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stream: do not unconditionally call `_read()` on `resume()`

`readable.resume()` calls `.read(0)`, which in turn previously set
`needReadable = true`, and so a subsequent `.read()` call would
call `_read()` even though enough data was already available.

This can lead to elevated memory usage, because calling `_read()`
when enough data is in the readable buffer means that backpressure
is not being honoured.

Fixes: #26957

PR-URL: #26965
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
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addaleax authored and BethGriggs committed Mar 28, 2019
1 parent 044e753 commit 274b97c4eaeb1de0eb41bdfcafd3e0eb0d015266
Showing with 22 additions and 1 deletion.
  1. +1 −1 lib/_stream_readable.js
  2. +21 −0 test/parallel/test-stream-readable-resume-hwm.js
@@ -469,7 +469,7 @@ Readable.prototype.read = function(n) {
ret = null;

if (ret === null) {
state.needReadable = true;
state.needReadable = state.length <= state.highWaterMark;
n = 0;
} else {
state.length -= n;
@@ -0,0 +1,21 @@
'use strict';
const common = require('../common');
const { Readable } = require('stream');

// readable.resume() should not lead to a ._read() call being scheduled
// when we exceed the high water mark already.

const readable = new Readable({
read: common.mustNotCall(),
highWaterMark: 100
});

// Fill up the internal buffer so that we definitely exceed the HWM:
for (let i = 0; i < 10; i++)
readable.push('a'.repeat(200));

// Call resume, and pause after one chunk.
// The .pause() is just so that we don’t empty the buffer fully, which would
// be a valid reason to call ._read().
readable.resume();
readable.once('data', common.mustCall(() => readable.pause()));

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