Add connPool implementation and connection pool errors

Introduce connPool struct and implement the Pool interface. Add error handling for invalid pool capacity settings and provide a constructor for creating new connection pools with specified capacities.

connpool.go

@@ -4,12 +4,20 @@
 
 package mail
 
-import "net"
+import (
+	"errors"
+	"net"
+	"sync"
+)
 
 // Parts of the connection pool code is forked from https://github.com/fatih/pool/
 // Thanks to Fatih Arslan and the project contributors for providing this great
 // concurrency template.
 
+var (
+	ErrPoolInvalidCap = errors.New("invalid connection pool capacity settings")
+)
+
 // Pool interface describes a connection pool implementation. A Pool is
 // thread-/go-routine safe.
 type Pool interface {
@@ -25,3 +33,57 @@ type Pool interface {
 	// Len returns the current number of connections of the pool.
 	Len() int
 }
+
+// connPool implements the Pool interface
+type connPool struct {
+	// mutex is used to synchronize access to the connection pool to ensure thread-safe operations
+	mutex sync.RWMutex
+	// conns is a channel used to manage and distribute net.Conn objects within the connection pool
+	conns chan net.Conn
+	// dialCtx represents the actual net.Conn returned by the DialContextFunc
+	dialCtx DialContextFunc
+}
+
+// NewConnPool returns a new pool based on buffered channels with an initial
+// capacity and maximum capacity. The DialContextFunc is used when the initial
+// capacity is greater than zero to fill the pool. A zero initialCap doesn't
+// fill the Pool until a new Get() is called. During a Get(), if there is no
+// new connection available in the pool, a new connection will be created via
+// the corresponding DialContextFunc() method.
+func NewConnPool(initialCap, maxCap int, dialCtxFunc DialContextFunc) (Pool, error) {
+	if initialCap < 0 || maxCap <= 0 || initialCap > maxCap {
+		return nil, ErrPoolInvalidCap
+	}
+
+	pool := &connPool{
+		conns:   make(chan net.Conn, maxCap),
+		dialCtx: dialCtxFunc,
+	}
+
+	// create initial connections, if something goes wrong,
+	// just close the pool error out.
+	for i := 0; i < initialCap; i++ {
+		/*
+			conn, err := dialCtxFunc()
+			if err != nil {
+				pool.Close()
+				return nil, fmt.Errorf("factory is not able to fill the pool: %s", err)
+			}
+			c.conns <- conn
+
+		*/
+	}
+
+	return pool, nil
+}
+
+func (c *connPool) Get() (net.Conn, error) {
+	return nil, nil
+}
+func (c *connPool) Close() {
+	return
+}
+
+func (c *connPool) Len() int {
+	return 0
+}