container: remove PATH lookup behaviour

This is way higher level than the container package and does not even work unless every path is mounted in the exact same location.

This behaviour causes nothing but confusion and problems,

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/container.go

@@ -27,8 +27,6 @@ type (
 	// Container represents a container environment being prepared or run.
 	// None of [Container] methods are safe for concurrent use.
 	Container struct {
-		// Name of initial process in the container.
-		name string
 		// Cgroup fd, nil to disable.
 		Cgroup *int
 		// ExtraFiles passed through to initial process in the container,
@@ -190,31 +188,12 @@ func (p *Container) Serve() error {
 	setup := p.setup
 	p.setup = nil
 
-	if p.Path != "" && !path.IsAbs(p.Path) {
+	if !path.IsAbs(p.Path) {
 		p.cancel()
 		return msg.WrapErr(EINVAL,
 			fmt.Sprintf("invalid executable path %q", p.Path))
 	}
 
-	if p.Path == "" {
-		if p.name == "" {
-			p.Path = os.Getenv("SHELL")
-			if !path.IsAbs(p.Path) {
-				p.cancel()
-				return msg.WrapErr(EBADE,
-					"no command specified and $SHELL is invalid")
-			}
-			p.name = path.Base(p.Path)
-		} else if path.IsAbs(p.name) {
-			p.Path = p.name
-		} else if v, err := exec.LookPath(p.name); err != nil {
-			p.cancel()
-			return msg.WrapErr(err, err.Error())
-		} else {
-			p.Path = v
-		}
-	}
-
 	if p.SeccompRules == nil {
 		// do not transmit nil
 		p.SeccompRules = make([]seccomp.NativeRule, 0)
@@ -251,8 +230,15 @@ func (p *Container) ProcessState() *os.ProcessState {
 	return p.cmd.ProcessState
 }
 
-func New(ctx context.Context, name string, args ...string) *Container {
-	return &Container{name: name, ctx: ctx,
-		Params: Params{Args: append([]string{name}, args...), Dir: FHSRoot, Ops: new(Ops)},
-	}
+// New returns the address to a new instance of [Container] that requires further initialisation before use.
+func New(ctx context.Context) *Container {
+	return &Container{ctx: ctx, Params: Params{Dir: FHSRoot, Ops: new(Ops)}}
+}
+
+// NewCommand calls [New] and initialises the [Params.Path] and [Params.Args] fields.
+func NewCommand(ctx context.Context, pathname, name string, args ...string) *Container {
+	z := New(ctx)
+	z.Path = pathname
+	z.Args = append([]string{name}, args...)
+	return z
 }