feat: 为执行后端增加独立容量池

internal/backendpool/pool.go

@@ -0,0 +1,50 @@
+// Package backendpool manages runtime capacity independently for each executor backend.
+package backendpool
+
+import "sync"
+
+type Pool struct {
+	mu       sync.Mutex
+	capacity int
+	active   map[string]struct{}
+}
+
+func New(capacity int) *Pool {
+	return &Pool{capacity: capacity, active: make(map[string]struct{})}
+}
+
+// Acquire reserves a backend slot without blocking. Redelivery of the same
+// assignment is idempotent and succeeds even while the pool is full.
+func (p *Pool) Acquire(assignmentID string) bool {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if _, exists := p.active[assignmentID]; exists {
+		return true
+	}
+	if assignmentID == "" || len(p.active) >= p.capacity {
+		return false
+	}
+	p.active[assignmentID] = struct{}{}
+	return true
+}
+
+func (p *Pool) Restore(assignmentID string) {
+	if assignmentID == "" {
+		return
+	}
+	p.mu.Lock()
+	p.active[assignmentID] = struct{}{}
+	p.mu.Unlock()
+}
+
+func (p *Pool) Release(assignmentID string) {
+	p.mu.Lock()
+	delete(p.active, assignmentID)
+	p.mu.Unlock()
+}
+
+func (p *Pool) Active() int {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	return len(p.active)
+}