Update err for single conn pool when context.Done() return first (#1981)

* Update err from ctx if reason is nil

* Add a unit test

* Move pool object to the test itself

* Rename a var from cl to cancel

* Add comment for explaining why ctx.Err() is used

internal/pool/pool_single.go

@@ -37,6 +37,11 @@ func (p *SingleConnPool) Put(ctx context.Context, cn *Conn) {}
 func (p *SingleConnPool) Remove(ctx context.Context, cn *Conn, reason error) {
 	p.cn = nil
 	p.stickyErr = reason
+	// If ctx is cancelled without a reason(error) value,
+	// then the ctx.Error is used as the reason for why the p.cn is assigned nil.
+	if reason == nil && ctx != nil {
+		p.stickyErr = ctx.Err()
+	}
 }
 
 func (p *SingleConnPool) Close() error {

internal/pool/pool_single_test.go

@@ -0,0 +1,25 @@
+package pool_test
+
+import (
+	"context"
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+
+	"github.com/go-pg/pg/v10/internal/pool"
+)
+
+var _ = Describe("SingleConnPool", func() {
+	It("remove a conn due to context is cancelled", func() {
+		p := pool.NewSingleConnPool(nil, &pool.Conn{})
+		ctx, cancel := context.WithCancel(context.TODO())
+		cn, err := p.Get(nil)
+		Expect(err).To(BeNil())
+		Expect(cn).ToNot(BeNil())
+
+		cancel()
+		p.Remove(ctx, cn, nil)
+		cn, err = p.Get(nil)
+		Expect(cn).To(BeNil())
+		Expect(err).ToNot(BeNil())
+	})
+})