This program demonstrates that Packer hangs when long lines need to be logged.

_instructions.md

1. go build -o packer-builder-bug
2. PACKER_LOG=1 ./packer build test.json

main.go

package main

import (
        "context"
        "log"
        "strings"
        "time"

        "github.com/hashicorp/packer/packer"
        "github.com/hashicorp/packer/packer/plugin"
)

func main() {
        server, err := plugin.Server()
        if err != nil {
                panic(err)
        }

        server.RegisterBuilder(new(testBuilder))
        server.Serve()
}

type testBuilder struct{}

func (*testBuilder) Prepare(...interface{}) ([]string, error) { return nil, nil }
func (*testBuilder) Run(context.Context, packer.Ui, packer.Hook) (packer.Artifact, error) {
        time.Sleep(time.Second)
        log.Print(strings.Repeat("Logging test - ", 10000))
        time.Sleep(time.Second)
        return nil, nil
}

test.json

{
  "builders":[
    {"type": "bug"}
  ]
}