opa build panics on second end-of-line comment in function call

[anbrsap]

Expected Behavior

The following rego code should compile, run and opa build.

testcase.rego:

package testcase

rule1 = contains(
    "", # first comment
    "", # second comment lets `opa build` panic
)

Actual Behavior

opa build panics:

$ opa build testcase.rego 
panic: overwriting non-nil beforeEnd

goroutine 1 [running]:
github.com/open-policy-agent/opa/format.(*writer).beforeLineEnd(...)
	/src/format/format.go:1044
github.com/open-policy-agent/opa/format.(*writer).insertComments(0xc000114c00, {0xc00011e500, 0x1426700, 0xc00021f1b8}, 0xc0001473c0)
	/src/format/format.go:378 +0x117
github.com/open-policy-agent/opa/format.(*writer).writeTermParens(0xc000114c00, 0x3a, 0xc0004192f0, {0xc00011e500, 0x412afd, 0xc00011e4b0})
	/src/format/format.go:517 +0x57
github.com/open-policy-agent/opa/format.(*writer).writeTerm(...)
	/src/format/format.go:513
github.com/open-policy-agent/opa/format.(*writer).writeFunctionCallPlain(0xc000114c00, {0xc000419278, 0x3, 0x8}, {0xc000412e50, 0x2, 0x2})
	/src/format/format.go:498 +0x1e6
github.com/open-policy-agent/opa/format.(*writer).writeCall(0xc000114c00, 0x0, {0xc000419278, 0x3, 0x3}, {0xc000412e50, 0x2, 0x2})
	/src/format/format.go:602 +0x26d
github.com/open-policy-agent/opa/format.(*writer).writeTermParens(0xc000114c00, 0x54, 0xc000419260, {0xc000412e40, 0x13eb120, 0xc000412e40})
	/src/format/format.go:548 +0x405
github.com/open-policy-agent/opa/format.(*writer).writeTerm(...)
	/src/format/format.go:513
github.com/open-policy-agent/opa/format.(*writer).writeHead(0xc000114c00, 0xc00017e460, 0x0, 0x1, {0xc000412e40, 0x2, 0x2})
	/src/format/format.go:366 +0x312
github.com/open-policy-agent/opa/format.(*writer).writeRule(0xc000114c00, 0xc000147580, 0x0, {0xc000412e40, 0x2, 0x2})
	/src/format/format.go:248 +0x20a
github.com/open-policy-agent/opa/format.(*writer).writeRules(0xc000412de0, {0xc00011e4e8, 0x1, 0x0}, {0xc000412e30, 0x0, 0xc00021f5f8})
	/src/format/format.go:223 +0x72
github.com/open-policy-agent/opa/format.(*writer).writeModule(0xc000114c00, 0xc0004021c0)
	/src/format/format.go:190 +0x2d3
github.com/open-policy-agent/opa/format.Ast({0xc75320, 0xc000402150})
	/src/format/format.go:74 +0x193
github.com/open-policy-agent/opa/format.Source({0x7fff4a746288, 0xd}, {0xc000174200, 0xffffffffffffffff, 0x4a2c91})
	/src/format/format.go:25 +0x7d
github.com/open-policy-agent/opa/bundle.(*Bundle).FormatModules(0xc00014a580, 0x0)
	/src/bundle/bundle.go:778 +0xeb
github.com/open-policy-agent/opa/compile.(*Compiler).Build(0xc00021fb98, {0xe3a790, 0xc0000340c0})
	/src/compile/compile.go:200 +0x11f
github.com/open-policy-agent/opa/cmd.dobuild({0xc000130168, 0xc000118480, 0x0, 0x0, {{0x0, 0x0, 0x0}, 0x0}, {0xd0f294, 0xd}, ...}, ...)
	/src/cmd/build.go:293 +0x70d
github.com/open-policy-agent/opa/cmd.init.1.func2(0xc00013c280, {0xc00011c590, 0x1, 0x1})
	/src/cmd/build.go:210 +0x6c
github.com/spf13/cobra.(*Command).execute(0xc00013c280, {0xc00011c560, 0x1, 0x1})
	/src/vendor/github.com/spf13/cobra/command.go:860 +0x5f8
github.com/spf13/cobra.(*Command).ExecuteC(0x13ddb80)
	/src/vendor/github.com/spf13/cobra/command.go:974 +0x3bc
github.com/spf13/cobra.(*Command).Execute(...)
	/src/vendor/github.com/spf13/cobra/command.go:902
main.main()
	/src/main.go:15 +0x25

However, the code compiles and runs:

$ opa eval -d testcase.rego 'data.testcase.rule1'
{
  "result": [
    {
      "expressions": [
        {
          "value": true,
          "text": "data.testcase.rule1",
          "location": {
            "row": 1,
            "col": 1
          }
        }
      ]
    }
  ]
}

Steps to Reproduce the Problem

Save testcase.rego to disk and run the commands shown above.

• OPA version: 0.32.1

Additional Info

[srenatus]

FWIW this one and #3832 are related to formatting (pretty-printing) policies. This happens as one of the steps in using opa build. The same panics can be triggered by using opa fmt, which would only do the formatting: opa fmt testcase.rego

[srenatus]

So what's happening here is that it attempts to make it a one-line statement:

rule1 = contains("", "") # first comment

and doesn't know what to do with the second comment.