fs: reduce memory retention when streaming small files

Fixes: https://github.com/nodejs/node/issues/21967

PR-URL: https://github.com/nodejs/node/pull/21968
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/test/parallel/test-fs-read-stream-concurrent-reads.js b/test/parallel/test-fs-read-stream-concurrent-reads.js
new file mode 100644
index 0000000000..32a6cd6236
--- /dev/null
+++ b/test/parallel/test-fs-read-stream-concurrent-reads.js
@@ -0,0 +1,47 @@
+'use strict';
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const assert = require('assert');
+const fs = require('fs');
+
+// Test that concurrent file read streams don’t interfere with each other’s
+// contents, and that the chunks generated by the reads only retain a
+// 'reasonable' amount of memory.
+
+// Refs: https://github.com/nodejs/node/issues/21967
+
+const filename = fixtures.path('loop.js');  // Some small non-homogeneous file.
+const content = fs.readFileSync(filename);
+
+const N = 1000;
+let started = 0;
+let done = 0;
+
+const arrayBuffers = new Set();
+
+function startRead() {
+  ++started;
+  const chunks = [];
+  fs.createReadStream(filename)
+    .on('data', (chunk) => {
+      chunks.push(chunk);
+      arrayBuffers.add(chunk.buffer);
+      if (started < N)
+        startRead();
+    })
+    .on('end', common.mustCall(() => {
+      assert.deepStrictEqual(Buffer.concat(chunks), content);
+      if (++done === N) {
+        const retainedMemory =
+          [...arrayBuffers].map((ab) => ab.byteLength).reduce((a, b) => a + b);
+        assert(retainedMemory / (N * content.length) <= 3,
+               `Retaining ${retainedMemory} bytes in ABs for ${N} ` +
+               `chunks of size ${content.length}`);
+      }
+    }));
+}
+
+// Don’t start the reads all at once – that way we would have to allocate
+// a large amount of memory upfront.
+for (let i = 0; i < 4; ++i)
+  startRead();