From 201d8561f93bd7f4693b709441ec299e4147efe8 Mon Sep 17 00:00:00 2001
From: Jack Christensen <jack@jackchristensen.com>
Date: Sat, 22 Dec 2018 16:40:42 -0600
Subject: [PATCH] Initial commit

---
 pool.go      | 116 +++++++++++++++++++++++++++++++++++++++++++++++++++
 pool_test.go | 104 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 220 insertions(+)
 create mode 100644 pool.go
 create mode 100644 pool_test.go

diff --git a/pool.go b/pool.go
new file mode 100644
index 0000000..6063294
--- /dev/null
+++ b/pool.go
@@ -0,0 +1,116 @@
+package pool
+
+import (
+	"context"
+	"sync"
+)
+
+const (
+	resourceStatusCreating  = 0
+	resourceStatusAvailable = iota
+	resourceStatusBorrowed  = iota
+)
+
+type CreateFunc func() (res interface{}, err error)
+
+type resourceWrapper struct {
+	resource interface{}
+	status   byte
+}
+
+// Pool is a thread-safe resource pool.
+type Pool struct {
+	cond *sync.Cond
+
+	allResources       map[interface{}]*resourceWrapper
+	availableResources []*resourceWrapper
+
+	create CreateFunc
+}
+
+func New(create CreateFunc) *Pool {
+	return &Pool{
+		cond:         sync.NewCond(new(sync.Mutex)),
+		allResources: make(map[interface{}]*resourceWrapper),
+		create:       create,
+	}
+}
+
+// Get gets a resource from the pool. If no resources are available and the pool
+// is not at maximum capacity it will create a new resource. If the pool is at
+// maximum capacity it will block until a resource is available. ctx can be used
+// to cancel the Get.
+func (p *Pool) Get(ctx context.Context) (interface{}, error) {
+	if doneChan := ctx.Done(); doneChan != nil {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		default:
+		}
+	}
+
+	p.cond.L.Lock()
+
+	if len(p.availableResources) > 0 {
+		rw := p.availableResources[len(p.availableResources)-1]
+		p.availableResources = p.availableResources[:len(p.availableResources)-1]
+		if rw.status != resourceStatusAvailable {
+			panic("BUG: unavailable resource gotten from availableResources")
+		}
+		rw.status = resourceStatusBorrowed
+		p.cond.L.Unlock()
+		return rw.resource, nil
+	}
+
+	// if can create resource
+
+	var localVal int
+	placeholder := &localVal
+	p.allResources[placeholder] = &resourceWrapper{resource: placeholder, status: resourceStatusCreating}
+	p.cond.L.Unlock()
+
+	resChan := make(chan interface{})
+	errChan := make(chan error)
+
+	go func() {
+		res, err := p.create()
+		if err != nil {
+			errChan <- err
+		}
+		resChan <- res
+	}()
+
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case err := <-errChan:
+		p.cond.L.Lock()
+		delete(p.allResources, placeholder)
+		p.cond.L.Unlock()
+		return nil, err
+	case res := <-resChan:
+		p.cond.L.Lock()
+		delete(p.allResources, placeholder)
+		p.allResources[res] = &resourceWrapper{resource: res, status: resourceStatusBorrowed}
+		p.cond.L.Unlock()
+		return res, nil
+	}
+
+}
+
+// Return returns res to the the pool. If res is not part of the pool Return
+// will panic.
+func (p *Pool) Return(res interface{}) {
+	p.cond.L.Lock()
+
+	rw, present := p.allResources[res]
+	if !present {
+		p.cond.L.Unlock()
+		panic("Return called on resource that does not belong to pool")
+	}
+
+	rw.status = resourceStatusAvailable
+	p.availableResources = append(p.availableResources, rw)
+
+	p.cond.L.Unlock()
+}
diff --git a/pool_test.go b/pool_test.go
new file mode 100644
index 0000000..2365ad8
--- /dev/null
+++ b/pool_test.go
@@ -0,0 +1,104 @@
+package pool_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/jackc/pool"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPoolGet_CreatesResourceWhenNoneAvailable(t *testing.T) {
+	createCalls := 0
+	createFunc := func() (interface{}, error) {
+		createCalls += 1
+		return createCalls, nil
+	}
+	pool := pool.New(createFunc)
+
+	res, err := pool.Get(context.Background())
+	require.NoError(t, err)
+	assert.Equal(t, 1, res)
+
+	pool.Return(res)
+}
+
+func TestPoolGet_ReturnsErrorFromFailedResourceCreate(t *testing.T) {
+	errCreateFailed := errors.New("create failed")
+	createFunc := func() (interface{}, error) {
+		return nil, errCreateFailed
+	}
+	pool := pool.New(createFunc)
+
+	res, err := pool.Get(context.Background())
+	assert.Equal(t, errCreateFailed, err)
+	assert.Nil(t, res)
+}
+
+func TestPoolGet_ReusesResources(t *testing.T) {
+	createCalls := 0
+	createFunc := func() (interface{}, error) {
+		createCalls += 1
+		return createCalls, nil
+	}
+	pool := pool.New(createFunc)
+
+	res, err := pool.Get(context.Background())
+	require.NoError(t, err)
+	assert.Equal(t, 1, res)
+
+	pool.Return(res)
+
+	res, err = pool.Get(context.Background())
+	require.NoError(t, err)
+	assert.Equal(t, 1, res)
+
+	pool.Return(res)
+
+	assert.Equal(t, 1, createCalls)
+}
+
+func TestPoolGet_ContextAlreadyCanceled(t *testing.T) {
+	createFunc := func() (interface{}, error) {
+		panic("should never be called")
+	}
+	pool := pool.New(createFunc)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	res, err := pool.Get(ctx)
+	assert.Equal(t, context.Canceled, err)
+	assert.Nil(t, res)
+}
+
+func TestPoolGet_ContextCanceledDuringCreate(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+
+	createCalls := 0
+
+	createFunc := func() (interface{}, error) {
+		cancel()
+		time.Sleep(1 * time.Second)
+		createCalls += 1
+		return createCalls, nil
+	}
+	pool := pool.New(createFunc)
+
+	res, err := pool.Get(ctx)
+	assert.Equal(t, context.Canceled, err)
+	assert.Nil(t, res)
+}
+
+func TestPoolReturnPanicsIfResourceNotPartOfPool(t *testing.T) {
+	createCalls := 0
+	createFunc := func() (interface{}, error) {
+		createCalls += 1
+		return createCalls, nil
+	}
+	pool := pool.New(createFunc)
+
+	assert.Panics(t, func() { pool.Return(42) })
+}