ModelRouter/internal/lua/plugin_shard_config_test.go

package lua

import (
	"os"
	"path/filepath"
	"testing"
)

// A shard VM is booted from the same plugin SOURCE but has no state file and no
// plugin identity, so anything a plugin reads from its `prices` table
// (configuration, as opposed to accumulated state) is empty unless the pool puts
// it there.
//
// That asymmetry is invisible until a plugin's behaviour depends on its
// configuration. The OpenDesign adapter is the first: it reads
// prices.control_key, and without seeding it concluded "not configured" for every
// host and reported a skip — while the identical call through the shared VM
// worked.
//
// It mattered because the two paths do different jobs: the page probes through
// the FANOUT (shard) path and records results through the shared one. A feature
// broken on the shard path is dead in production and green in every test that
// calls the shared VM.

const shardConfigProbe = `
local P = {}
P.name = "cfgprobe"
P.version = "1.0.0"
P.exports = { "read_config" }
P.read_config = function(payload)
  local c = P.prices
  return { key = (type(c) == "table") and c.control_key or nil }, nil
end
return P
`

func shardConfigPlugins(t *testing.T) *Plugins {
	t.Helper()
	dir := t.TempDir()
	pluginDir := filepath.Join(dir, "plugins")
	if err := os.MkdirAll(pluginDir, 0o755); err != nil {
		t.Fatal(err)
	}
	vm := NewVM(filepath.Join(dir, "adapters"))
	if err := vm.Start(); err != nil {
		t.Fatal(err)
	}
	t.Cleanup(vm.Stop)
	if err := os.WriteFile(filepath.Join(pluginDir, "cfgprobe.lua"),
		[]byte(shardConfigProbe), 0o644); err != nil {
		t.Fatal(err)
	}
	ps := NewPlugins(vm, pluginDir)
	t.Cleanup(ps.Close)
	if err := ps.LoadDir(); err != nil {
		t.Fatal(err)
	}
	return ps
}

func configKeyOf(t *testing.T, out interface{}) string {
	t.Helper()
	m, ok := out.(map[string]interface{})
	if !ok {
		t.Fatalf("result is %T (%v), want a table", out, out)
	}
	s, _ := m["key"].(string)
	return s
}

func TestShardSeesPluginConfiguration(t *testing.T) {
	ps := shardConfigPlugins(t)
	if err := ps.SetState("cfgprobe", map[string]interface{}{
		"prices": map[string]interface{}{"control_key": "secret-value"},
	}); err != nil {
		t.Fatal(err)
	}

	// The shared path is the control: every pre-existing test exercises it, and
	// it passes whether or not the pool seeds anything.
	out, err := ps.Call("cfgprobe", "read_config", map[string]interface{}{})
	if err != nil {
		t.Fatalf("shared call: %v", err)
	}
	if got := configKeyOf(t, out); got != "secret-value" {
		t.Fatalf("shared VM cannot read prices.control_key (%q) — the test itself "+
			"is wrong, not the shard path", got)
	}

	// The shard path is the one the page uses.
	outs, err := ps.ShardCall("cfgprobe", "read_config",
		[]map[string]interface{}{{}, {}, {}})
	if err != nil {
		t.Fatalf("ShardCall: %v", err)
	}
	for i, o := range outs {
		if got := configKeyOf(t, o); got != "secret-value" {
			t.Fatalf("shard %d read prices.control_key as %q, want the configured "+
				"value — a plugin that branches on its configuration behaves "+
				"differently on the path the page actually uses", i, got)
		}
	}
}

// A pooled VM is reused, so the seeding has to happen on every checkout rather
// than only on boot, or a configuration change would be invisible to the pool
// until the process restarted.
func TestShardConfigFollowsAConfigurationChange(t *testing.T) {
	ps := shardConfigPlugins(t)
	if err := ps.SetState("cfgprobe", map[string]interface{}{
		"prices": map[string]interface{}{"control_key": "first"},
	}); err != nil {
		t.Fatal(err)
	}
	if _, err := ps.ShardCall("cfgprobe", "read_config", []map[string]interface{}{{}}); err != nil {
		t.Fatal(err)
	}
	if err := ps.SetState("cfgprobe", map[string]interface{}{
		"prices": map[string]interface{}{"control_key": "second"},
	}); err != nil {
		t.Fatal(err)
	}
	outs, err := ps.ShardCall("cfgprobe", "read_config",
		[]map[string]interface{}{{}, {}, {}, {}})
	if err != nil {
		t.Fatal(err)
	}
	for i, o := range outs {
		if got := configKeyOf(t, o); got != "second" {
			t.Fatalf("shard %d still reads %q after the configuration changed to "+
				"%q — pooled VMs keep a stale configuration", i, got, "second")
		}
	}
}

// Reading must not disturb the shared value. That is what makes it acceptable to
// seed configuration into a VM whose state is otherwise throwaway, and it is why
// the seeding is a one-way copy rather than a reference to the shared table.
func TestShardConfigSeedingDoesNotAliasSharedState(t *testing.T) {
	ps := shardConfigPlugins(t)
	if err := ps.SetState("cfgprobe", map[string]interface{}{
		"prices": map[string]interface{}{"control_key": "keep-me"},
	}); err != nil {
		t.Fatal(err)
	}
	outs, err := ps.ShardCall("cfgprobe", "read_config", []map[string]interface{}{{}})
	if err != nil {
		t.Fatal(err)
	}
	if got := configKeyOf(t, outs[0]); got != "keep-me" {
		t.Fatalf("shard read %q", got)
	}
	out, err := ps.Call("cfgprobe", "read_config", map[string]interface{}{})
	if err != nil {
		t.Fatal(err)
	}
	if got := configKeyOf(t, out); got != "keep-me" {
		t.Fatalf("the shared configuration changed to %q after a shard read", got)
	}
}