63 lines
4.6 KiB
Markdown
63 lines
4.6 KiB
Markdown
---
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id: "bug-1890"
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status: "fixed"
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status_updated: "2026-09-29"
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status_note: ""
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component: "lib"
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severity: "noise"
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timestamp: "2026-09-29T14:01:08Z"
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last_seen: "2026-09-29T14:01:08Z"
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occurrences: 1
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files: ["src/lib/netio.c (socket_create, socket_connect, endpoint_accept)"]
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tags: ["netio", "tcp", "latency", "ha", "snapshot"]
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related: ["bug-1871"]
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fix_commits: ["8b561547"]
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gitea_issue: 336
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gitea_synced_status: "fixed"
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gitea_synced_hash: "6ce3dfb46b4d"
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---
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# bug-1890: netio sockets leave Nagle on: last TLS records of a message burst arrive ~40 ms late
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## Description
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**Observed:** a 9-chunk HA snapshot transfer is fully written by the primary within 1 ms, but the standby reads the last chunks ~40 ms later. Primary trace (xce-1871-fs1, ctrl-1) shows the final send drained at 13:20:33.967 (`bug1871 upd ... 0x5->0x1 outq_empty=1 fsm_out=5`); the standby logs:
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```
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[2026-09-29 13:20:33,969] ... ha_handle_snapshot_chunk: chunk crc32 checkpoint_lsn=114 chunk_index=6 ...
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[2026-09-29 13:20:34,010] ... ha_handle_snapshot_chunk: chunk crc32 checkpoint_lsn=114 chunk_index=7 ...
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```
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**Expected:** on loopback the whole burst arrives within a few ms (most runs: max inter-chunk gap 1-4 ms).
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**Impact:** latency only, ~40 ms on the tail of a burst (snapshot/WAL batches, request-reply pairs after bulk data); no loss. `grep -rn TCP_NODELAY src/lib` finds nothing, so every netio TCP socket runs with Nagle, and a small trailing TLS record waits for the receiver's delayed ACK.
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**Found by:** soak -- bug-1871's instrumented force-snapshot runs, 2026-09-29. Suspected (not yet proven with TCP_NODELAY on): Nagle + delayed ACK.
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UPDATE (2026-09-29): confirmed -- with TCP_NODELAY set the ~40 ms tail gaps disappear (see Verification), so the Nagle + delayed-ACK suspicion was right.
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## Environment
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- OS: no /etc/os-release; Docker container; NixOS kernel; kernel Linux 6.18.33 x86_64
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- libc: glibc 2.42-47 (interpreter /nix/store/wb6rhpznjfczwlwx23zmdrrw74bayxw4-glibc-2.42-47/lib/ld-linux-x86-64.so.2)
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- Toolchain: gcc (GCC) 15.2.0, meson 1.9.1
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- Build: asan (src/xce.build.bug1871, with temporary netio TRACE instrumentation)
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- Code: scheduler-framework @ 3444cf16
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- Deployment: local 3-node HA controller cluster (xce-test-ha --force-snapshot), localhost:9991-9993, 32 CPUs
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## Reproduction
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1. `nix develop ./src --command bash .claude/skills/xce-test-ha/run-ha-soak.sh --force-snapshot <dir>`
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2. On the rejoined standby's log, compute the gap between consecutive `ha_handle_snapshot_chunk: chunk crc32` lines of one transfer; a ~40 ms gap (while the primary finished sending 1 ms earlier) is the symptom.
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- Frequency: 5/40 transfers had a 39-41 ms gap, 2026-09-29 (xce-1871-fs1..fs16)
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- Evidence: /home/claude/tmp/xce-1871-fs1, /home/claude/tmp/xce-1871-fs5, /home/claude/tmp/xce-1871-fs16 (ctrl-*.log)
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## Root cause
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netio never set TCP_NODELAY (socket_create() and socket_accept() in src/lib/netio.c set only SO_REUSEADDR/FD_CLOEXEC), so every controller/worker/scheduler/apiserver TCP connection ran with Nagle's algorithm. netio writes a message as separate TLS records (an 8-byte size header, then the body split into <=16 KB records); when a small record follows data the peer has not ACKed yet, Nagle holds it until the ACK arrives, and the receiver delays that ACK (Linux delayed ACK, ~40 ms). The tail of a burst -- the last short chunk of a snapshot, a header, a reply -- therefore arrived ~40 ms after the sender had finished writing.
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## Fix
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src/lib/netio.c: new socket_set_nodelay() sets TCP_NODELAY; socket_create() calls it for AF_INET stream sockets (outbound endpoints from endpoint_init, and listeners) and socket_accept() for every accepted connection. Small records are sent immediately instead of waiting for the peer's delayed ACK. Best effort: a failure leaves the socket working as before.
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## Verification
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- Test: test_outbound_socket_has_nodelay and test_accepted_socket_has_nodelay in src/lib/testsuite/test_netio_nodelay.c check TCP_NODELAY on an endpoint_init socket and an endpoint_accept socket: RED before the fix (`bug-1890: outbound netio socket runs with Nagle on`, same for accepted), GREEN after
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- Suites: xcelib_test 699/699 and controller 1256/1256, asan, no sanitizer errors
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- Live: xce-test-ha --force-snapshot, 16 runs (2026-09-29, xce-1890-nd1..16): 40/40 snapshot transfers complete, max inter-chunk gap 4 ms, 0 gaps >= 20 ms -- versus 5/40 transfers with a 39-41 ms gap before the fix. 15/16 PASS; the one FAIL (nd5) is bug-1891 (snapshot sent to the wrong standby), unrelated to this change and seen before it
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## References
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- Fix commit: 8b561547
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- Related: bug-1871 (found while instrumenting it), bug-1891
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- Gitea: #336
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