stream: do not chunk strings and Buffer in Readable.from

PR-URL: nodejs#30912
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: mcollina

doc/api/stream.md

@@ -1642,6 +1642,10 @@ readable.on('data', (chunk) => {
 });
 ```
 
+Calling `Readable.from(string)` or `Readable.from(buffer)` will not have
+the strings or buffers be iterated to match the other streams semantics
+for performance reasons.
+
 ## API for Stream Implementers
 
 <!--type=misc-->

lib/internal/streams/from.js

@@ -4,13 +4,25 @@ const {
   Symbol,
   SymbolIterator
 } = primordials;
+const { Buffer } = require('buffer');
 
 const {
   ERR_INVALID_ARG_TYPE
 } = require('internal/errors').codes;
 
 function from(Readable, iterable, opts) {
   let iterator;
+  if (typeof iterable === 'string' || iterable instanceof Buffer) {
+    return new Readable({
+      objectMode: true,
+      ...opts,
+      read() {
+        this.push(iterable);
+        this.push(null);
+      }
+    });
+  }
+
   if (iterable && iterable[Symbol.asyncIterator])
     iterator = iterable[Symbol.asyncIterator]();
   else if (iterable && iterable[SymbolIterator])

test/parallel/test-readable-from.js

@@ -56,13 +56,23 @@ async function toReadablePromises() {
 async function toReadableString() {
   const stream = Readable.from('abc');
 
-  const expected = ['a', 'b', 'c'];
+  const expected = ['abc'];
 
   for await (const chunk of stream) {
     strictEqual(chunk, expected.shift());
   }
 }
 
+async function toReadableBuffer() {
+  const stream = Readable.from(Buffer.from('abc'));
+
+  const expected = ['abc'];
+
+  for await (const chunk of stream) {
+    strictEqual(chunk.toString(), expected.shift());
+  }
+}
+
 async function toReadableOnData() {
   async function* generate() {
     yield 'a';
@@ -154,6 +164,7 @@ Promise.all([
   toReadableSyncIterator(),
   toReadablePromises(),
   toReadableString(),
+  toReadableBuffer(),
   toReadableOnData(),
   toReadableOnDataNonObject(),
   destroysTheStreamWhenThrowing(),