feat(stream): label a session to split the metrics zone below its listening address

[AlinsRan]

Why

apisix_stream_metrics_zone counts active sessions and bytes per stream listening address. The slots are claimed once, per address, and a session is merged into its address's slot with nothing else recorded about it. A consumer that wants the same counters split by something only known at runtime — for example the service a session was routed to — cannot recover that split from the aggregated totals.

What

A session can now be labelled, typically from preread_by_lua* once it is known what it belongs to:

local metrics = require("resty.apisix.stream.metrics")
local ok, err = metrics.set_labels({ "svc-a", "order" })

From then on the session's active count and the bytes it moves are accounted on the slot of (listening address, label values), which dump() reports as its own entry with the values in a labels array. The values are ordered, as the caller's own metric declares its labels; the label names stay with the caller.

• The per address total is preserved. Bytes stay on the entry they were counted on: what a session moved before its first label, and every session that is never labelled, stays on the unlabelled entry (labels = {}), so the entries of one address always add up to what the zone reported before this change. Labelling flushes the pending delta to the old slot first, then raises the new slot's active before lowering the old one.
• An empty array moves the session back to the unlabelled entry.
• Encoding. The C side keys a slot by one byte string; the Lua side joins the values with \31 and splits them back in dump(), so a value cannot contain \31.
• Slot claiming. Labelled slots are claimed at runtime by whichever worker first sees that set of values, so claiming now takes the zone's slab mutex. Lookups stay lock free: slots are still append-only, and the existing publish barrier is kept. Each worker keeps a small direct-mapped cache of (address slot, labels) -> slot, so labelling a session does not scan the zone.
• Capacity. Slot cap raised from 512 to 8192; the actual count is still derived from the zone size. The joined values take up to 512 bytes, room for an object id (at most 256 bytes) together with a name. A slot grows from about 176 to about 690 bytes, so 1m holds about 760 slots. Labels may take at most three quarters of them; the rest is kept for listening addresses, so an address added by a later reload is still counted when labels have filled the zone. When the zone is full, set_labels returns nil, "stream metrics zone is full" and the session stays where it was. Logging it is left to the caller.
• FFI.
  • New ngx_stream_apisix_metrics_set_labels() and ngx_stream_apisix_metrics_size().
  • The request is declared as void * so the Lua module still loads in http, where dump() is also called.
  • dump() sizes its buffer from size() instead of a fixed 512 entries.
  • The entry layout gained labels_len and labels. The C side and resty.apisix.stream.metrics ship together, so there is no mixed-version pairing to support.

Without a zone, or on an address that is not accounted for (unix sockets), set_labels returns nil, "not accounted".

Testing

• t/stream/metrics.t TEST 11–18 cover:
  • labelling moves the active count and later bytes;
  • an empty array moves the session back;
  • one slot per address per set of values, reused across sessions;
  • invalid labels and the http subsystem are refused;
  • no zone configured;
  • a full zone keeps the session on its slot;
  • bytes stay on the entry they were counted on when the labels change;
  • several values are one slot, returned in order, empty values keeping their position.
• t/stream/metrics-reload.t (HUP) fills the zone with labels, then reloads with a new listening address. The new address is still counted, and the labels survive the reload. It fails without the listen share.
• Built locally against openresty 1.29.2.4 with -Werror: t/stream/metrics.t and t/stream/metrics-reload.t pass (73 tests), luacheck clean. Only 1.29.2.4 was built locally; CI covers 1.25.3.1 with ASAN.
• In t/stream/xrpc/{downstream,upstream}.t, 5 cases fail locally both with and without this change.

Summary by CodeRabbit

• New Features
  • Stream metrics are tracked separately by listening address and label set. Empty labels select the unlabelled counter.
  • Sessions can be assigned or switched to labels. Bytes already counted remain attributed to the label set active at the time.
  • Metrics dumps include labels, and entry capacity reflects the configured metrics zone.
  • Label assignment reports errors for invalid labels, unaccounted sessions, and full zones.
• Documentation
  • Added details on label behavior, byte accounting, limits, capacity, and related errors.

