sandbox: override underlying command function

Mostly useful for test instrumentation or otherwise nifty environments.

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

internal/sandbox/container.go

@@ -60,6 +60,8 @@ type (
 		InitParams
 		// Custom [exec.Cmd] initialisation function.
 		CommandContext func(ctx context.Context) (cmd *exec.Cmd)
+		// Underlying [exec.CommandContext] function called by CommandContext.
+		RawCommandContext *func(ctx context.Context, name string, args ...string) (cmd *exec.Cmd)
 
 		// param encoder for shim and init
 		setup *gob.Encoder
@@ -220,12 +222,15 @@ func (p *Container) String() string {
 }
 
 func New(ctx context.Context, name string, args ...string) *Container {
-	return &Container{name: name, ctx: ctx,
-		InitParams: InitParams{Args: append([]string{name}, args...), Dir: "/", Ops: new(Ops)},
-		CommandContext: func(ctx context.Context) (cmd *exec.Cmd) {
-			cmd = exec.CommandContext(ctx, internal.MustExecutable())
-			cmd.Args = []string{"init"}
-			return
-		},
+	commandContext := exec.CommandContext
+	container := &Container{name: name, ctx: ctx,
+		InitParams:        InitParams{Args: append([]string{name}, args...), Dir: "/", Ops: new(Ops)},
+		RawCommandContext: &commandContext,
 	}
+	container.CommandContext = func(ctx context.Context) (cmd *exec.Cmd) {
+		cmd = (*container.RawCommandContext)(ctx, internal.MustExecutable())
+		cmd.Args = []string{"init"}
+		return
+	}
+	return container
 }