dns: coalesce identical concurrent lookup() requests

[orgads]

When multiple callers issue dns.lookup() for the same (hostname, family, hints, order) concurrently, only one getaddrinfo call is now dispatched to the libuv threadpool. All callers share the result.

getaddrinfo is a blocking call that runs on the libuv threadpool (capped at 4 threads by default, with a slow I/O concurrency limit of 2). When DNS resolution is slow - e.g. ~10-20 s per call due to a misbehaving resolver - identical requests queue behind each other, causing timeouts that grow linearly with the number of concurrent callers:

Before: 100 parallel lookup('host') -> 50 batches × 10 s = 500+ s
After: 100 parallel lookup('host') -> 1 getaddrinfo call = ~10 s

This is particularly severe on WSL, where the DNS relay rewrites QNAMEs in responses (appending the search domain), causing glibc to discard them as non-matching and wait for a 5s timeout per retry.

The coalescing is keyed on (hostname, family, hints, order) so lookups with different options still get separate getaddrinfo calls. Each caller independently post-processes the shared raw result (applying the 'all' flag, constructing address objects, etc.).

Fixes: #62503

[nodejs-github-bot]

Review requested:

• @nodejs/net

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.72%. Comparing base (12249cc) to head (610a0c3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62599      +/-   ##
==========================================
- Coverage   91.53%   89.72%   -1.81%     
==========================================
  Files         352      695     +343     
  Lines      147833   214543   +66710     
  Branches    23148    41076   +17928     
==========================================
+ Hits       135321   192503   +57182     
- Misses      12255    14095    +1840     
- Partials      257     7945    +7688     

Files with missing lines	Coverage Δ
lib/dns.js	99.48% <100.00%> (+0.83%)	⬆️
lib/internal/dns/promises.js	99.02% <100.00%> (+1.46%)	⬆️

... and 463 files with indirect coverage changes

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[mcollina]

I think this might break async_hooks continuation, could you verify? Adding an AsyncResource on coalescing would be enough.

[orgads]

Great observation! Should be fixed now.

[mcollina]

I have a feeling that the failing test on GHA is relevant, can you take a look?

[orgads]

I have a feeling that the failing test on GHA is relevant, can you take a look?

It doesn't look related. Similar failures happened here and here. Also, this test calls dns.resolve (c-ares query request), which wasn't touched in this change. Only dns.lookup (getaddrinfo) was touched.

The test is flaky, since it relies on external systems which may refuse or time out.