[9.2](backport #47846) Fix SetPaths to be idempotent for global processors

[mergify]

Proposed commit message

When multiple inputs connect to the same pipeline, each input calls SetPaths on global processors. Previously, the second call would fail with 'attempt to set paths twice'.

This fix makes SetPaths idempotent by storing the paths pointer and returning nil (instead of an error) when called again with the same pointer. This is the expected behavior for global processors shared across multiple inputs within the same beat instance.

Changes:

• libbeat/processors/safe_processor.go: Store paths pointer and allow subsequent calls with the same pointer to succeed
• libbeat/processors/safe_processor_test.go: Add test cases for idempotent SetPaths behavior
• filebeat/tests/integration/cache_processor_test.go: Add integration test that verifies global cache processor works with multiple inputs

Fixes #48193

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability. -- I did use the stresstest script to verify the new integration test
• I have added an entry in ./changelog/fragments using the changelog tool.

How to test this PR locally

Option 1: Run the Integration Test

cd filebeat
mage buildSystemTestBinary
go test -v -tags integration -run "TestGlobalCacheProcessorMultipleInputs" ./tests/integration/

Expected result: Test passes with all 30 events processed from 3 inputs.
Without the fix: Test fails with:

"error while connecting to output with pipeline: failed setting paths for global processors: attempt to set paths twice"

Option 2: Manual Test with Config File

filebeat.inputs:
  - type: filestream
    id: input-a
    paths:
      - /tmp/logs/a.log
    prospector.scanner.fingerprint.enabled: false

  - type: filestream
    id: input-b
    paths:
      - /tmp/logs/b.log
    prospector.scanner.fingerprint.enabled: false

processors:
  - cache:
      backend:
        file:
          id: test-cache
          write_interval: 1s
        capacity: 1000
      put:
        key_field: message
        value_field: message
        ttl: 1h
      ignore_missing: true

output.console:
  enabled: true

mkdir -p /tmp/logs
for i in $(seq 1 100); do echo "{\"message\":\"test $i\"}"; done > /tmp/logs/a.log
for i in $(seq 1 100); do echo "{\"message\":\"test $i\"}"; done > /tmp/logs/b.log

./filebeat -e -c config_cache_global.yml 2>&1 | grep -E "error|twice|initialized|harvester.*started"

Expected result (with fix):

• "initialized cache processor" appears once
• Both inputs start successfully
• Events are processed from both inputs

Without the fix:

"error while connecting to output with pipeline: failed setting paths for global processors: attempt to set paths twice"

Only input-a processes events; input-b fails to connect.

Related issues

• [cache-processor] Set beat paths #47353

Logs

Logs

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---

This is an automatic backport of pull request #47846 done by [Mergify](https://mergify.com).

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @orestisfl? 🙏

[pierrehilbert]

Will be failing until Kibana team bumped the version to 9.2.4.