net: don't return IPv4 unspecified addr for Resolve*Addr of [::] or [::]:n

ResolveTCPAddr, ResolveUDPAddr, and ResolveIPAddr return at most one
address. When given a name like "golang.org" to resolve that might
have more than 1 address, the net package has historically preferred
IPv4 addresses, with the assumption that many users don't yet have
IPv6 connectivity and randomly selecting between an IPv4 address and
an IPv6 address at runtime wouldn't be a good experience for IPv4-only
users.

In CL 45088 (78cf0e56) I modified the resolution of the
unspecified/empty address to internally resolve to both IPv6 "::" and
0.0.0.0 to fix issue #18806.

That code has 3 other callers I hadn't considered, though: the
Resolve*Addr functions. Since they preferred IPv4, any Resolve*Addr of
"[::]:port" or "::" (for ResolveIPAddr) would internally resolve both
"::" and 0.0.0.0 and then prefer 0.0.0.0, even though the user was
looking up an IPv6 literal.

Add tests and fix it, not by undoing the fix to #18806 but by
selecting the preference function for Resolve*Addr more explicitly: we
still prefer IPv4, but if the address being looked up was an IPv6
literal, prefer IPv6.

The tests are skipped on machines without IPv6.

Fixes #20911

Change-Id: Ib7036cc43182ae4118cd1390c254e17c04a251a3
Reviewed-on: https://go-review.googlesource.com/47554
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/net/iprawsock.go

@@ -91,7 +91,7 @@ func ResolveIPAddr(network, address string) (*IPAddr, error) {
 	if err != nil {
 		return nil, err
 	}
-	return addrs.first(isIPv4).(*IPAddr), nil
+	return addrs.forResolve(network, address).(*IPAddr), nil
 }
 
 // IPConn is the implementation of the Conn and PacketConn interfaces

src/net/ipsock.go

@@ -50,7 +50,7 @@ func supportsIPv4map() bool {
 // An addrList represents a list of network endpoint addresses.
 type addrList []Addr
 
-// isIPv4 returns true if the Addr contains an IPv4 address.
+// isIPv4 reports whether addr contains an IPv4 address.
 func isIPv4(addr Addr) bool {
 	switch addr := addr.(type) {
 	case *TCPAddr:
@@ -63,6 +63,28 @@ func isIPv4(addr Addr) bool {
 	return false
 }
 
+// isNotIPv4 reports whether addr does not contain an IPv4 address.
+func isNotIPv4(addr Addr) bool { return !isIPv4(addr) }
+
+// forResolve returns the most appropriate address in address for
+// a call to ResolveTCPAddr, ResolveUDPAddr, or ResolveIPAddr.
+// IPv4 is preferred, unless addr contains an IPv6 literal.
+func (addrs addrList) forResolve(network, addr string) Addr {
+	var want6 bool
+	switch network {
+	case "ip":
+		// IPv6 literal (addr does NOT contain a port)
+		want6 = count(addr, ':') > 0
+	case "tcp", "udp":
+		// IPv6 literal. (addr contains a port, so look for '[')
+		want6 = count(addr, '[') > 0
+	}
+	if want6 {
+		return addrs.first(isNotIPv4)
+	}
+	return addrs.first(isIPv4)
+}
+
 // first returns the first address which satisfies strategy, or if
 // none do, then the first address of any kind.
 func (addrs addrList) first(strategy func(Addr) bool) Addr {

src/net/main_test.go

@@ -89,6 +89,12 @@ func setupTestData() {
 		resolveTCPAddrTests = append(resolveTCPAddrTests, resolveTCPAddrTest{"tcp6", "localhost:3", &TCPAddr{IP: IPv6loopback, Port: 3}, nil})
 		resolveUDPAddrTests = append(resolveUDPAddrTests, resolveUDPAddrTest{"udp6", "localhost:3", &UDPAddr{IP: IPv6loopback, Port: 3}, nil})
 		resolveIPAddrTests = append(resolveIPAddrTests, resolveIPAddrTest{"ip6", "localhost", &IPAddr{IP: IPv6loopback}, nil})
+
+		// Issue 20911: don't return IPv4 addresses for
+		// Resolve*Addr calls of the IPv6 unspecified address.
+		resolveTCPAddrTests = append(resolveTCPAddrTests, resolveTCPAddrTest{"tcp", "[::]:4", &TCPAddr{IP: IPv6unspecified, Port: 4}, nil})
+		resolveUDPAddrTests = append(resolveUDPAddrTests, resolveUDPAddrTest{"udp", "[::]:4", &UDPAddr{IP: IPv6unspecified, Port: 4}, nil})
+		resolveIPAddrTests = append(resolveIPAddrTests, resolveIPAddrTest{"ip", "::", &IPAddr{IP: IPv6unspecified}, nil})
 	}
 
 	ifi := loopbackInterface()

src/net/tcpsock.go

@@ -77,7 +77,7 @@ func ResolveTCPAddr(network, address string) (*TCPAddr, error) {
 	if err != nil {
 		return nil, err
 	}
-	return addrs.first(isIPv4).(*TCPAddr), nil
+	return addrs.forResolve(network, address).(*TCPAddr), nil
 }
 
 // TCPConn is an implementation of the Conn interface for TCP network

src/net/udpsock.go

@@ -80,7 +80,7 @@ func ResolveUDPAddr(network, address string) (*UDPAddr, error) {
 	if err != nil {
 		return nil, err
 	}
-	return addrs.first(isIPv4).(*UDPAddr), nil
+	return addrs.forResolve(network, address).(*UDPAddr), nil
 }
 
 // UDPConn is the implementation of the Conn and PacketConn interfaces