stream: fast path single-destination pipe

[trivikr]

Avoid EventEmitter dispatch for the internal readable.pipe() data handler
when it is the only data listener on the source stream.

If any additional data listener is present, the stream falls back to normal
data event emission so listener ordering is preserved.

Benchmark:

                          confidence improvement accuracy (*)   (**)  (***)
streams/pipe.js n=5000000        ***     11.96 %       +/-2.00% +/-2.66% +/-3.47%

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[ronag]

This looks to me like it will break stuff. What happens if a data listener is added after pipe? Can you add a test for that.

[trivikr]

What happens if a data listener is added after pipe?

If a data listener is added after pipe(), the fast path stops applying because _events.data is no longer the single internal pipe handler. Subsequent chunks go through normal stream.emit('data'), preserving EventEmitter listener order.

Can you add a test for that?

Added a test in 3184889 (this PR)

The added test starts with r.pipe(w), then adds a data listener during the first destination write(). It asserts the listener does not see the in-flight first chunk, but does see the second chunk after the pipe write, matching normal EventEmitter behavior.

[ronag]

This might still break stuff. Some streams override emit. @mcollina wdyt?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.32%. Comparing base (13feb34) to head (3184889).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #63592   +/-   ##
=======================================
  Coverage   90.32%   90.32%           
=======================================
  Files         730      730           
  Lines      234653   234666   +13     
  Branches    43942    43943    +1     
=======================================
+ Hits       211959   211973   +14     
+ Misses      14415    14411    -4     
- Partials     8279     8282    +3     

Files with missing lines	Coverage Δ
lib/internal/streams/readable.js	97.25% <100.00%> (+0.01%)	⬆️

... and 31 files with indirect coverage changes

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[trivikr]

Benchmark CI: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1855/