toposort: stack overflow panic in vertexFeatures.addEdges

[mvdan]

# Without toposort.
env CUE_EXPERIMENT=evalv3=0,toposort=0
exec cue export input.cue

# With toposort.
env CUE_EXPERIMENT=evalv3=0,toposort=1
exec cue export input.cue

-- input.cue --
package p

#Exporter: {
	_objects: {...}
	_imports: [...]

	objects: [
		for _, objs in _objects {
			objs
		}
		for import in _imports for obj in import._export.objects {
			obj
		}
	]
}

out: #Exporter & {
	_imports: [_app1]
}
_app1: {
	_export: #Exporter & {
		_imports: [_app2]
	}
}
_app2: {
	_export: #Exporter & {
		_objects: leaf: leafField: "leaf value"
	}
}

As of 870bfc2:

# Without toposort. (0.010s)
> env CUE_EXPERIMENT=evalv3=0,toposort=0
> exec cue export input.cue
[stdout]
{
    "out": {
        "objects": [
            {
                "leafField": "leaf value"
            }
        ]
    }
}
# With toposort. (0.361s)
> env CUE_EXPERIMENT=evalv3=0,toposort=1
> exec cue export input.cue
[stderr]
runtime: goroutine stack exceeds 1000000000-byte limit
runtime: sp=0xc020610328 stack=[0xc020610000, 0xc040610000]
fatal error: stack overflow

runtime stack:
runtime.throw({0x109ea13?, 0x755b36f2cd38?})
	/home/mvdan/tip/src/runtime/panic.go:1096 +0x48 fp=0x755b36f2cd08 sp=0x755b36f2ccd8 pc=0x47a9e8
runtime.newstack()
	/home/mvdan/tip/src/runtime/stack.go:1107 +0x5bb fp=0x755b36f2ce40 sp=0x755b36f2cd08 pc=0x45e2db
runtime.morestack()
	/home/mvdan/tip/src/runtime/asm_amd64.s:621 +0x7a fp=0x755b36f2ce48 sp=0x755b36f2ce40 pc=0x48095a

goroutine 1 gp=0xc000002380 m=8 mp=0xc000580008 [running]:
cuelang.org/go/cue/token.(*File).PositionFor(0xc0002601c0?, {0xc0002601c0?, 0xab3?}, 0x1?)
	/home/mvdan/src/c/cue/cue/token/position.go:465 +0xdf fp=0xc020610338 sp=0xc020610330 pc=0x61537f
cuelang.org/go/cue/token.(*File).Position(...)
	/home/mvdan/src/c/cue/cue/token/position.go:479
cuelang.org/go/cue/token.Pos.Position(...)
	/home/mvdan/src/c/cue/cue/token/position.go:107
cuelang.org/go/cue/token.Pos.Filename({0xc0002601c0?, 0x0?})
	/home/mvdan/src/c/cue/cue/token/position.go:100 +0x45 fp=0xc0206103b8 sp=0xc020610338 pc=0x613e25
cuelang.org/go/internal/core/toposort.(*vertexFeatures).addEdges(0xc04060e718, {0x0, 0x0, 0x0}, 0xc0004455a0)
	/home/mvdan/src/c/cue/internal/core/toposort/vertex.go:454 +0x57 fp=0xc020610508 sp=0xc0206103b8 pc=0x7617f7
cuelang.org/go/internal/core/toposort.(*vertexFeatures).addEdges(0xc04060e718, {0x0, 0x0, 0x0}, 0xc0004455a0)
	/home/mvdan/src/c/cue/internal/core/toposort/vertex.go:515 +0x83f fp=0xc020610658 sp=0xc020610508 pc=0x761fdf
cuelang.org/go/internal/core/toposort.(*vertexFeatures).addEdges(0xc04060e718, {0x0, 0x0, 0x0}, 0xc0004455a0)
	/home/mvdan/src/c/cue/internal/core/toposort/vertex.go:515 +0x83f fp=0xc0206107a8 sp=0xc020610658 pc=0x761fdf
[...]

[mvdan]

Note that this only affects the old evaluator. Editing this testscript to always use evalv3=1 makes the panic go away.

[mvdan]

Thanks to @nxcc for reporting this issue via Unity!