hostinfo: use Foundation APIs to get hostname on macOS

Updates tailscale/corp#22332

This PR changes the way we determine the current hostname on Darwin && !iOS builds. We now use the native NSHost.currentHost API to get the hostname, instead of the os.Hostname() call which in Go uses `sysctl kern.hostname`. In Sequoia, this appears to always return `Mac.home`, (`home` comes from my LAN).

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

hostinfo/hostinfo.go

@@ -34,7 +34,7 @@
 
 // New returns a partially populated Hostinfo for the current host.
 func New() *tailcfg.Hostinfo {
-	hostname, _ := os.Hostname()
+	hostname := GetHostname()
 	hostname = dnsname.FirstLabel(hostname)
 	return &tailcfg.Hostinfo{
 		IPNVersion:      version.Long(),

hostinfo/hostinfo_test.go

@@ -21,6 +21,15 @@ func TestNew(t *testing.T) {
 	t.Logf("Got: %s", j)
 }
 
+// TestGetHostname asserts that GetHostname always returns a non-empty string.
+func TestGetHostname(t *testing.T) {
+	h := GetHostname()
+	if h == "" {
+		t.Fatalf("empty hostname")
+	}
+	t.Logf("Got: %s", h)
+}
+
 func TestOSVersion(t *testing.T) {
 	if osVersion == nil {
 		t.Skip("not available for OS")

hostinfo/hostname.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !(cgo && darwin && !ios)
+
+package hostinfo
+
+import "os"
+
+func GetHostname() string {
+	h, _ := os.Hostname()
+	return h
+}

hostinfo/hostname_macos.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build cgo && darwin && !ios
+
+package hostinfo
+
+/*
+#cgo CFLAGS: -x objective-c
+#cgo LDFLAGS: -framework Foundation
+#import <Foundation/Foundation.h>
+
+const char *getHostname() {
+	NSString *hostname = [[NSHost currentHost] localizedName];
+    if (hostname != nil) {
+        const char *hostnameCString = [hostname UTF8String];
+        if (hostnameCString != NULL) {
+            return strdup(hostnameCString);
+        }
+    }
+    return NULL;
+}
+*/
+import "C"
+
+func GetHostname() string {
+	chn := C.getHostname()
+	hostname := C.GoString(chn)
+	return hostname
+}