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📝 Walkthrough

Walkthrough

Stream metric slots are keyed by listening address and label set. The new set_labels(labels) API assigns labels to a stream session. Metric dumps include decoded labels and size their buffer from the reported slot count.

Changes

Labeled stream metrics

Layer / File(s)	Summary
Address-and-label slot model src/stream/ngx_stream_apisix_metrics_module.h, src/stream/ngx_stream_apisix_metrics_module.c	Native metric slots store labels and are keyed by listening address and label set. The slot capacity is 8,192. Slot creation uses the shared-memory slab mutex, and each listening address has an unlabelled slot.
Native label assignment and metric export src/stream/ngx_stream_apisix_metrics_module.c, src/stream/ngx_stream_apisix_metrics_module.h	The native API resolves and caches labeled slots. When a session changes slots, it flushes byte counters and transfers the active count. The API exposes the used slot count, and metric entries include labels.
Lua API, dumps, tests, and documentation lib/resty/apisix/stream/metrics.lua, t/stream/metrics.t, README.md	The Lua module validates labels, maps native results to API responses, and grows its dump buffer to the reported slot count. Tests cover labeled and unlabelled accounting, address-specific slots, validation, and full-zone behavior. The README documents label limits and slot behavior.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant StreamSession
  participant MetricsLua as metrics.set_labels
  participant NativeMetrics as ngx_stream_apisix_metrics_set_labels
  participant MetricsSlots as shared metrics slots
  StreamSession->>MetricsLua: provide label values
  MetricsLua->>NativeMetrics: request and encoded labels
  NativeMetrics->>MetricsSlots: resolve address-and-label slot
  MetricsSlots-->>NativeMetrics: slot or capacity status
  NativeMetrics-->>MetricsLua: native result

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Merge Risk: ⚪ Minimal · up to 54579

The labeled metrics changes are mergeable with no identified current-head correctness, availability, or data-integrity risk.

🚥 Pre-merge checks | ✅ 5 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
E2e Test Quality Review	⚠️ Warning	The PR adds real end-to-end coverage through stream Lua, proxy traffic, shared-memory metrics, and HTTP dump. The tests cover labels, empty arrays, invalid inputs, exact 512-byte limits, multiple addr…	Add E2E tests that send traffic before calling set_labels and assert those bytes remain on labels = {} while later bytes use the labelled entry. Add a Unix-listener test that calls set_labels in the stream phase and asserts `nil, "not…

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Security Check	✅ Passed	No security-check failure was introduced. The changed code only adds internal stream metrics labels and FFI access. It does not add database persistence, mutating HTTP endpoints, ownership-sensitive r…
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: adding session labels to split stream metrics below the listening address.

Full details: E2e Test Quality Review

Explanation

The PR adds real end-to-end coverage through stream Lua, proxy traffic, shared-memory metrics, and HTTP dump. The tests cover labels, empty arrays, invalid inputs, exact 512-byte limits, multiple addresses, full-zone failure, and relabelling. However, two core flows remain untested: bytes sent before the first label must remain on the unlabelled entry, and set_labels on an unaccounted Unix listener must return "not accounted". The current relabelling test starts with svc-a, and the no-zone test does not exercise an unaccounted listener. This is a critical scenario-coverage gap under the review criteria.

Resolution

Add E2E tests that send traffic before calling set_labels and assert those bytes remain on labels = {} while later bytes use the labelled entry. Add a Unix-listener test that calls set_labels in the stream phase and asserts nil, "not accounted". Keep the existing real upstream and dump assertions in those tests.

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- Around line 89-90: Clarify the accounting description around “Everything
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Review comments at @lib/resty/apisix/stream/metrics.lua:
- Around line 159-161: Update set_labels to validate that labels contains only
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