cmd/entrypoint: do not interpret anything after --

The way the `flag` package works, it can eat the flag
"terminator" (aka the double dash `--`). This means that, in some very
specific cases — where the first item after the `--` is also a
subcommand, it would execute the entrypoint subcommand instead of the
actual command.

For example :

```
$ /ko-app/entrypoint -- init a b
$ /ko-app/entrypoint init a b
```

This is fixed by making sure we remove anything after `--` for the
subcommand processing. And then we pass the rest (after `--`) to the
entrypointer to be executed.

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

cmd/entrypoint/main.go

@@ -78,9 +78,28 @@ func main() {
 	gitcreds.AddFlags(flag.CommandLine)
 	dockercreds.AddFlags(flag.CommandLine)
 
-	flag.Parse()
+	// Detect if `--` is present, if it is, parse only the one before.
+	terminatorIndex := -1
+	commandArgs := []string{}
+	args := os.Args[1:]
+	for i, a := range os.Args {
+		if a == "--" {
+			terminatorIndex = i
+			break
+		}
+	}
+	if terminatorIndex > 0 {
+		commandArgs = os.Args[terminatorIndex+1:]
+		args = os.Args[1:terminatorIndex]
+	} else {
+		args = os.Args[1:]
+	}
 
-	if err := subcommands.Process(flag.Args()); err != nil {
+	// We are using the global variable flag.CommandLine here to be able
+	// to define what args it should parse.
+	// flag.Parse() does flag.CommandLine.Parse(os.Args[1:])
+	flag.CommandLine.Parse(args)
+	if err := subcommands.Process(flag.CommandLine.Args()); err != nil {
 		log.Println(err.Error())
 		switch err.(type) {
 		case subcommands.SubcommandSuccessful:
@@ -123,7 +142,7 @@ func main() {
 	}
 
 	e := entrypoint.Entrypointer{
-		Command:             append(cmd, flag.Args()...),
+		Command:             append(cmd, commandArgs...),
 		WaitFiles:           strings.Split(*waitFiles, ","),
 		WaitFileContent:     *waitFileContent,
 		PostFile:            *postFile,

examples/v1beta1/taskruns/5080-entrypoint-init-regression.yaml

@@ -0,0 +1,27 @@
+apiVersion: tekton.dev/v1beta1
+kind: TaskRun
+metadata:
+  generateName: tkn-arg-test-
+spec:
+  params:
+  - name: ARGS
+    value:
+    - init
+    - foo
+    - bar
+  taskSpec:
+    description: >-
+      Test consuming args, acts as a regression test for bug 5080
+    params:
+    - name: ARGS
+      description: The terraform cli commands to tun
+      type: array
+      default:
+      - "--help"
+    steps:
+      - name: echo-cli
+        image: registry.access.redhat.com/ubi9/ubi-minimal:9.0.0-1580
+        workingDir: /tekton/home
+        args:
+        - "$(params.ARGS)"
+        command: ["echo"]