stream: avoid duplicate writes in toWritable

[trivikr]

toWritable() currently treats writeSync() / writevSync() returning
false as meaning the data was not accepted and retries through the async
writer method.

For PushWriter with backpressure: 'block', false can instead mean the
chunk was accepted and backpressure is active. Retrying through
write() / writev() writes the same data twice.

This updates the adapter to recognize that accepted-backpressure case and wait
for drain instead of retrying the chunk.

Fixes: #63359

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 87.14286% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.05%. Comparing base (1f371fc) to head (c5132e7).
⚠️ Report is 78 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/classic.js	80.00%	9 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #63360 +/- ##
=======================================
Coverage 90.05% 90.05% =======================================
Files 714 714 Lines 225704 225768 +64 Branches 42714 42727 +13 =======================================
+ Hits 203250 203321 +71 - Misses 14225 14242 +17 + Partials 8229 8205 -24 

Files with missing lines	Coverage Δ
lib/internal/streams/iter/push.js	92.20% <100.00%> (+0.12%)	⬆️
lib/internal/streams/iter/types.js	100.00% <100.00%> (ø)
lib/internal/streams/iter/classic.js	88.91% <80.00%> (-0.54%)	⬇️

... and 42 files with indirect coverage changes

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CI: https://ci.nodejs.org/job/node-test-pull-request/73613/ 💛

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----------------------------------- PR info ------------------------------------
Title stream: avoid duplicate writes in toWritable (#63360)
⚠ Could not retrieve the email or name of the PR author's from user's GitHub profile!
Branch trivikr:stream-iter-towritable-write -> nodejs:main
Labels stream, author ready, needs-ci
Commits 2
- stream: avoid duplicate writes in toWritable
- doc: describe kSyncWriteAcceptedOnFalse stream sentinel
Committers 1
- Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
PR-URL: https://github.com/nodejs/node/pull/63360
Fixes: https://github.com/nodejs/node/issues/63359
Reviewed-By: James M Snell <jasnell@gmail.com>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/63360
Fixes: https://github.com/nodejs/node/issues/63359
Reviewed-By: James M Snell <jasnell@gmail.com>
--------------------------------------------------------------------------------
ℹ This PR was created on Sat, 16 May 2026 02:41:57 GMT
✔ Approvals: 1
✔ - James M Snell (@jasnell) (TSC): https://github.com/nodejs/node/pull/63360#pullrequestreview-4342429217
✘ This PR needs to wait 13 more hours to land (or 0 minutes if there is one more approval)
✔ Last GitHub CI successful
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Landed in 8d3245e