stream: increase MAX_HWM

ronag · BethGriggs · commit 77e0318849aa · 2020-01-07T13:57:28.000Z
MAX_HWM was added in 9208c89 where the highwatermark was changed to always increase in steps of highest power of 2 to prevent increasing hwm excessivly in tiny amounts. Why a limit was added on the highwatermark is unclear but breaks existing usage where a larger read size is used. The invariant for read(n) is that a buffer of size n is always returned. Considering a maximum ceiling on the buffer size breaks this invariant. This PR significantly increases the limit to make it less likely to break the previous invariant and also documents the limit. Fixes: #29933 PR-URL: #29938 Reviewed-By: Anna Henningsen <anna@addaleax.net> Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
diff --git a/doc/api/stream.md b/doc/api/stream.md
@@ -1010,6 +1010,8 @@ buffer will be returned.
 If the `size` argument is not specified, all of the data contained in the
 internal buffer will be returned.
 
+The `size` argument must be less than or equal to 1 GB.
+
 The `readable.read()` method should only be called on `Readable` streams
 operating in paused mode. In flowing mode, `readable.read()` is called
 automatically until the internal buffer is fully drained.
diff --git a/lib/_stream_readable.js b/lib/_stream_readable.js
@@ -340,10 +340,11 @@ Readable.prototype.setEncoding = function(enc) {
   return this;
 };
 
-// Don't raise the hwm > 8MB
-const MAX_HWM = 0x800000;
+// Don't raise the hwm > 1GB
+const MAX_HWM = 0x40000000;
 function computeNewHighWaterMark(n) {
   if (n >= MAX_HWM) {
+    // TODO(ronag): Throw ERR_VALUE_OUT_OF_RANGE.
     n = MAX_HWM;
   } else {
     // Get the next highest power of 2 to prevent increasing hwm excessively in
diff --git a/test/parallel/test-readable-large-hwm.js b/test/parallel/test-readable-large-hwm.js
@@ -0,0 +1,27 @@
+'use strict';
+const common = require('../common');
+const { Readable } = require('stream');
+
+// Make sure that readable completes
+// even when reading larger buffer.
+const bufferSize = 10 * 1024 * 1024;
+let n = 0;
+const r = new Readable({
+  read() {
+    // Try to fill readable buffer piece by piece.
+    r.push(Buffer.alloc(bufferSize / 10));
+
+    if (n++ > 10) {
+      r.push(null);
+    }
+  }
+});
+
+r.on('readable', () => {
+  while (true) {
+    const ret = r.read(bufferSize);
+    if (ret === null)
+      break;
+  }
+});
+r.on('end', common.mustCall());
