Packer intermittent crashes: panic: ConfigSpec failed: connection is shut down [recovered]

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#### Overview of the Issue

Intermittent crashes with packer build. See subject/title. Packer requested I report this crash:

!!!!!!!!!!!!!!!!!!!!!!!!!!! PACKER CRASH !!!!!!!!!!!!!!!!!!!!!!!!!!!!


Packer crashed! This is always indicative of a bug within Packer.

A crash log has been placed at "crash.log" relative to your current

working directory. It would be immensely helpful if you could please

report the crash with Packer[1] so that we can fix this.

[1]: https://github.com/hashicorp/packer/issues

!!!!!!!!!!!!!!!!!!!!!!!!!!! PACKER CRASH !!!!!!!!!!!!!!!!!!!!!!!!!!!!

#### Reproduction Steps

packer build .

### Packer version

1.8.5

### Simplified Packer Template

Need private way to provide this. Also the crash.log.

### Operating system and Environment details

Ubuntu 22.04, x86_64

### Log Fragments and crash.log files

see above.

Set the env var `PACKER_LOG=1` for maximum log detail.

tail of crash log:

2023/02/27 17:57:27 ConfigSpec failed: connection is shut down

panic: ConfigSpec failed: connection is shut down [recovered]

        panic: ConfigSpec failed: connection is shut down


goroutine 772 [running]:

log.Panic({0xc0014ccf88?, 0x1?, 0xc000bfc460?})

        /opt/hostedtoolcache/go/1.18.9/x64/src/log/log.go:385 +0x65

github.com/hashicorp/packer/packer.(*cmdProvisioner).checkExit(0xc0004ab730?, {0x4017300, 0xc00088ed30}, 0x0)

        /home/runner/work/packer/packer/packer/cmd_provisioner.go:47 +0x7f

github.com/hashicorp/packer/packer.(*cmdProvisioner).ConfigSpec.func1()

        /home/runner/work/packer/packer/packer/cmd_provisioner.go:19 +0x39

panic({0x4017300, 0xc00088ed30})

        /opt/hostedtoolcache/go/1.18.9/x64/src/runtime/panic.go:838 +0x207

github.com/hashicorp/packer-plugin-sdk/rpc.(*commonClient).ConfigSpec(0xc001095620)

        /home/runner/go/pkg/mod/github.com/hashicorp/packer-plugin-sdk@v0.3.2/rpc/common.go:44 +0x297

github.com/hashicorp/packer/packer.(*cmdProvisioner).ConfigSpec(0x48fe4c0?)

        /home/runner/work/packer/packer/packer/cmd_provisioner.go:22 +0x65

github.com/hashicorp/packer/hcl2template.decodeHCL2Spec({0x5927740, 0xc000c60600}, 0x50e5c69?, {0x7f89868582d8?, 0xc00109bce0?})

        /home/runner/work/packer/packer/hcl2template/decode.go:17 +0x39

github.com/hashicorp/packer/hcl2template.(*HCL2Provisioner).HCL2Prepare(0xc0010cc450, 0xc0010a0ea0?)

        /home/runner/work/packer/packer/hcl2template/types.hcl_provisioner.go:51 +0x447

github.com/hashicorp/packer/hcl2template.(*HCL2Provisioner).Provision(0xc0010cc450, {0x5926da0, 0xc000cc0600}, {0x592bb88, 0xc0010a0e70}, {0x5929f10, 0xc000cceee0}, 0xc000cceee0?)

        /home/runner/work/packer/packer/hcl2template/types.hcl_provisioner.go:71 +0x4e

github.com/hashicorp/packer/packer.(*ProvisionHook).Run(0xc000665160?, {0x5926da0, 0xc000cc0600}, {0x7f89ad6fd108?, 0x20?}, {0x592bb88, 0xc0010a0e70}, {0x5929f10?, 0xc000cceee0}, {0x4335480, ...})

        /home/runner/work/packer/packer/packer/provisioner.go:122 +0x1cb

github.com/hashicorp/packer-plugin-sdk/packer.(*DispatchHook).Run(0x0?, {0x5926da0, 0xc000cc0600}, {0xc000665160, 0x10}, {0x592bb88, 0xc0010a0e70}, {0x5929f10, 0xc000cceee0}, {0x4335480, ...})

        /home/runner/go/pkg/mod/github.com/hashicorp/packer-plugin-sdk@v0.3.2/packer/hook.go:47 +0x172

github.com/hashicorp/packer-plugin-sdk/rpc.(*HookServer).Run(0xc000c3ddc0, 0xc0010a0c60, 0xc00088ea20)

        /home/runner/go/pkg/mod/github.com/hashicorp/packer-plugin-sdk@v0.3.2/rpc/hook.go:75 +0x263

reflect.Value.call({0xc000bfe2a0?, 0xc001553d28?, 0x13?}, {0x50e42f9, 0x4}, {0xc0007c6ef8, 0x3, 0x3?})

        /opt/hostedtoolcache/go/1.18.9/x64/src/reflect/value.go:556 +0x845

reflect.Value.Call({0xc000bfe2a0?, 0xc001553d28?, 0x0?}, {0xc0010f56f8, 0x3, 0x3})

        /opt/hostedtoolcache/go/1.18.9/x64/src/reflect/value.go:339 +0xbf

net/rpc.(*service).call(0xc000c3de00, 0x0?, 0xc000a58fc0?, 0xc000664f80, 0xc001530480, 0xc0010f57d0?, {0x3e6bbe0?, 0xc0010a0c60?, 0xc0006ba540?}, {0x3edfbe0, ...}, ...)

        /opt/hostedtoolcache/go/1.18.9/x64/src/net/rpc/server.go:381 +0x226

created by net/rpc.(*Server).ServeCodec

        /opt/hostedtoolcache/go/1.18.9/x64/src/net/rpc/server.go:478 +0x3fe

Start with packer/cmd_provisioner.go and trace the ConfigSpec call through hcl2template/decode.go and hcl2template/types.hcl_provisioner.go, using the reported stack and packer build . as the entry point. Compare this path with packer-plugin-sdk/rpc/common.go and reproduce with a shareable template and crash log. Done means the intermittent ConfigSpec failure no longer causes the reported panic, with regression coverage for the failure path.

Written by the indexing model from the issue text.