Merge pull request #1770 from nextcloud/bugfix/dns-cache-blocking-thread

BugFix - DNSCache Lock

Author: alperozturk96

library/src/main/java/com/nextcloud/common/DNSCache.kt

@@ -2,11 +2,13 @@
  * Nextcloud Android Library
  *
  * SPDX-FileCopyrightText: 2022-2024 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-FileCopyrightText: 2025 Alper Ozturk <[email protected]>
  * SPDX-FileCopyrightText: 2022 Álvaro Brey <[email protected]>
  * SPDX-License-Identifier: MIT
  */
 package com.nextcloud.common
 
+import android.os.Build
 import androidx.annotation.VisibleForTesting
 import com.nextcloud.android.lib.core.Clock
 import com.nextcloud.android.lib.core.ClockImpl
@@ -15,6 +17,7 @@ import java.net.Inet4Address
 import java.net.Inet6Address
 import java.net.InetAddress
 import java.net.UnknownHostException
+import java.util.concurrent.ConcurrentHashMap
 
 /**
  * DNS Cache which prefers IPv6 unless otherwise specified
@@ -24,12 +27,15 @@ object DNSCache {
 
     // 30 seconds is the Java default. Let's keep it.
     @VisibleForTesting
+    @Volatile
     var ttlMillis: Long = DEFAULT_TTL
 
     @VisibleForTesting
+    @Volatile
     var clock: Clock = ClockImpl()
 
     @VisibleForTesting
+    @Volatile
     var dns: Dns = Dns.SYSTEM
 
     data class DNSInfo(
@@ -40,49 +46,47 @@ object DNSCache {
         fun isExpired(): Boolean = clock.currentTimeMillis - timestamp > ttlMillis
     }
 
-    private val cache: MutableMap<String, DNSInfo> = HashMap()
+    private val cache: ConcurrentHashMap<String, DNSInfo> = ConcurrentHashMap()
 
     @Throws(UnknownHostException::class)
-    @Synchronized
     @JvmStatic
     fun lookup(hostname: String): List<InetAddress> {
         val entry = cache[hostname]
         if (entry?.addresses?.isNotEmpty() == true && !entry.isExpired()) {
             return entry.addresses
         }
-        val preferIPV4 =
-            when (entry) {
-                null -> false
-                else -> entry.preferIPV4
-            }
 
         val addresses = dns.lookup(hostname).toMutableList()
         if (addresses.isEmpty()) {
             throw UnknownHostException("Unknown host $hostname")
         }
-        val sortedAddresses = sortAddresses(addresses, preferIPV4)
 
-        val newEntry = DNSInfo(sortedAddresses, preferIPV4)
-        cache[hostname] = newEntry
+        val preferIPV4 = entry?.preferIPV4 ?: false
+        val sortedAddresses = sortAddresses(addresses, preferIPV4)
+        cache[hostname] = DNSInfo(sortedAddresses, preferIPV4)
 
         return sortedAddresses
     }
 
     /**
      * Set IP version preference for a hostname, and re-sort addresses if needed
      */
-    @Synchronized
     @JvmStatic
     fun setIPVersionPreference(
         hostname: String,
         preferIPV4: Boolean
     ) {
-        val entry = cache[hostname]
-        if (entry != null) {
-            val addresses = sortAddresses(entry.addresses, preferIPV4)
-            cache[hostname] = DNSInfo(addresses, preferIPV4)
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
+            cache.compute(hostname) { _, old ->
+                val addresses =
+                    old?.addresses?.let {
+                        sortAddresses(it, preferIPV4)
+                    } ?: emptyList()
+                DNSInfo(addresses, preferIPV4)
+            }
         } else {
-            cache[hostname] = DNSInfo(emptyList(), preferIPV4)
+            val addresses = cache[hostname]?.addresses?.let { sortAddresses(it, preferIPV4) } ?: emptyList()
+            cache[hostname] = DNSInfo(addresses, preferIPV4)
         }
     }
 
@@ -92,7 +96,6 @@ object DNSCache {
      *  - The first address is an IPv6 address
      *  - There are IPv4 addresses available too
      */
-    @Synchronized
     @JvmStatic
     fun isIPV6First(hostname: String): Boolean {
         val firstV6 = cache[hostname]?.addresses?.firstOrNull() is Inet6Address
@@ -103,7 +106,6 @@ object DNSCache {
     /**
      * Clears the cache
      */
-    @Synchronized
     @JvmStatic
     fun clear() {
         cache.clear()