input: set FLB_IO_TCP_KA flag in flb_input_upstream_set

[labros-mediaalpha]

This PR description was AI-assisted (Claude Code).

What does this PR do?

flb_input_upstream_set() copies net.* config into the upstream's
net_setup struct, but never sets the FLB_IO_TCP_KA stream flag that
flb_upstream_conn_get() actually checks when deciding a new
connection's initial recycle state. This mirrors the equivalent, already
correct logic in flb_output_upstream_set().

Companion PR (#12095) adds the missing flb_input_upstream_set() call
to in_prometheus_scrape; neither PR alone restores connection reuse —
verified empirically, see #12094.

Impact

Affects every input plugin calling flb_input_upstream_set():
currently in_prometheus_scrape (companion PR), in_kubernetes_events,
and in_calyptia_fleet. The latter two were not independently
re-tested against this fix — flagging for maintainer visibility since
their existing "fix" may not have been fully effective either.

Testing

Verified via -vv debug logging and tcpdump packet capture on a live
v5.0.7 instance: without this fix, in_prometheus_scrape connections
are torn down and recreated every scrape cycle even with net.keepalive
on and the companion PR's upstream_set() call in place. With this fix,
connections are created once and recycled on subsequent scrapes.

Ref #12094

Summary by CodeRabbit

• Bug Fixes
  • Improved TCP connection reliability by consistently applying keepalive settings when initializing upstream connections.
  • Ensures configured keepalive behavior is honored for input connections.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 419e822c-a5ce-414b-bb3d-e7d65f622687

📥 Commits

Reviewing files that changed from the base of the PR and between 5c5b8ce and a799075.

📒 Files selected for processing (1)

• src/flb_input.c

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📝 Walkthrough

Walkthrough

Input upstream setup now enables the FLB_IO_TCP_KA stream flag when net.keepalive is configured, aligning input behavior with output upstream initialization.

Changes

Input upstream keepalive

Layer / File(s)	Summary
Apply keepalive during upstream setup src/flb_input.c	flb_input_upstream_set() enables FLB_IO_TCP_KA on the upstream base when input keepalive is enabled.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related issues

• in_prometheus_scrape never reuses TCP connections; root cause also affects flb_input_upstream_set() #12094 — Adds FLB_IO_TCP_KA handling to flb_input_upstream_set() for the identified keepalive bug.

Suggested reviewers: edsiper, cosmo0920, edbingo, niedbalski, pwhelan

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main code change in flb_input_upstream_set.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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