From 4014e4825a5e9f561c2e7b610be4d2a623fbb791 Mon Sep 17 00:00:00 2001
From: Patrick Ellul <ellulpatrick@gmail.com>
Date: Mon, 3 Feb 2020 11:14:49 +1100
Subject: [PATCH] CreateResource constructs a new resource without acquiring
 it.

---
 pool.go      | 26 ++++++++++++++++++++++++++
 pool_test.go | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+)

diff --git a/pool.go b/pool.go
index 54acf30..1a3fb30 100644
--- a/pool.go
+++ b/pool.go
@@ -373,6 +373,32 @@ func (p *Pool) AcquireAllIdle() []*Resource {
 	return resources
 }
 
+// CreateResource constructs a new resource without acquiring it.
+// It goes straight in the IdlePool. It does not check against maxSize.
+// It can be useful to maintain warm resources under little load.
+func (p *Pool) CreateResource(ctx context.Context) error {
+
+	value, err := p.constructResourceValue(ctx)
+	if err != nil {
+		return err
+	}
+
+	res := &Resource{
+		pool:         p,
+		creationTime: time.Now(),
+		status:       resourceStatusIdle,
+		value:        value,
+	}
+
+	p.cond.L.Lock()
+	p.allResources = append(p.allResources, res)
+	p.idleResources = append(p.idleResources, res)
+	p.destructWG.Add(1)
+	p.cond.L.Unlock()
+
+	return nil
+}
+
 // releaseAcquiredResource returns res to the the pool.
 func (p *Pool) releaseAcquiredResource(res *Resource, lastUsedNano int64) {
 	p.cond.L.Lock()
diff --git a/pool_test.go b/pool_test.go
index c822a67..1df1486 100644
--- a/pool_test.go
+++ b/pool_test.go
@@ -245,6 +245,44 @@ func TestPoolAcquireAllIdle(t *testing.T) {
 	resources[3].Release()
 }
 
+func TestPoolCreateResource(t *testing.T) {
+	constructor, counter := createConstructor()
+	pool := puddle.NewPool(constructor, stubDestructor, 10)
+	defer pool.Close()
+
+	var err error
+
+	err = pool.CreateResource(context.Background())
+	require.NoError(t, err)
+
+	allIdle := pool.AcquireAllIdle()
+	assert.Equal(t, counter.Value(), allIdle[0].Value())
+	allIdle[0].ReleaseUnused()
+
+	stats := pool.Stat()
+	assert.EqualValues(t, 1, stats.IdleResources())
+
+	res, err := pool.Acquire(context.Background())
+	require.NoError(t, err)
+	assert.Equal(t, 1, res.Value())
+	assert.WithinDuration(t, time.Now(), res.CreationTime(), time.Second)
+	res.Release()
+
+	stats = pool.Stat()
+	assert.EqualValues(t, 0, stats.EmptyAcquireCount())
+}
+
+func TestPoolCreateResourceReturnsErrorFromFailedResourceCreate(t *testing.T) {
+	errCreateFailed := errors.New("create failed")
+	constructor := func(ctx context.Context) (interface{}, error) {
+		return nil, errCreateFailed
+	}
+	pool := puddle.NewPool(constructor, stubDestructor, 10)
+
+	err := pool.CreateResource(context.Background())
+	assert.Equal(t, errCreateFailed, err)
+}
+
 func TestPoolCloseClosesAllIdleResources(t *testing.T) {
 	constructor, _ := createConstructor()