[zhanglongtu]

This PR introduces tracing channels support to pg-pool and pg querying methods as discussed in #3619

I haven't yet run a benchmark, but without active tracing or subscribers the overhead is non-existent so existing users shouldn't be affected. The overhead would come from the consumers of the diag channels published here which is done anyways via monkey patching.

I added the following channels:

Channel	Type	Package	Description
pg:query	TracingChannel	pg	Query lifecycle (start, end, error with async context)
pg:connection	TracingChannel	pg	Client connection lifecycle
pg:pool:connect	TracingChannel	pg-pool	Pool connection acquisition lifecycle
pg:pool:release	Channel	pg-pool	Client released back to pool
pg:pool:remove	Channel	pg-pool	Client removed from pool

I understand there may be hesitation given that AI tooling was used in the implementation. To be clear, this isn't a drive-by contribution. I designed the channel layout, wrote the proposal document, and iterated through multiple rounds to handle real edge cases (Node 18 compat, preserving custom Promise types, hasSubscribers guards). The AI assisted with the code, but the approach and decisions are mine.

Usage

Instrumentations will only need to subscribe to tracing channels to create traces, logs or metrics:

import { tracingChannel } from 'diagnostics_channel'

tracingChannel('pg:query').subscribe({
  start({ query, client }) {
    // start span
    // query: { text, name, rowMode }
    // client: { database, host, port, user, processID, ssl }
  },
  asyncEnd({ query, result }) {
    // end span
    // result: { rowCount, command }
  },
  error({ query, error }) {
    // record error on span
  },
})

This is part of a broader initiative to bring TracingChannel support to the most widely used Node.js database and cache libraries. The same pattern has already been merged and shipped in:

• mysql2
• node-redis
• ioredis

I have been directly involved in those prior implementations and I'm happy to own this through to release, address any feedback, and provide whatever support is needed. Would love to hear your thoughts.

Supersedes #3624.

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[Copilot]

🟡 Changes recommended

Critical packaging and callback-tracing issues remain unresolved.

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Pull request overview

Adds optional diagnostics_channel tracing for pg queries/connections and pg-pool lifecycle events.

Changes:

• Adds tracing and diagnostic channels.
• Instruments client and pool lifecycle operations.
• Adds diagnostics unit tests.

File summaries

File	Summary
packages/pg/test/unit/client/diagnostics-tests.js	Adds client diagnostics tests.
packages/pg/lib/diagnostics.js	Defines client tracing channels; builtin-module loading needs correction.
packages/pg/lib/client.js	Instruments connections and queries; callback tracing and native-client coverage require changes.
packages/pg-pool/test/diagnostics.js	Adds pool diagnostics tests.
packages/pg-pool/index.js	Publishes pool lifecycle events; the diagnostics module must be included in the package allowlist.
packages/pg-pool/diagnostics.js	Defines pool channels; builtin-module loading needs correction.

Review details

Suppressed comments (4)

packages/pg-pool/diagnostics.js:33

• On Node 18 the comment above says the aggregate hasSubscribers value is undefined; with !== false, this returns true even when every underlying tracing channel has no subscribers. Each pool connection acquisition then still goes through traceCallback, so the claimed no-subscriber zero-overhead path is not true on a supported runtime; check the component channels before tracing.

function shouldTrace(channel) {
  return channel.hasSubscribers !== false
}

packages/pg-pool/index.js:223

• traceCallback expects context immediately after the function, but this passes 0 first. As a result pool acquisition callbacks are not wrapped correctly when tracing is enabled, preventing the caller's callback/promise from completing and making the pg:pool:connect lifecycle unusable. Pass context as the second argument.

        0,
        context,

packages/pg/lib/client.js:12

• The new channels are wired only into the pure-JavaScript Client; pg.native and NODE_PG_FORCE_NATIVE use lib/native/client.js, whose connect/query paths never publish these channels. Instrumentation subscribing to the documented pg:query and pg:connection channels will therefore silently miss supported native clients; add equivalent hooks or explicitly scope the API to the JS client.

const { queryChannel, connectionChannel, shouldTrace } = require('./diagnostics')

packages/pg/lib/diagnostics.js:29

• On Node 18 the comment above says the aggregate hasSubscribers value is undefined; with !== false, this returns true even when every underlying tracing channel has no subscribers. Each query/connect then still goes through traceCallback, so the claimed no-subscriber zero-overhead path is not true on a supported runtime; check the component channels before tracing.

function shouldTrace(channel) {
  return channel.hasSubscribers !== false
}

• Files reviewed: 6/6 changed files
• Comments generated: 5
• Review effort level: Lite

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