8 pc=0x43c29b
runtime.goexit()
        /home/goprojects/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc0000337e0 sp=0xc0000337d8 pc=0x47e401
created by runtime.gcenable
        /home/goprojects/go/src/runtime/mgc.go:217 +0x5c

goroutine 4 [GC scavenge wait]:
runtime.gopark(0x68d798, 0x80f740, 0x140d, 0x1)
        /home/goprojects/go/src/runtime/proc.go:306 +0xe5 fp=0xc000033f78 sp=0xc000033f58 pc=0x44fda5
runtime.goparkunlock(...)
        /home/goprojects/go/src/runtime/proc.go:312
runtime.bgscavenge(0xc00001e1c0)
        /home/goprojects/go/src/runtime/mgcscavenge.go:314 +0x29c fp=0xc000033fd8 sp=0xc000033f78 pc=0x43a3fc
runtime.goexit()
        /home/goprojects/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc000033fe0 sp=0xc000033fd8 pc=0x47e401
created by runtime.gcenable
        /home/goprojects/go/src/runtime/mgc.go:218 +0x7e

goroutine 18 [finalizer wait]:
runtime.gopark(0x68d798, 0x84a1a0, 0xffff1410, 0x1)
        /home/goprojects/go/src/runtime/proc.go:306 +0xe5 fp=0xc000032758 sp=0xc000032738 pc=0x44fda5
runtime.goparkunlock(...)
        /home/goprojects/go/src/runtime/proc.go:312
runtime.runfinq()
        /home/goprojects/go/src/runtime/mfinal.go:175 +0xa9 fp=0xc0000327e0 sp=0xc000032758 pc=0x431189
runtime.goexit()
        /home/goprojects/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc0000327e8 sp=0xc0000327e0 pc=0x47e401
created by runtime.createfing
        /home/goprojects/go/src/runtime/mfinal.go:156 +0x65

goroutine 19 [GC worker (idle)]:
runtime.gopark(0x68d658, 0xc000550000, 0x1418, 0x0)
        /home/goprojects/go/src/runtime/proc.go:306 +0xe5 fp=0xc00002e760 sp=0xc00002e740 pc=0x44fda5
runtime.gcBgMarkWorker(0xc000024000)
        /home/goprojects/go/src/runtime/mgc.go:1891 +0xff fp=0xc00002e7d8 sp=0xc00002e760 pc=0x434c5f
runtime.goexit()
        /home/goprojects/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc00002e7e0 sp=0xc00002e7d8 pc=0x47e401
created by runtime.gcBgMarkStartWorkers
        /home/goprojects/go/src/runtime/mgc.go:1839 +0x77

goroutine 34 [GC worker (idle)]:
runtime.gopark(0x68d658, 0xc000550010, 0x1418, 0x0)
        /home/goprojects/go/src/runtime/proc.go:306 +0xe5 fp=0xc000558760 sp=0xc000558740 pc=0x44fda5
runtime.gcBgMarkWorker(0xc000026800)
        /home/goprojects/go/src/runtime/mgc.go:1891 +0xff fp=0xc0005587d8 sp=0xc000558760 pc=0x434c5f
runtime.goexit()
        /home/goprojects/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc0005587e0 sp=0xc0005587d8 pc=0x47e401
created by runtime.gcBgMarkStartWorkers
        /home/goprojects/go/src/runtime/mgc.go:1839 +0x77

goroutine 5 [GC worker (idle)]:
runtime.gopark(0x68d658, 0xc0000200a0, 0x1418, 0x0)
        /home/goprojects/go/src/runtime/proc.go:306 +0xe5 fp=0xc000034760 sp=0xc000034740 pc=0x44fda5
runtime.gcBgMarkWorker(0xc000029000)
        /home/goprojects/go/src/runtime/mgc.go:1891 +0xff fp=0xc0000347d8 sp=0xc000034760 pc=0x434c5f
runtime.goexit()
        /home/goprojects/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc0000347e0 sp=0xc0000347d8 pc=0x47e401
created by runtime.gcBgMarkStartWorkers
        /home/goprojects/go/src/runtime/mgc.go:1839 +0x77

goroutine 35 [GC worker (idle)]:
runtime.gopark(0x68d658, 0xc000550020, 0x1418, 0x0)
        /home/goprojects/go/src/runtime/proc.go:306 +0xe5 fp=0xc000558f60 sp=0xc000558f40 pc=0x44fda5
runtime.gcBgMarkWorker(0xc00002b800)
        /home/goprojects/go/src/runtime/mgc.go:1891 +0xff fp=0xc000558fd8 sp=0xc000558f60 pc=0x434c5f
runtime.goexit()
        /home/goprojects/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc000558fe0 sp=0xc000558fd8 pc=0x47e401
created by runtime.gcBgMarkStartWorkers
        /home/goprojects/go/src/runtime/mgc.go:1839 +0x77

Process finished with exit code 2