feat(dns): add Lua scripting for DNS queries

common/lua/pool.go

@@ -0,0 +1,138 @@
+package lua
+
+import (
+	"context"
+	"errors"
+	"sync"
+
+	glua "github.com/yuin/gopher-lua"
+)
+
+const maxIdleStates = 16
+
+// LStateFactory must initialize a state fully and observe ctx while doing so.
+// The pool owns any non-nil state it returns, even when it also returns an error.
+type LStateFactory func(ctx context.Context) (*glua.LState, error)
+
+// Pool lends each state to one caller at a time. It grows on contention and
+// keeps up to maxIdleStates idle states until Close. Callers decide whether a
+// state is reusable.
+type Pool struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+
+	factory LStateFactory
+	idle    []*glua.LState
+
+	mu     sync.Mutex
+	active sync.WaitGroup
+	closed bool
+}
+
+// NewPool initializes one state before returning, so top-level errors surface at startup.
+func NewPool(ctx context.Context, factory LStateFactory) (*Pool, error) {
+	poolCtx, cancel := context.WithCancel(ctx)
+
+	// Create one state now to catch factory errors at startup.
+	state, err := factory(poolCtx)
+	if err != nil {
+		cancel()
+		if state != nil {
+			state.Close()
+		}
+		return nil, err
+	}
+	if state == nil {
+		cancel()
+		return nil, errors.New("Lua state factory returned nil")
+	}
+	if err := poolCtx.Err(); err != nil {
+		state.Close()
+		cancel()
+		return nil, err
+	}
+
+	return &Pool{ctx: poolCtx, cancel: cancel, factory: factory, idle: []*glua.LState{state}}, nil
+}
+
+// Context is cancelled by Close. Query contexts should derive from it.
+func (p *Pool) Context() context.Context {
+	return p.ctx
+}
+
+// Acquire returns an initialized exclusive state, growing the pool if necessary.
+func (p *Pool) Acquire() (*glua.LState, error) {
+	p.mu.Lock()
+	if p.closed || p.ctx.Err() != nil {
+		p.mu.Unlock()
+		return nil, p.ctx.Err()
+	}
+
+	p.active.Add(1)
+
+	n := len(p.idle)
+	if n != 0 {
+		state := p.idle[n-1]
+		p.idle = p.idle[:n-1]
+		p.mu.Unlock()
+		return state, nil
+	}
+	p.mu.Unlock()
+
+	// TODO: Limit the total number of states. When the limit is reached, wait
+	// for a Release instead of creating another state; allow the wait to be
+	// cancelled by the caller or by Close.
+	state, err := p.factory(p.ctx)
+	if err == nil && state == nil {
+		err = errors.New("Lua state factory returned nil")
+	}
+	if err != nil {
+		if state != nil {
+			state.Close()
+		}
+		p.active.Done()
+		return nil, err
+	}
+	if err := p.ctx.Err(); err != nil {
+		state.Close()
+		p.active.Done()
+		return nil, err
+	}
+
+	return state, nil
+}
+
+// Release returns a healthy state to the pool and closes a failed or cancelled one.
+func (p *Pool) Release(state *glua.LState, reusable bool) {
+	if reusable {
+		p.mu.Lock()
+		if !p.closed && p.ctx.Err() == nil && len(p.idle) < maxIdleStates {
+			p.idle = append(p.idle, state)
+		} else {
+			reusable = false
+		}
+		p.mu.Unlock()
+	}
+
+	if !reusable {
+		state.Close()
+	}
+
+	p.active.Done()
+}
+
+// Close cancels active work, closes idle states, and waits for borrowed states.
+func (p *Pool) Close() {
+	p.mu.Lock()
+	if !p.closed {
+		p.closed = true
+		p.cancel()
+		for _, state := range p.idle {
+			state.Close()
+		}
+		p.idle = nil
+	}
+	p.mu.Unlock()
+
+	p.active.Wait()
+}