tls: Fix read wait buffer sizing

common/tls/apple_client_platform.go

@@ -520,10 +520,6 @@ func (w *appleTLSReadWaiter) WaitReadBuffer() (*buf.Buffer, error) {
 	if c.readEOF {
 		return nil, io.EOF
 	}
-	maximumLen := readWaitFreeLen(w.options)
-	if maximumLen <= 0 {
-		return nil, io.ErrShortBuffer
-	}
 	timeoutMs, err := c.prepareReadTimeout()
 	if err != nil {
 		return nil, err
@@ -533,11 +529,18 @@ func (w *appleTLSReadWaiter) WaitReadBuffer() (*buf.Buffer, error) {
 		return nil, err
 	}
 	defer c.releaseClient()
+	buffer := w.options.NewBuffer()
+	if buffer.IsFull() {
+		buffer.Release()
+		return nil, io.ErrShortBuffer
+	}
+	maximumLen := buffer.FreeLen()
 
 	handle := cgo.NewHandle(w)
 	defer handle.Delete()
 	var errorPtr *C.char
 	if !bool(C.box_apple_tls_client_read_async(client, C.size_t(maximumLen), C.uintptr_t(handle), &errorPtr)) {
+		buffer.Release()
 		return nil, c.errorFromPointer(errorPtr)
 	}
 
@@ -553,22 +556,18 @@ func (w *appleTLSReadWaiter) WaitReadBuffer() (*buf.Buffer, error) {
 			if result != nil {
 				C.box_apple_tls_read_result_free(result)
 			}
+			buffer.Release()
 			c.markReadTimedOut()
 			return nil, os.ErrDeadlineExceeded
 		}
 	} else {
 		result = <-w.results
 	}
-	return c.readWaitResultToBuffer(result, w.options)
+	return c.readWaitResultToBuffer(result, buffer, w.options)
 }
 
-func (c *appleTLSConn) readWaitResultToBuffer(result *C.box_apple_tls_read_result_t, options N.ReadWaitOptions) (*buf.Buffer, error) {
+func (c *appleTLSConn) readWaitResultToBuffer(result *C.box_apple_tls_read_result_t, buffer *buf.Buffer, options N.ReadWaitOptions) (*buf.Buffer, error) {
 	defer C.box_apple_tls_read_result_free(result)
-	buffer := options.NewBuffer()
-	if buffer.IsFull() {
-		buffer.Release()
-		return nil, io.ErrShortBuffer
-	}
 	startLen := buffer.Len()
 	var eof C.bool
 	var errorPtr *C.char
@@ -593,16 +592,6 @@ func (c *appleTLSConn) readWaitResultToBuffer(result *C.box_apple_tls_read_resul
 	return buffer, nil
 }
 
-func readWaitFreeLen(options N.ReadWaitOptions) int {
-	if options.IncreaseBuffer {
-		return 65535 - options.FrontHeadroom - options.RearHeadroom
-	}
-	if options.MTU > 0 {
-		return options.MTU
-	}
-	return buf.BufferSize - options.FrontHeadroom - options.RearHeadroom
-}
-
 //export box_apple_tls_read_callback
 func box_apple_tls_read_callback(callbackHandle C.uintptr_t, result *C.box_apple_tls_read_result_t) {
 	handle := cgo.Handle(callbackHandle)