Call getifaddrs and freeifaddrs dynamically on Android

Foundation's Host.swift uses getifaddrs and freeifaddrs, which currently causes Android 6.0 or below to fail to link.

Change these functions so they are called using dlsym, which allows Android 6.0 to link while providing full support on later versions of Android.

Author: Molanda

Sources/Foundation/Host.swift

@@ -24,6 +24,32 @@ import CoreFoundation
     private func getnameinfo(_ addr: UnsafePointer<sockaddr>?, _ addrlen: socklen_t, _ host: UnsafeMutablePointer<Int8>?, _ hostlen: socklen_t, _ serv: UnsafeMutablePointer<Int8>?, _ servlen: socklen_t, _ flags: Int32) -> Int32 {
         return Glibc.getnameinfo(addr, addrlen, host, Int(hostlen), serv, Int(servlen), flags)
     }
+
+    // getifaddrs and freeifaddrs are not available in Android 6.0 or earlier, so call these functions dynamically.
+    // This only happens during the initial lookup of the addresses and then the results are cached and _resolved is marked true.
+    // If this API changes so these functions are called more frequently, it might be beneficial to cache the function pointers.
+
+    private typealias GetIfAddrsFunc = @convention(c) (UnsafeMutablePointer<UnsafeMutablePointer<ifaddrs>?>) -> Int32
+    private func getifaddrs(_ ifap: UnsafeMutablePointer<UnsafeMutablePointer<ifaddrs>?>) -> Int32 {
+        var result: Int32 = -1
+        if let handle = dlopen("libc.so", RTLD_NOLOAD) {
+            if let entry = dlsym(handle, "getifaddrs") {
+                result = unsafeBitCast(entry, to: GetIfAddrsFunc.self)(ifap)
+            }
+            dlclose(handle)
+        }
+        return result
+    }
+
+    private typealias FreeIfAddrsFunc = @convention(c) (UnsafeMutablePointer<ifaddrs>?) -> Void
+    private func freeifaddrs(_ ifa: UnsafeMutablePointer<ifaddrs>?) {
+        if let handle = dlopen("libc.so", RTLD_NOLOAD) {
+            if let entry = dlsym(handle, "freeifaddrs") {
+                unsafeBitCast(entry, to: FreeIfAddrsFunc.self)(ifa)
+            }
+            dlclose(handle)
+        }
+    }
 #endif
 
 open class Host: NSObject {