[hamzakargin]

fs.createReadStream() and filehandle.createReadStream() ignore
stream.setDefaultHighWaterMark() because ReadStream hardcodes a 64 KiB
default.

ReadStream now uses the value set with setDefaultHighWaterMark() and
falls back to 64 KiB when it was never called. Using
getDefaultHighWaterMark() directly would lower the default to 16 KiB on
Windows, so the fallback stays as it is.

Only code that calls setDefaultHighWaterMark() sees a change.

Fixes: #66937

cc @mertcanaltin

I used an AI coding assistant for this change. I verified it myself: reproduced the bug, confirmed the new test fails without the fix, and ran the related tests and linters locally.

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.59%. Comparing base (1bffd1d) to head (dfa7cb1).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66956      +/-   ##
==========================================
- Coverage   90.60%   90.59%   -0.02%     
==========================================
  Files         797      797              
  Lines      278276   278334      +58     
  Branches    53498    53516      +18     
==========================================
+ Hits       252133   252148      +15     
- Misses      16506    16555      +49     
+ Partials     9637     9631       -6     

Files with missing lines	Coverage Δ
lib/internal/fs/streams.js	93.12% <100.00%> (+0.01%)	⬆️
lib/internal/streams/state.js	98.27% <100.00%> (+0.23%)	⬆️

... and 40 files with indirect coverage changes

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[mertcanaltin]

Thanks for the pr. I tested it locally, and the issue is fixed.

I have two questions:

After setDefaultHighWaterMark(false, 0), fs.createReadStream reads nothing:

const { setDefaultHighWaterMark } = require('stream');
const fs = require('fs');
setDefaultHighWaterMark(false, 0);
let n = 0;
fs.createReadStream('README.md')
  .on('data', (d) => n += d.length)
  .on('end', () => console.log('read', n, 'size', fs.statSync('README.md').size));
// read 0 size 42695

Can this PR fall back to 64 KiB for 0? Or should we open a new issue?

This PR changes fs read streams for code that uses setDefaultHighWaterMark. Should it be semver-major?

[hamzakargin]

Thanks for testing!

1. This isn't new with this PR. An explicit highWaterMark: 0 already reads nothing on current Node (v24.22.0):

   fs.createReadStream('README.md', { highWaterMark: 0 });
   // read 0 size 42695

   ReadStream.prototype._read gets n = 0 and hits if (n <= 0) this.push(null), so the stream ends before reading. Falling back to 64 KiB only for the default would leave the explicit case broken, so I think it should be fixed in _read for both, in a separate issue.

2. I see it as a fix, since setDefaultHighWaterMark() is documented as setting the default for streams. But it does change the documented 64 * 1024 default for fs read streams, so semver-major is fine with me if collaborators prefer it.