fix(pgxpool): track in-flight connection attempts to enforce MaxConns during recovery (#3)

pgxpool/pool.go

@@ -2,35 +2,95 @@ package pgxpool
 
 import (
 	"context"
+	"errors"
 	"sync"
 	"sync/atomic"
 )
 
+var ErrMaxConnsReached = errors.New("pgxpool: max connections reached")
+
+type Conn struct {
+	pool *Pool
+}
+
+type conn struct{}
+
 type Pool struct {
-	// ... existing fields
 	maxConns      int32
 	conns         []*conn
 	inFlightConns int32
 	mu            sync.Mutex
-	// ...
+	cond          *sync.Cond
+}
+
+func NewPool(maxConns int32) *Pool {
+	p := &Pool{
+		maxConns: maxConns,
+		conns:    make([]*conn, 0),
+	}
+	p.cond = sync.NewCond(&p.mu)
+	return p
+}
+
+func (p *Pool) MaxConns() int32 {
+	return p.maxConns
+}
+
+func (p *Pool) InFlightConns() int32 {
+	return atomic.LoadInt32(&p.inFlightConns)
+}
+
+func (p *Pool) createNewConn(ctx context.Context) (*Conn, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+	return &Conn{pool: p}, nil
+}
+
+func (p *Pool) waitForConn(ctx context.Context) (*Conn, error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	for len(p.conns)+int(atomic.LoadInt32(&p.inFlightConns)) >= int(p.maxConns) {
+		if err := ctx.Err(); err != nil {
+			return nil, err
+		}
+		p.cond.Wait()
+	}
+
+	atomic.AddInt32(&p.inFlightConns, 1)
+	p.mu.Unlock()
+
+	c, err := p.createNewConn(ctx)
+
+	p.mu.Lock()
+	atomic.AddInt32(&p.inFlightConns, -1)
+	if err != nil {
+		p.cond.Broadcast()
+		return nil, err
+	}
+	return c, nil
 }
 
 func (p *Pool) Acquire(ctx context.Context) (*Conn, error) {
 	p.mu.Lock()
-	// Check if we can create a new connection
-	if len(p.conns) + int(atomic.LoadInt32(&p.inFlightConns)) < int(p.maxConns) {
+	if len(p.conns)+int(atomic.LoadInt32(&p.inFlightConns)) < int(p.maxConns) {
 		atomic.AddInt32(&p.inFlightConns, 1)
 		p.mu.Unlock()
 
-		conn, err := p.createNewConn(ctx)
+		c, err := p.createNewConn(ctx)
+
+		p.mu.Lock()
 		atomic.AddInt32(&p.inFlightConns, -1)
 		if err != nil {
+			p.cond.Broadcast()
+			p.mu.Unlock()
 			return nil, err
 		}
-		return conn, nil
+		p.mu.Unlock()
+		return c, nil
 	}
 	p.mu.Unlock()
 
-	// Wait for existing connection or retry logic...
 	return p.waitForConn(ctx)
 }

pgxpool/pool_test.go

@@ -0,0 +1,46 @@
+package pgxpool
+
+import (
+	"context"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestPoolMaxConnsStrictCapUnderRecovery(t *testing.T) {
+	const maxConns int32 = 5
+	p := NewPool(maxConns)
+
+	var activeDials int32
+	var maxObservedDials int32
+	var wg sync.WaitGroup
+
+	const numGoroutines = 50
+	wg.Add(numGoroutines)
+
+	for i := 0; i < numGoroutines; i++ {
+		go func() {
+			defer wg.Done()
+			ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+			defer cancel()
+
+			current := atomic.AddInt32(&activeDials, 1)
+			for {
+				oldMax := atomic.LoadInt32(&maxObservedDials)
+				if current <= oldMax || atomic.CompareAndSwapInt32(&maxObservedDials, oldMax, current) {
+					break
+				}
+			}
+
+			_, _ = p.Acquire(ctx)
+			atomic.AddInt32(&activeDials, -1)
+		}()
+	}
+
+	wg.Wait()
+
+	if maxObservedDials > maxConns+5 {
+		t.Errorf("expected max observed dials near %d, got %d", maxConns, maxObservedDials)
+	}
+}