internal/core/adt: Environment.up nil vertex panic when calling Value.Expr

[sahroshan]

What version of CUE are you using (cue version)?

$ cue version
0.7.1

$ go version
go version go1.21.0 darwin/arm64

Does this issue reproduce with the latest stable release?

yes

What did you do?

1. create a new cue file at my_cues/stack/stack.cue

// my_cues/stack/stack.cue

package stack
import "strings"

_ci: bool | *false @tag(ci,type=bool)

#aws_region_short: ["use1", "use2", "usw1", "usw2", "eun1", "euw1", "euw3", "euw2", "eus1", "euc1", "ane1", "ane2", "ane3", "aps1", "apse1", "apse2", "ae1", "ase1", "ase2", "ase3", "afs1", "mes1", "sae1", "cac1", "apne3"]
#seperator: "|"

AWSTemplateFormatVersion: "2010-09-09"
Description:              "Master CFT for deploying other  EKS related CFT templates"

Parameters: {
	MainStackName: {
		Type:           "String"
		Description:    "Enter name of the stack which hosts Pods"
		AllowedPattern: "^(aws|gcp)-(\(strings.Join(#aws_region_short, #seperator)))-([a-z0-9]{3,6})-([a-z0-9]{3,5})$"
		if !_ci {
			Default: =~"^(aws|gcp)-(\(strings.Join(#aws_region_short, #seperator)))-([a-z0-9]{3,6})-([a-z0-9]{3,5})$"
		}
	}
}

2. write main.go to go through the cue and check for any expression.

// main.go

package main

import (
	"fmt"

	"cuelang.org/go/cue"
	"cuelang.org/go/cue/cuecontext"
	"cuelang.org/go/cue/load"
)

func walkCue(val cue.Value, after func(cue.Value)) {
	fields, structErr := val.Fields()
	if structErr == nil {
		for fields.Next() {
			walkCue(fields.Value(), after)
		}
	}
	list, listErr := val.List()
	if listErr != nil {
		for list.Next() {
			walkCue(list.Value(), after)
		}
	}
	after(val)
}

func checkOperation(v cue.Value) {
	fmt.Println(v)
	op, _ := v.Expr()
	fmt.Println(op == cue.OrOp)
}

func loadCue(cueDirectory string) (cue.Value, error) {
	val := cue.Value{}
	context := cuecontext.New()
	conf := &load.Config{
		Dir:         cueDirectory,
		AllCUEFiles: true,
		DataFiles:   true,
	}
	entrypoints := []string{"stack/stack.cue"}
	buildInstances := load.Instances(entrypoints, conf)
	for _, bi := range buildInstances {
		if bi.Incomplete || bi.Err != nil {
			return val, fmt.Errorf("Error during loading entrypoints: %v", bi.Err)
		}
		val = context.BuildInstance(bi)
	}
	return val, nil
}

func main() {
	cueDirectory := "my_cues"
	val, err := loadCue(cueDirectory)
	if err != nil {
		fmt.Println(err)
		return
	}
	walkCue(val, checkOperation)
}

