work-unit: logfwd-io/tcp_input — P0/P1 TCP input hardening

[strawgate]

Purpose

Fix three bugs in the TCP input that cause data corruption and OOM under load.

Why this is one agent run

• All three are in tcp_input.rs (398 lines)
• bug: TCP input merges all connections into one stream — cross-connection data corruption #803 (per-connection buffering) is prerequisite to correctly fixing bug: TCP client disconnect loses partial line — no EndOfFile emitted #804 and P0: TCP/UDP inputs have no backpressure — will OOM under sustained load #576
• Same test surface — TCP connection lifecycle

Repo footprint

• crates/logfwd-io/src/tcp_input.rs

Related issues

#	Type	Why included
#576	P0 bug	TCP/UDP inputs have no backpressure — will OOM under sustained load
#803	bug	TCP input merges all connections into one stream — cross-connection data corruption
#804	bug	TCP client disconnect loses partial line — no EndOfFile emitted

Non-goals

• TLS support (bug: TCP ingress needs TLS/mTLS support (UDP DTLS out of scope) #579) — requires rustls integration, separate concern
• UDP backpressure — similar pattern but different file (udp_input.rs)
• TCP multiline framing (P1: newline-only TCP framing breaks multiline messages and enables log injection #580) — protocol-level design decision
• Async migration (feat: migrate TCP/UDP inputs and outputs to tokio async I/O #571) — architectural change, not a bug fix

Merge-safety notes

• Blocked by PR fix: TCP cross-connection corruption, fingerprint collision, per-file read fairness #868 — PR fix: TCP cross-connection corruption, fingerprint collision, per-file read fairness #868 is fixing TCP cross-connection corruption (bug: TCP input merges all connections into one stream — cross-connection data corruption #803). Wait for fix: TCP cross-connection corruption, fingerprint collision, per-file read fairness #868 to merge. If fix: TCP cross-connection corruption, fingerprint collision, per-file read fairness #868 resolves bug: TCP input merges all connections into one stream — cross-connection data corruption #803, this work-unit reduces to P0: TCP/UDP inputs have no backpressure — will OOM under sustained load #576 + bug: TCP client disconnect loses partial line — no EndOfFile emitted #804.

Exit criteria

• bug: TCP input merges all connections into one stream — cross-connection data corruption #803 fixed — each TCP connection gets its own buffer and SourceId
• bug: TCP client disconnect loses partial line — no EndOfFile emitted #804 fixed — TCP disconnect emits EndOfFile with partial line
• P0: TCP/UDP inputs have no backpressure — will OOM under sustained load #576 fixed — memory budget check prevents OOM (reject or drop connections over limit)
• Test: multiple concurrent TCP clients don't corrupt each other's data

[strawgate]

Adding #580 (TCP framing / line splitting) to this work-unit scope — it's a TCP input correctness issue.