stream: validate writable defaultEncoding

lib/internal/streams/writable.js

@@ -122,7 +122,14 @@ function WritableState(options, stream, isDuplex) {
   // Crypto is kind of old and crusty.  Historically, its default string
   // encoding is 'binary' so we have to make this configurable.
   // Everything else in the universe uses 'utf8', though.
-  this.defaultEncoding = (options && options.defaultEncoding) || 'utf8';
+  const defaultEncoding = options?.defaultEncoding;
+  if (defaultEncoding == null) {
+    this.defaultEncoding = 'utf8';
+  } else if (Buffer.isEncoding(defaultEncoding)) {
+    this.defaultEncoding = defaultEncoding;
+  } else {
+    throw new ERR_UNKNOWN_ENCODING(defaultEncoding);
+  }
 
   // Not an actual buffer we keep track of, but a measurement
   // of how much we're waiting to get pushed to some underlying

test/parallel/test-stream-writable-decoded-encoding.js

@@ -56,3 +56,50 @@ class MyWritable extends stream.Writable {
   m.write('some-text', 'utf8');
   m.end();
 }
+
+{
+  assert.throws(() => {
+    const m = new MyWritable(null, {
+      defaultEncoding: 'my invalid encoding',
+    });
+    m.end();
+  }, {
+    code: 'ERR_UNKNOWN_ENCODING',
+  });
+}
+
+{
+  const w = new MyWritable(function(isBuffer, type, enc) {
+    assert(!isBuffer);
+    assert.strictEqual(type, 'string');
+    assert.strictEqual(enc, 'hex');
+  }, {
+    defaultEncoding: 'hex',
+    decodeStrings: false
+  });
+  w.write('asd');
+  w.end();
+}
+
+{
+  const w = new MyWritable(function(isBuffer, type, enc) {
+    assert(!isBuffer);
+    assert.strictEqual(type, 'string');
+    assert.strictEqual(enc, 'utf8');
+  }, {
+    defaultEncoding: null,
+    decodeStrings: false
+  });
+  w.write('asd');
+  w.end();
+}
+
+{
+  const m = new MyWritable(function(isBuffer, type, enc) {
+    assert.strictEqual(type, 'object');
+    assert.strictEqual(enc, 'utf8');
+  }, { defaultEncoding: 'hex',
+       objectMode: true });
+  m.write({ foo: 'bar' }, 'utf8');
+  m.end();
+}