Panic on printing some YAML as JSON

[haguenau]

Describe the bug

Some YAML documents cause yq to panic. I would expect either a clear error message (assuming input is invalid), or no error at all (assuming input is valid).

Version of yq: 4.29.2
Operating system: macOS
Installed via: binary release

Input Yaml

input.yml:

---
a: &a
  - 0

b: &b
  - 1

c:
  <<: [*a, *b]

Command
The command you ran:

yq-4.29.2 . <in.yml -o json

Actual behavior

panic: runtime error: index out of range [1] with length 1

goroutine 1 [running]:
github.com/mikefarah/yq/v4/pkg/yqlib.applyAlias(0xc000287310?, 0xc000354e60, 0x1486786?, {0xc00034e780, 0xc00034e7e0, 0x0, {0x0, 0x0}})
	/home/runner/work/yq/yq/pkg/yqlib/operator_anchors_aliases.go:250 +0x1b3
github.com/mikefarah/yq/v4/pkg/yqlib.reconstructAliasedMap(0xc000355040, {0xc00034e660, 0x0, 0x0, {0x0, 0x0}})
	/home/runner/work/yq/yq/pkg/yqlib/operator_anchors_aliases.go:163 +0x599
github.com/mikefarah/yq/v4/pkg/yqlib.explodeNode(0xc000355040, {0xc00034e660, 0x0, 0x0, {0x0, 0x0}})
	/home/runner/work/yq/yq/pkg/yqlib/operator_anchors_aliases.go:222 +0x30c
github.com/mikefarah/yq/v4/pkg/yqlib.explodeNode(0xc000354b40, {0xc00034e660, 0x0, 0x0, {0x0, 0x0}})
	/home/runner/work/yq/yq/pkg/yqlib/operator_anchors_aliases.go:232 +0x3d0
github.com/mikefarah/yq/v4/pkg/yqlib.explodeNode(0xc000200780?, {0xc00034e660, 0x0, 0x0, {0x0, 0x0}})
	/home/runner/work/yq/yq/pkg/yqlib/operator_anchors_aliases.go:192 +0x29b
github.com/mikefarah/yq/v4/pkg/yqlib.explodeOperator(0xc000200780?, {0xc00034e660, 0x0, 0x0, {0x0, 0x0}}, 0xc0002143c0)
	/home/runner/work/yq/yq/pkg/yqlib/operator_anchors_aliases.go:133 +0x2fe
github.com/mikefarah/yq/v4/pkg/yqlib.(*dataTreeNavigator).GetMatchingNodes(0xc000200780?, {0xc00034e660, 0x0, 0x0, {0x0, 0x0}}, 0xc0002143c0)
	/home/runner/work/yq/yq/pkg/yqlib/data_tree_navigator.go:37 +0x25c
github.com/mikefarah/yq/v4/pkg/yqlib.(*resultsPrinter).PrintResults(0xc0002064e0, 0xc00034e660)
	/home/runner/work/yq/yq/pkg/yqlib/printer.go:96 +0x203
github.com/mikefarah/yq/v4/pkg/yqlib.(*streamEvaluator).Evaluate(0xc000287c90, {0x1479d0d, 0x1}, {0x1546c20?, 0xc0002065a0?}, 0x1f53308?, {0x1548f78, 0xc0002064e0}, {0x1548210, 0xc00034e450})
	/home/runner/work/yq/yq/pkg/yqlib/stream_evaluator.go:112 +0x339
github.com/mikefarah/yq/v4/pkg/yqlib.(*streamEvaluator).EvaluateFiles(0xc0000a0008?, {0x7ff7bfefefd2, 0x1}, {0xc000226c10, 0x1, 0xc00021e500?}, {0x1548f78, 0xc0002064e0}, {0x1548210, 0xc00034e450})
	/home/runner/work/yq/yq/pkg/yqlib/stream_evaluator.go:65 +0x1af
github.com/mikefarah/yq/v4/cmd.evaluateSequence(0xc00022c300?, {0xc000226c00, 0x1, 0x3})
	/home/runner/work/yq/yq/cmd/evalute_sequence_command.go:132 +0x899
github.com/spf13/cobra.(*Command).execute(0xc00022c300, {0xc000226ba0, 0x3, 0x3})
	/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:916 +0x862
github.com/spf13/cobra.(*Command).ExecuteC(0xc00022c000)
	/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:1044 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
	/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:968
main.main()
	/home/runner/work/yq/yq/yq.go:22 +0x1e5

Expected behavior

{
  "a": [
    0
  ],
  "b": [
    1
  ],
  "c": [
    0,
    1
  ]
}

Additional context
I don't have a strong opinion on whether the input YAML is valid. Note that if YAML output is requested, no panic occurs; instead, slightly different YAML text is emitted:

---
a: &a
  - 0
b: &b
  - 1
c:
  !!merge <<: [*a, *b]

This document also causes a panic with yq -o json.

[mikefarah]

Oh I see - this is a problem with the yaml, merge anchors only work with maps and both 'a' and 'b' are sequences. I will update yq to return a proper error message.

[mikefarah]

Fixed in 4.30.1

[haguenau]

Thanks for the impressively quick fix! Indeed, I get a proper error message this morning on Homebrew version 4.30.1.