panic: runtime error: invalid memory address or nil pointer dereference when docker-compose.yaml empty

[ivosh]

By accident, docker-compose.yaml happened to be an empty file.
IMO docker compose plugin should not crash on this singularity but rather complain with a useful error message.

How to reproduce:

1. Install docker engine and docker compose plugin. I used RHEL 8.2 with the latest docker CE and docker-compose-plugin:

$ docker version
Client: Docker Engine - Community
 Version:           24.0.7
 API version:       1.43
 Go version:        go1.20.10
 Git commit:        afdd53b
 Built:             Thu Oct 26 09:09:18 2023
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          24.0.7
  API version:      1.43 (minimum version 1.12)
  Go version:       go1.20.10
  Git commit:       311b9ff
  Built:            Thu Oct 26 09:08:20 2023
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.6.24
  GitCommit:        61f9fd88f79f081d64d6fa3bb1a0dc71ec870523
 runc:
  Version:          1.1.9
  GitCommit:        v1.1.9-0-gccaecfc
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

$ docker compose version
Docker Compose version v2.21.0

2. Create an empty docker-compose.yaml file:

touch docker-compose.yaml

3. Crash docker compose plugin with:

$ docker compose down
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x108bf33]

goroutine 1 [running]:
github.com/compose-spec/compose-go/loader.load({0x2638cb8, 0xc00011e010}, {{0x0, 0x0}, {0xc0001cd560, 0x1c}, {0xc0003cc120, 0x1, 0x1}, 0xc000131e90}, ...)
        github.com/compose-spec/[email protected]/loader/loader.go:343 +0x893
github.com/compose-spec/compose-go/loader.LoadWithContext({0x2638cb8, 0xc00011e010}, {{0x0, 0x0}, {0xc0001cd560, 0x1c}, {0xc0003cc120, 0x1, 0x1}, 0xc000131e90}, ...)
        github.com/compose-spec/[email protected]/loader/loader.go:242 +0x2a5
github.com/compose-spec/compose-go/cli.ProjectFromOptions(0xc0000cbe00)
        github.com/compose-spec/[email protected]/cli/options.go:402 +0x4f2
github.com/docker/compose/v2/cmd/compose.(*ProjectOptions).ToProject(0xc0000cb700, {0x0, 0x0, 0x0}, {0xc00052f748?, 0x375e558?, 0x375e558?})
        github.com/docker/compose/v2/cmd/compose/compose.go:221 +0xfb
github.com/docker/compose/v2/cmd/compose.(*ProjectOptions).projectOrName(0x417e50?, {0x0?, 0xc00052f828?, 0x417e50?})
        github.com/docker/compose/v2/cmd/compose/compose.go:180 +0x5c
github.com/docker/compose/v2/cmd/compose.runDown({0x2638c80, 0xc0002d6d70}, {0x2651338, 0xc0000c6600}, {0xc0000cb700, 0x0, 0x0, 0x0, 0x0, {0x0, ...}}, ...)
        github.com/docker/compose/v2/cmd/compose/down.go:80 +0x75
github.com/docker/compose/v2/cmd/compose.downCommand.func2({0x2638c80?, 0xc0002d6d70?}, {0x375e558?, 0xc0000061a0?, 0x1d12c3a?})
        github.com/docker/compose/v2/cmd/compose/down.go:59 +0x65
github.com/docker/compose/v2/cmd/compose.Adapt.func1({0x2638c80?, 0xc0002d6d70?}, 0x2?, {0x375e558?, 0x1?, 0xc00052f950?})
        github.com/docker/compose/v2/cmd/compose/compose.go:108 +0x36
github.com/docker/compose/v2/cmd/compose.AdaptCmd.func1(0xc000004c00, {0x375e558, 0x0, 0x0})
        github.com/docker/compose/v2/cmd/compose/compose.go:87 +0x21c
github.com/docker/compose/v2/cmd/cmdtrace.wrapRunE.func2(0xc000004c00?, {0x375e558?, 0x0?, 0x0?})
        github.com/docker/compose/v2/cmd/cmdtrace/cmd_span.go:80 +0x73
github.com/spf13/cobra.(*Command).execute(0xc000004c00, {0xc000098150, 0x0, 0x0})
        github.com/spf13/[email protected]/command.go:940 +0x862
github.com/spf13/cobra.(*Command).ExecuteC(0xc000282f00)
        github.com/spf13/[email protected]/command.go:1068 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
        github.com/spf13/[email protected]/command.go:992
github.com/docker/cli/cli-plugins/plugin.RunPlugin(0x188?, 0xc000004600, {{0x22d0b8e, 0x5}, {0x22daca9, 0xb}, {0x2611938, 0x7}, {0x0, 0x0}, ...})
        github.com/docker/[email protected]+incompatible/cli-plugins/plugin/plugin.go:51 +0x130
github.com/docker/cli/cli-plugins/plugin.Run(0x240e410, {{0x22d0b8e, 0x5}, {0x22daca9, 0xb}, {0x2611938, 0x7}, {0x0, 0x0}, {0x0, ...}, ...})
        github.com/docker/[email protected]+incompatible/cli-plugins/plugin/plugin.go:64 +0xee
main.pluginMain()
        github.com/docker/compose/v2/cmd/main.go:37 +0xdf
main.main()
        github.com/docker/compose/v2/cmd/main.go:75 +0x198

[ndeloof]

This has been fixed by 2a83d1c
I also proposed a test case to cover this scenario to prevent any regression with this

[ivosh]

Thank you, @ndeloof!