Fix create resource concurrently with Stat call race

https://github.com/jackc/pgx/issues/1148#issuecomment-1304898720

pool.go

@@ -427,8 +427,12 @@ func (p *Pool[T]) initResourceValue(ctx context.Context, res *Resource[T]) (*Res
 			return
 		}
 
+		// The resource is already in p.allResources where it might be read. So we need to acquire the lock to update its
+		// status.
+		p.mux.Lock()
 		res.value = value
 		res.status = resourceStatusAcquired
+		p.mux.Unlock()
 
 		// This select works because the channel is unbuffered.
 		select {

pool_test.go

@@ -985,6 +985,11 @@ func TestStress(t *testing.T) {
 				res.Release()
 			}
 		},
+		// Stat
+		func() {
+			stat := pool.Stat()
+			assert.NotNil(t, stat)
+		},
 	}
 
 	workerCount := int(poolSize) * 2