net: simplify non-cgo DNS exchange

Also does less buffer allocation in case of TCP fallback.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12925043

diff --git a/src/pkg/net/dnsclient_unix.go b/src/pkg/net/dnsclient_unix.go
index 38fbf322cf2622a1d7a45e990c2986e8220c5511..8548f50354770bef31e4860614ddbf84c2965757 100644 (file)
--- a/src/pkg/net/dnsclient_unix.go
+++ b/src/pkg/net/dnsclient_unix.go
@@ -26,13 +26,7 @@ import (
 // Send a request on the connection and hope for a reply.
 // Up to cfg.attempts attempts.
 func exchange(cfg *dnsConfig, c Conn, name string, qtype uint16) (*dnsMsg, error) {
-       var useTCP bool
-       switch c.(type) {
-       case *UDPConn:
-               useTCP = false
-       case *TCPConn:
-               useTCP = true
-       }
+       _, useTCP := c.(*TCPConn)
        if len(name) >= 256 {
                return nil, &DNSError{Err: "name too long", Name: name}
        }
@@ -69,8 +63,11 @@ func exchange(cfg *dnsConfig, c Conn, name string, qtype uint16) (*dnsMsg, error
                                        continue
                                }
                        }
-                       buf = make([]byte, uint16(buf[0])<<8+uint16(buf[1]))
-                       n, err = io.ReadFull(c, buf)
+                       mlen := int(buf[0])<<8 | int(buf[1])
+                       if mlen > len(buf) {
+                               buf = make([]byte, mlen)
+                       }
+                       n, err = io.ReadFull(c, buf[:mlen])
                } else {
                        n, err = c.Read(buf)
                }
@@ -80,7 +77,7 @@ func exchange(cfg *dnsConfig, c Conn, name string, qtype uint16) (*dnsMsg, error
                        }
                        return nil, err
                }
-               buf = buf[0:n]
+               buf = buf[:n]
                in := new(dnsMsg)
                if !in.Unpack(buf) || in.id != out.id {
                        continue