fix(multistream): isolate per-stream write() errors so remaining streams still receive log data

[LeSingh1]

Problem

pino.multistream().write() iterates over all registered streams and calls stream.write(data) on each. There is no error isolation around individual writes: if any stream's synchronous write() throws, the iteration loop aborts immediately and every stream that comes after the failing one in the sorted list never receives the log line. Logs are silently dropped with no indication that anything went wrong.

Minimal reproduction:

const pino = require('pino')

let received = 0
const goodStream = { write () { received++ } }
const badStream  = { write () { throw new Error('boom') } }

const multi = pino.multistream([
  { level: 'info', stream: badStream  },   // sorted first (same level)
  { level: 'info', stream: goodStream }
])

const log = pino({ level: 'info' }, multi)
log.info('hello')

console.log(received) // prints 0 — goodStream never received the line!

Fix

Wrap each stream.write(data) call in a try/catch inside multistream.write(). The caught error is surfaced via a dedicated EventEmitter (errorEmitter) so callers can observe it with res.on('error', fn) — without interfering with the existing res.emit() method, which propagates events like 'message' down to every child stream for pino's internal config protocol.

When no 'error' listener is registered the error is swallowed, preserving the existing non-throwing contract of multistream.write().

What changed in lib/multistream.js:

• Added an internal errorEmitter (EventEmitter) instance.
• Exposed on / once / removeListener / off on res delegating to errorEmitter.
• Wrapped stream.write(data) in try/catch; emits 'error' on errorEmitter when there is at least one listener, otherwise swallows.

New tests (in test/multistream.test.js)

1. multistream.write continues writing to remaining streams when one stream throws synchronously — asserts that a stream ordered after a throwing stream still receives all log lines and that an 'error' event is emitted for each throwing write.
2. multistream.write does not throw when a stream write() throws and no error listener is registered — asserts doesNotThrow when no listener is attached.

Test results

All 32 multistream tests pass (including the 2 new ones), and the full basic + serializer suites (126 tests) continue to pass.

Checklist

• Existing tests pass
• New regression tests added
• No change to the res.emit() stream-propagation behaviour
• Backward compatible (callers that do not attach a res.on('error') listener see no behaviour change)

[mcollina]

I think the best outcome would be to move the res object to
be a full EventEmitter instead.

[mcollina]

Ahum, this doesn't look a good idea after all. Do we need the return value to have an emit property?

If we can't remove this, the previous approach was in fact better. My bad.