BugFix - DNSCache Lock

[alperozturk96]

Issue

DNSCache used @Synchronized on all public methods to protect access to a MutableMap.

All threads must acquire the lock, even for read operations like isIPV6First() or lookup()

Methods like lookup() perform DNS resolution, which can be slow. Holding the @Synchronized lock this increases the risk of blocking critical threads like the UI or Android system event receivers.

Changes

Replaced MutableMap with ConcurrentHashMap to enable lock-free concurrent access.
Removed unnecessary @Synchronized annotations from all methods.
UseConcurrentHashMap.compute(), ensuring atomic read–modify–write operations without external locking.

Issue Logs

Logs:
"main" tid=1 Blocked
 at com.nextcloud.common.DNSCache.clear (unavailable:2)
 at [com.owncloud.android](http://com.owncloud.android/).utils.ReceiversHelper$1.onReceive ([ReceiversHelper.java:49](http://receivershelper.java:49/))
 at [android.app](http://android.app/).LoadedApk$ReceiverDispatcher$Args.lambda$getRunnable$0 ([LoadedApk.java:1821](http://loadedapk.java:1821/))
 at [android.app](http://android.app/).LoadedApk$ReceiverDispatcher$Args.$r8$lambda$gDuJqgxY6Zb-ifyeubKeivTLAwk (unavailable)
 at [android.app](http://android.app/).LoadedApk$ReceiverDispatcher$Args$$ExternalSyntheticLambda0.run (unavailable:2)
 at android.os.Handler.handleCallback ([Handler.java:1000](http://handler.java:1000/))
 at android.os.Handler.dispatchMessage ([Handler.java:104](http://handler.java:104/))
 at android.os.Looper.loopOnce ([Looper.java:242](http://looper.java:242/))
 at android.os.Looper.loop ([Looper.java:362](http://looper.java:362/))
 at [android.app](http://android.app/).ActivityThread.main ([ActivityThread.java:8393](http://activitythread.java:8393/))
 at java.lang.reflect.Method.invoke (Native method)
 at [com.android](http://com.android/).internal.os.RuntimeInit$MethodAndArgsCaller.run ([RuntimeInit.java:552](http://runtimeinit.java:552/))
 at [com.android](http://com.android/).internal.os.ZygoteInit.main ([ZygoteInit.java:992](http://zygoteinit.java:992/))

"androidx.work-3" tid=41 Native
 #00  pc 0x00000000000ec824  /apex/com.android.runtime/lib64/bionic/libc.so (read+4)
 #01  pc 0x00000000000f8990  /apex/com.android.runtime/lib64/bionic/libc.so (__sread+48)
 #02  pc 0x00000000000f887c  /apex/com.android.runtime/lib64/bionic/libc.so (__srefill+268)

"ReferenceQueueDaemon" tid=6 Waiting
 at java.lang.Object.wait (Native method)
 at java.lang.Object.wait ([Object.java:405](http://object.java:405/))

"FinalizerDaemon" tid=7 Waiting
 at java.lang.Object.wait (Native method)
 at java.lang.Object.wait ([Object.java:405](http://object.java:405/))

"MultiThreadedHttpConnectionManager cleanup" tid=51 Waiting
 at java.lang.Object.wait (Native method)

"androidx.work-1" tid=38 Waiting
 at jdk.internal.misc.Unsafe.park (Native method)
 at java.util.concurrent.locks.LockSupport.park ([LockSupport.java:341](http://locksupport.java:341/))
 at java.lang.Object.wait ([Object.java:405](http://object.java:405/))

...

[github-actions]

SpotBugs

Category	Base	New
Bad practice	34	34
Correctness	34	34
Dodgy code	26	26
Internationalization	6	6
Malicious code vulnerability	49	49
Multithreaded correctness	11	11
Performance	8	8
Total	168	168

[codecov]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.

Project coverage is 43.01%. Comparing base (ccc7ae5) to head (97125f6).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
...ary/src/main/java/com/nextcloud/common/DNSCache.kt	30.76%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1770      +/-   ##
============================================
- Coverage     43.06%   43.01%   -0.05%     
  Complexity      987      987              
============================================
  Files           235      235              
  Lines          8569     8569              
  Branches       1123     1124       +1     
============================================
- Hits           3690     3686       -4     
- Misses         4361     4365       +4     
  Partials        518      518              

Files with missing lines	Coverage Δ
...ary/src/main/java/com/nextcloud/common/DNSCache.kt	43.75% <30.76%> (-8.34%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

SpotBugs

Category	Base	New
Bad practice	34	34
Correctness	34	34
Dodgy code	26	26
Internationalization	6	6
Malicious code vulnerability	49	49
Multithreaded correctness	11	11
Performance	8	8
Total	168	168

[github-actions]

SpotBugs

Category	Base	New
Bad practice	34	34
Correctness	34	34
Dodgy code	26	26
Internationalization	6	6
Malicious code vulnerability	49	49
Multithreaded correctness	11	11
Performance	8	8
Total	168	168

[alperozturk96]

/backport to stable-2.21

[alperozturk96]

/backport to stable-2.21