Revert "RUM-6171 use FileObserver to limit calls to filesystem"

[aleksandr-gringauz]

RUM integration tests in instrumented/integration started failing with the following stacktrace:

java.lang.ClassCastException: com.google.gson.JsonNull cannot be cast to com.google.gson.JsonObject
at com.datadog.android.sdk.integration.rum.RumTestUtilsKt.rumPayloadToJsonList(RumTestUtils.kt:23)
at com.datadog.android.sdk.integration.rum.RumTest.verifyExpectedEvents(RumTest.kt:44)
at com.datadog.android.sdk.integration.rum.ConsentPendingGrantedActivityTrackingTest$verifyRumEvents$1.invoke(ConsentPendingGrantedActivityTrackingTest.kt:43)
at com.datadog.android.sdk.integration.rum.ConsentPendingGrantedActivityTrackingTest$verifyRumEvents$1.invoke(ConsentPendingGrantedActivityTrackingTest.kt:45)
at com.datadog.tools.unit.ConditionWatcher.doWait(ConditionWatcher.kt:42)
at com.datadog.android.sdk.integration.rum.ConsentPendingGrantedActivityTrackingTest.verifyRumEvents(ConsentPendingGrantedActivityTrackingTest.kt:45)

It can be reproduced locally if you run all the tests in rum folder.

This is most likely caused by #2720. I suggest reverting it and think about the proper fix later in future PRs.

I added some logging to better understand the problem https://github.com/DataDog/dd-sdk-android/tree/aleksandr-gringauz/file-observer-concurrency

2026-01-13 14:17:08.608 23340-23410 WAHAHA                  com.datadog.android.sdk.integration  W  DataUploadRunnable.run
2026-01-13 14:17:08.609 23340-23410 WAHAHA                  com.datadog.android.sdk.integration  W  DataUploadRunnable.handleNextBatch
2026-01-13 14:17:08.610 23340-23410 WAHAHA                  com.datadog.android.sdk.integration  W  DataUploadRunnable.consumeBatch /data/user/0/com.datadog.android.sdk.integration/cache/datadog-a588c719ae523df29023a954180d678b65b9f6c2469b68ce31c1d2a13674af36/rum-v2/1768310215860
2026-01-13 14:17:08.626 23340-23410 WAHAHA                  com.datadog.android.sdk.integration  W  confirmBatchRead /data/user/0/com.datadog.android.sdk.integration/cache/datadog-a588c719ae523df29023a954180d678b65b9f6c2469b68ce31c1d2a13674af36/rum-v2/1768310215860
2026-01-13 14:17:08.626 23340-23410 WAHAHA                  com.datadog.android.sdk.integration  W  deleteBatchFile 1768310215860
2026-01-13 14:17:08.627 23340-23410 WAHAHA                  com.datadog.android.sdk.integration  W  DataUploadRunnable.handleNextBatch
2026-01-13 14:17:08.627 23340-23410 WAHAHA                  com.datadog.android.sdk.integration  W  /data/user/0/com.datadog.android.sdk.integration/cache/datadog-a588c719ae523df29023a954180d678b65b9f6c2469b68ce31c1d2a13674af36/rum-v2/1768310215860 batchData.isEmpty()
2026-01-13 14:17:08.627 23340-23410 WAHAHA                  com.datadog.android.sdk.integration  W  DataUploadRunnable.consumeBatch /data/user/0/com.datadog.android.sdk.integration/cache/datadog-a588c719ae523df29023a954180d678b65b9f6c2469b68ce31c1d2a13674af36/rum-v2/1768310215860
2026-01-13 14:17:08.627 23340-23410 WAHAHA                  com.datadog.android.sdk.integration  W  empty
2026-01-13 14:17:08.628 23340-23378 WAHAHA                  com.datadog.android.sdk.integration  W  removing 1768310215860

The following happens:

1. DatadogUploadRunnable consumes the batch, sends it and confirms that it read it here.
2. The batch file gets deleted here.
3. knownFiles in BatchFileOrchestrator isn't immediately updated, because FileObserver works asynchronously.
4. DatadogUploadRunnable reads next batch. It tries to read the same batch file as before, because the state in BatchFileOrchestrator isn't updated yet. We end up here.
5. However the batch file is deleted, that is why DatadogUploadRunnable sends the request body with an empty batch. It leads to the exception in the test.

[datadog-official]

🎯 Code Coverage
• Patch Coverage: 91.89%
• Overall Coverage: 64.70% (-1.21%)

View detailed report

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.74%. Comparing base (53a83b8) to head (bade6c9).

Files with missing lines	Patch %	Lines
...al/persistence/file/batch/BatchFileOrchestrator.kt	92.11%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3124      +/-   ##
===========================================
- Coverage    70.78%   70.74%   -0.04%     
===========================================
  Files          893      893              
  Lines        33014    33000      -14     
  Branches      5554     5549       -5     
===========================================
- Hits         23366    23343      -23     
+ Misses        8107     8101       -6     
- Partials      1541     1556      +15     

Files with missing lines	Coverage Δ
...al/persistence/file/batch/BatchFileOrchestrator.kt	92.90% <92.11%> (-1.77%)	⬇️

... and 39 files with indirect coverage changes

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