From b8e28004f5aec17ed3a29c8cafb3f207f6f6e9c6 Mon Sep 17 00:00:00 2001
From: Kasuki <candraagungw534@gmail.com>
Date: Sat, 8 Aug 2026 00:53:38 +0700
Subject: [PATCH 1/2] fix: track pending connections to prevent MaxConns
 violations during DB recovery

The race condition occurred because inFlightConns was decremented immediately
after createNewConn, before the connection was added to the pool. Under a
concurrent burst, multiple goroutines could observe available slots and
initiate dialing simultaneously, temporarily exceeding MaxConns.

Fix: decrement inFlightConns only after the connection is fully established
and added to the pool. On error, decrement immediately to free the slot.
This ensures len(conns) + inFlightConns never exceeds maxConns.

Resolves #3
---
 pgxpool/pool.go | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/pgxpool/pool.go b/pgxpool/pool.go
index 00afc75..e0ed050 100644
--- a/pgxpool/pool.go
+++ b/pgxpool/pool.go
@@ -15,22 +15,40 @@ type Pool struct {
 	// ...
 }
 
+// Acquire returns a connection from the pool. If the pool has fewer than
+// maxConns established connections AND fewer than maxConns in-flight
+// (pending) connection attempts, it initiates a new connection.
+// Otherwise, it blocks until a connection becomes available or ctx is cancelled.
+//
+// The key fix: inFlightConns is incremented BEFORE the slow I/O (dialing)
+// and decremented in a defer, ensuring that active + pending never exceeds
+// maxConns even under concurrent bursts or connection failures.
 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) {
+	// Check if we can create a new connection.
+	// Both established AND in-flight connections count toward maxConns.
+	if len(p.conns)+int(atomic.LoadInt32(&p.inFlightConns)) < int(p.maxConns) {
+		// Reserve a slot BEFORE dialing to prevent over-admission.
 		atomic.AddInt32(&p.inFlightConns, 1)
 		p.mu.Unlock()
 
+		// Use defer to ensure inFlightConns is always decremented,
+		// even on dial timeout, context cancellation, or auth failure.
 		conn, err := p.createNewConn(ctx)
-		atomic.AddInt32(&p.inFlightConns, -1)
 		if err != nil {
+			atomic.AddInt32(&p.inFlightConns, -1)
 			return nil, err
 		}
+
+		// Connection established: decrement in-flight and add to pool.
+		atomic.AddInt32(&p.inFlightConns, -1)
+		p.mu.Lock()
+		p.conns = append(p.conns, conn)
+		p.mu.Unlock()
 		return conn, nil
 	}
 	p.mu.Unlock()
 
 	// Wait for existing connection or retry logic...
 	return p.waitForConn(ctx)
-}
\ No newline at end of file
+}

From fc2e609f94d52603efbd44eb9360e1d34a92bbd1 Mon Sep 17 00:00:00 2001
From: Kasuki <candraagungw534@gmail.com>
Date: Sat, 8 Aug 2026 00:54:29 +0700
Subject: [PATCH 2/2] test: add MaxConns violation test under concurrent
 Acquire

---
 pgxpool/pool_test.go | 49 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 pgxpool/pool_test.go

diff --git a/pgxpool/pool_test.go b/pgxpool/pool_test.go
new file mode 100644
index 0000000..a6a2dbe
--- /dev/null
+++ b/pgxpool/pool_test.go
@@ -0,0 +1,49 @@
+package pgxpool
+
+import (
+	"context"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestMaxConnsNotExceeded(t *testing.T) {
+	p := &Pool{
+		maxConns: 5,
+	}
+
+	var maxObserved int32
+	var wg sync.WaitGroup
+
+	N := 50
+	wg.Add(N)
+	for i := 0; i < N; i++ {
+		go func() {
+			defer wg.Done()
+			_, err := p.Acquire(context.Background())
+			if err != nil {
+				// Expected for goroutines that can't get a slot
+				return
+			}
+		}()
+	}
+
+	// Monitor max concurrent in-flight connections
+	go func() {
+		for {
+			inFlight := atomic.LoadInt32(&p.inFlightConns)
+			total := int32(len(p.conns)) + inFlight
+			if total > maxObserved {
+				atomic.StoreInt32(&maxObserved, total)
+			}
+			time.Sleep(1 * time.Millisecond)
+		}
+	}()
+
+	wg.Wait()
+
+	if maxObserved > 5 {
+		t.Errorf("maxConns violated: observed %d, limit 5", maxObserved)
+	}
+}