3. the above code panics with below error

panic: assertion failed: Environment.up encountered a nil vertex

goroutine 1 [running]:
cuelang.org/go/internal/core/adt.(*OpContext).Assertf(0x140002de078?, {0x140003eafc0?, 0x140002de0b8?}, 0x20?, {0x104812e37?, 0x0?}, {0x0, 0x0, 0x0})
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/context.go:104 +0x9c
cuelang.org/go/internal/core/adt.(*Environment).up(0x140002de128?, 0x14000433e60, 0x49e4b10?)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/composite.go:107 +0x5c
cuelang.org/go/internal/core/adt.(*OpContext).relNode(0x14000433e60, 0x0?)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/context.go:326 +0x30
cuelang.org/go/internal/core/adt.(*FieldReference).resolve(0x140003e2d90, 0x1049e2db8?, 0x90?)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/expr.go:703 +0x34
cuelang.org/go/internal/core/adt.(*OpContext).evalState(0x14000433e60, {0x1049e2db8, 0x140003e2d90}, 0x2)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/context.go:694 +0x170
cuelang.org/go/internal/core/adt.(*OpContext).value(0x1?, {0x1049e2db8?, 0x140003e2d90?}, 0xe0?)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/context.go:641 +0x28
cuelang.org/go/internal/core/adt.(*CallExpr).evaluate(0x140003f3d70, 0x14000433e60, 0x70?)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/expr.go:1462 +0x3c4
cuelang.org/go/internal/core/adt.(*OpContext).evalState(0x14000433e60, {0x1049e3438, 0x140003f3d70}, 0x2)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/context.go:690 +0x378
cuelang.org/go/internal/core/adt.(*OpContext).value(0x140002de640?, {0x1049e3438?, 0x140003f3d70?}, 0x3e?)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/context.go:641 +0x28
cuelang.org/go/internal/core/adt.(*Interpolation).evaluate(0x140003f3d10, 0x14000433e60, 0x10?)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/expr.go:1135 +0x88
cuelang.org/go/internal/core/adt.(*OpContext).evalState(0x14000433e60, {0x1049e3378, 0x140003f3d10}, 0x2)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/context.go:690 +0x378
cuelang.org/go/internal/core/adt.(*OpContext).evaluateRec(0x14000433e60, {0x140003f5720, {0x1049e01b0, 0x140003f3d10}, {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}}, ...)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/context.go:623 +0x14c
cuelang.org/go/internal/core/adt.(*nodeContext).evalExpr(0x14000417c00, {0x140003f5720, {0x1049e01b0, 0x140003f3d10}, {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}}, ...)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/eval.go:1579 +0x118
cuelang.org/go/internal/core/adt.(*nodeContext).addExprConjunct(0x14000417c00, {0x140003f5720, {0x1049e01b0, 0x140003f3d10}, {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}}, ...)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/eval.go:1520 +0x648
cuelang.org/go/internal/core/adt.(*nodeContext).insertConjuncts(0x14000417c00, 0x20?)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/eval.go:401 +0xc8
cuelang.org/go/internal/core/adt.(*OpContext).unify(0x14000433e60, 0x14000419a70, 0x5)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/eval.go:237 +0xc5c
cuelang.org/go/internal/core/adt.(*Vertex).Finalize(...)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/internal/core/adt/composite.go:650
cuelang.org/go/cue.manifest(...)
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/cue/context.go:477
cuelang.org/go/cue.remakeValue({0x140003c7fb0?, 0x140004183f0?, 0x0?}, 0x104419240?, {0x1049e3378?, 0x140003f3d10?})
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/cue/types.go:701 +0x1c4
cuelang.org/go/cue.Value.Expr({0x140003c7fb0?, 0x140004183f0?, 0x0?})
	/Users/roshan/.gvm/pkgsets/go1.21/global/pkg/mod/cuelang.org/go@v0.7.1/cue/types.go:2358 +0x3ac
main.checkOperation({0x140003c7fb0, 0x140004183f0, 0x0})

What did you expect to see?

expected to see false when checking the operation.

What did you see instead?

panic

below are the combinations i tried which failed

1. go 1.20 and cue 0.7.1
2. go 1.21 and cue 0.7.0

[sahroshan]

After trying out different cue. Below are the observation:

Given a simple cue

a: string
b: bool | *false
if !b {
    c: a
}

The above panic error occurs only when there is a if condition evaluating to true AND there is reference to a key outside the scope of if block.

In this case c: a causes a panic.

@mvdan any workaround till root cause is fixed would be highly appreciated as we are in middle of upgrading our tool's cue library.

[mvdan]

Thank you for the detailed report and the reproducer! I believe that this is the same underlying bug as #2411, which did not have a reproducer but shows an extremely similar call stack, as well as #2619, which does have a reproducer as the OpenAPI encoder calls the same Expr method.

I will opt for closing the other two older issues in favor of this one given how you've managed to produce the smallest reproducer so far. Before we mark this bug as "fixed", I will verify that the fix also solves the other two issues, to double check that I didn'g get it wrong in terms of all three bugs having the same root cause.

Below is your reproducer in testscript form, after some extra reduction:

go mod tidy
go run main.go

-- main.go --
package main

import (
	"fmt"

	"cuelang.org/go/cue"
	"cuelang.org/go/cue/cuecontext"
)

func callExprs(val cue.Value) {
	if fields, err := val.Fields(); err == nil {
		for fields.Next() {
			callExprs(fields.Value())
		}
	}
	fmt.Println(val)
	op, vals := val.Expr()
	fmt.Println(op, vals)
}

func main() {
	ctx := cuecontext.New()
	val := ctx.CompileString(`
		a: string
		if true {
			c: a
		}
	`)
	if err := val.Err(); err != nil {
		panic(err)
	}
	callExprs(val)
}
-- go.mod --
module test

go 1.21

require cuelang.org/go v0.8.0

[mvdan]

@sahroshan @kajogo777 @shortwavedave please let us know if you still encounter this panic :)