Prevent deadlock on ConnPool.Acquire with Reset

If no connections are available, and the pool is reset, there will
be no connections at all in the pool. So the pool needs to be able
to create a connection after waiting for a connection to be released.

conn_pool.go

@@ -73,48 +73,49 @@ func NewConnPool(config ConnPoolConfig) (p *ConnPool, err error) {
 }
 
 // Acquire takes exclusive use of a connection until it is released.
-func (p *ConnPool) Acquire() (c *Conn, err error) {
+func (p *ConnPool) Acquire() (*Conn, error) {
 	p.cond.L.Lock()
-	defer p.cond.L.Unlock()
+	c, err := p.acquire()
+	p.cond.L.Unlock()
+	return c, err
+}
 
+// acquire performs acquision assuming pool is already locked
+func (p *ConnPool) acquire() (*Conn, error) {
 	if p.closed {
 		return nil, errors.New("cannot acquire from closed pool")
 	}
 
 	// A connection is available
 	if len(p.availableConnections) > 0 {
-		c = p.availableConnections[len(p.availableConnections)-1]
+		c := p.availableConnections[len(p.availableConnections)-1]
 		c.poolResetCount = p.resetCount
 		p.availableConnections = p.availableConnections[:len(p.availableConnections)-1]
-		return
+		return c, nil
 	}
 
 	// No connections are available, but we can create more
 	if len(p.allConnections) < p.maxConnections {
-		c, err = p.createConnection()
+		c, err := p.createConnection()
 		if err != nil {
-			return
+			return nil, err
 		}
 		c.poolResetCount = p.resetCount
 		p.allConnections = append(p.allConnections, c)
-		return
+		return c, nil
 	}
 
 	// All connections are in use and we cannot create more
-	if len(p.availableConnections) == 0 {
-		if p.logLevel >= LogLevelWarn {
-			p.logger.Warn("All connections in pool are busy - waiting...")
-		}
-		for len(p.availableConnections) == 0 {
-			p.cond.Wait()
-		}
+	if p.logLevel >= LogLevelWarn {
+		p.logger.Warn("All connections in pool are busy - waiting...")
 	}
 
-	c = p.availableConnections[len(p.availableConnections)-1]
-	c.poolResetCount = p.resetCount
-	p.availableConnections = p.availableConnections[:len(p.availableConnections)-1]
+	// Wait until there is an available connection OR room to create a new connection
+	for len(p.availableConnections) == 0 && len(p.allConnections) == p.maxConnections {
+		p.cond.Wait()
+	}
 
-	return
+	return p.acquire()
 }
 
 // Release gives up use of a connection.