hostinfo: retrieve OS version for Macs running the OSS client

Updates #18520

Change-Id: If86a1f702c704b003002aa7e2f5a6b1418b469cc
Signed-off-by: Alex Chan <alexc@tailscale.com>

hostinfo/hostinfo_darwin.go

@@ -8,14 +8,31 @@ package hostinfo
 import (
 	"os"
 	"path/filepath"
+
+	"golang.org/x/sys/unix"
+	"tailscale.com/types/ptr"
 )
 
 func init() {
+	osVersion = lazyOSVersion.Get
 	packageType = packageTypeDarwin
 }
 
+var (
+	lazyOSVersion = &lazyAtomicValue[string]{f: ptr.To(osVersionDarwin)}
+)
+
 func packageTypeDarwin() string {
 	// Using tailscaled or IPNExtension?
 	exe, _ := os.Executable()
 	return filepath.Base(exe)
 }
+
+// Returns the marketing version (e.g., "15.0.1" or "26.0.0")
+func osVersionDarwin() string {
+	version, err := unix.Sysctl("kern.osproductversion")
+	if err != nil {
+		return ""
+	}
+	return version
+}