feat: respect custom exit code from plugin

[knqyf263]

Description

Trivy doesn't respect exit codes from plugins now and always exits with 1 if the plugin fails. This PR adds support for it.

Before

The testplugin always exits with 2.

$ trivy testplugin; echo $?
Error: plugin error: plugin exec: exit status 2
Usage:
  trivy testplugin [flags]

Flags:
  -h, --help   help for testplugin

Global Flags:
      --cache-dir string          cache directory (default "/Users/teppei/Library/Caches/trivy")
  -c, --config string             config path (default "trivy.yaml")
  -d, --debug                     debug mode
      --generate-default-config   write the default config to trivy-default.yaml
      --insecure                  allow insecure server connections
  -q, --quiet                     suppress progress bar and log output
      --timeout duration          timeout (default 5m0s)
  -v, --version                   show version

2024-04-30T22:56:13+04:00       FATAL   Fatal error     plugin error: plugin exec: exit status 2
1

After

It exits with the code from a plugin and doesn't show usage and error anymore.

$ trivy testplugin; echo $?
2

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[simar7]

lgtm, should we consider the case if the plugin panics? I was thinking along the lines of a global panic handler.

[knqyf263]

lgtm, should we consider the case if the plugin panics? I was thinking along the lines of a global panic handler.

Trivy calls plugins via OS command. IIUC, panic is just converted into exit status 2.

I created a plugin causing panic.

$ cat main.go
package main

func main() {
        panic("error")
}

$ go build -o trivy-panic main.go

$ ./trivy-panic
panic: error

goroutine 1 [running]:
main.main()
        /tmp/panic/main.go:4 +0x2c
$ echo $?
2

Then, run the plugin through Trivy.

$ trivy panic; echo $?
panic: error

goroutine 1 [running]:
main.main()
        /tmp/panic/main.go:4 +0x2c
$ echo $?
2

I hope it answers your question.