Allow the adb server to bind on ::1

Currently the adb server can only bind on 127.0.0.1 or * if -a is
provided. This diff adds the ability for adb to bind on ::1 as
well for use cases where you might want to force adb to be IPv6
only.

To bind the adb server on ::1
```
$ ./adb -L tcp:[::1]:1234 server
$ lsof -nPi :1234
COMMAND ... NODE NAME
adb     ...  TCP [::1]:1234 (LISTEN)
```
The original behaviour is also retained, so this would only affect
users explicitly specifying ::1 in the socket spec
```
$ export ANDROID_ADB_SERVER_PORT=1234
$ ./adb server
$ lsof -nPi :1234
COMMAND ... NODE NAME
adb     ...  TCP 127.0.0.1:1234 (LISTEN)
```

Note: I've only implemented this behaviour for posix base systems,
due to my limited understanding of networking on windows. If
needed I can do some research to implement it there as well, but
there is currently no IPv6 support at all on the windows side.

Test: New unit test and the commands in the summary

Change-Id: I23e4531e8dfda4de9348124ad7491d728aecdbf7

adb/socket_spec.cpp

@@ -272,7 +272,9 @@ int socket_spec_listen(std::string_view spec, std::string* error, int* resolved_
         if (hostname.empty() && gListenAll) {
             result = network_inaddr_any_server(port, SOCK_STREAM, error);
         } else if (tcp_host_is_local(hostname)) {
-            result = network_loopback_server(port, SOCK_STREAM, error);
+            result = network_loopback_server(port, SOCK_STREAM, error, true);
+        } else if (hostname == "::1") {
+            result = network_loopback_server(port, SOCK_STREAM, error, false);
         } else {
             // TODO: Implement me.
             *error = "listening on specified hostname currently unsupported";

adb/sysdeps/network.h

@@ -19,4 +19,4 @@
 #include <string>
 
 int network_loopback_client(int port, int type, std::string* error);
-int network_loopback_server(int port, int type, std::string* error);
+int network_loopback_server(int port, int type, std::string* error, bool prefer_ipv4);

adb/sysdeps/posix/network.cpp

@@ -119,12 +119,15 @@ static int _network_loopback_server(bool ipv6, int port, int type, std::string*
     return s.release();
 }
 
-int network_loopback_server(int port, int type, std::string* error) {
-    int rc = _network_loopback_server(false, port, type, error);
+int network_loopback_server(int port, int type, std::string* error, bool prefer_ipv4) {
+    int rc = -1;
+    if (prefer_ipv4) {
+        rc = _network_loopback_server(false, port, type, error);
+    }
 
-    // Only attempt to listen on IPv6 if IPv4 is unavailable.
+    // Only attempt to listen on IPv6 if IPv4 is unavailable or prefer_ipv4 is false
     // We don't want to start an IPv6 server if there's already an IPv4 one running.
-    if (rc == -1 && (errno == EADDRNOTAVAIL || errno == EAFNOSUPPORT)) {
+    if (rc == -1 && (errno == EADDRNOTAVAIL || errno == EAFNOSUPPORT || !prefer_ipv4)) {
         return _network_loopback_server(true, port, type, error);
     }
     return rc;

adb/sysdeps_win32.cpp

@@ -920,7 +920,8 @@ static int _network_server(int port, int type, u_long interface_address, std::st
     return fd;
 }
 
-int network_loopback_server(int port, int type, std::string* error) {
+int network_loopback_server(int port, int type, std::string* error, bool prefer_ipv4) {
+    // TODO implement IPv6 support on windows
     return _network_server(port, type, INADDR_LOOPBACK, error);
 }
 
@@ -1132,7 +1133,7 @@ int adb_socketpair(int sv[2]) {
     int local_port = -1;
     std::string error;
 
-    server = network_loopback_server(0, SOCK_STREAM, &error);
+    server = network_loopback_server(0, SOCK_STREAM, &error, true);
     if (server < 0) {
         D("adb_socketpair: failed to create server: %s", error.c_str());
         goto fail;

adb/test_adb.py

@@ -281,6 +281,37 @@ class ServerTest(unittest.TestCase):
             subprocess.check_output(["adb", "-P", str(port), "kill-server"],
                                     stderr=subprocess.STDOUT)
 
+    @unittest.skipUnless(
+        os.name == "posix",
+        "adb doesn't yet support IPv6 on Windows",
+    )
+    def test_starts_on_ipv6_localhost(self):
+        """
+        Tests that the server can start up on ::1 and that it's accessible
+        """
+        server_port = 5037
+        # Kill any existing server on this non-default port.
+        subprocess.check_output(
+            ["adb", "-P", str(server_port), "kill-server"],
+            stderr=subprocess.STDOUT,
+        )
+        try:
+            subprocess.check_output(
+                ["adb", "-L", "tcp:[::1]:{}".format(server_port), "server"],
+                stderr=subprocess.STDOUT,
+            )
+            with fake_adbd() as (port, _):
+                with adb_connect(self, serial="localhost:{}".format(port)):
+                    pass
+        finally:
+            # If we started a server, kill it.
+            subprocess.check_output(
+                ["adb", "-P", str(server_port), "kill-server"],
+                stderr=subprocess.STDOUT,
+            )
+
+
+
 
 class EmulatorTest(unittest.TestCase):
     """Tests for the emulator connection."""