Add stream.isReadonly property to detect read-only streams

Fixes #1927

Author: sindresorhus

documentation/3-streams.md

@@ -21,7 +21,7 @@ This constructor takes the same arguments as the Got promise.
 > To filter which headers are copied, listen to the `response` event and modify `response.headers` before piping to the destination.
 
 **Note:**
-> If the `body`, `json` or `form` option is used, this stream will be read-only.
+> If the `body`, `json` or `form` option is used, this stream will be read-only. Check [`stream.isReadonly`](#streamisreadonly) to detect this condition.
 
 **Note:**
 > - While `got.post('https://example.com')` resolves, `got.stream.post('https://example.com')` will hang indefinitely until a body is provided.
@@ -169,6 +169,12 @@ Whether the response has been fetched from cache.
 
 Whether the socket was used for other previous requests.
 
+### `stream.isReadonly`
+
+**Type: `boolean`**
+
+Whether the stream is read-only. Returns `true` when `body`, `json`, or `form` options are provided.
+
 ## Events
 
 ### `stream.on('response', …)`

source/core/index.ts

@@ -1471,14 +1471,14 @@ export default class Request extends Duplex implements RequestEvents<Request> {
 
 			if (isClientRequest(requestOrResponse!)) {
 				this._onRequest(requestOrResponse);
-			} else if (this.writable) {
+			} else if (this.writableEnded) {
+				void this._onResponse(requestOrResponse as IncomingMessageWithTimings);
+			} else {
 				this.once('finish', () => {
 					void this._onResponse(requestOrResponse as IncomingMessageWithTimings);
 				});
 
 				this._sendBody();
-			} else {
-				void this._onResponse(requestOrResponse as IncomingMessageWithTimings);
 			}
 		} catch (error) {
 			if (error instanceof CacheableCacheError) {
@@ -1642,4 +1642,11 @@ export default class Request extends Duplex implements RequestEvents<Request> {
 	get reusedSocket(): boolean | undefined {
 		return this._request?.reusedSocket;
 	}
+
+	/**
+	Whether the stream is read-only. Returns `true` when `body`, `json`, or `form` options are provided.
+	*/
+	get isReadonly(): boolean {
+		return !is.undefined(this.options?.body) || !is.undefined(this.options?.json) || !is.undefined(this.options?.form);
+	}
 }

test/stream.ts

@@ -657,3 +657,40 @@ test('validates content-length for gzip compressed responses', withServer, async
 		},
 	);
 });
+
+test('isReadonly is false for streams without body', withServer, async (t, server, got) => {
+	server.post('/', postHandler);
+
+	const stream = got.stream.post('');
+	t.false(stream.isReadonly);
+
+	stream.end('data');
+	await getStream(stream);
+});
+
+test('isReadonly is true when body option is provided', withServer, async (t, server, got) => {
+	server.post('/', postHandler);
+
+	const stream = got.stream.post({body: 'test data'});
+	t.true(stream.isReadonly);
+
+	await t.notThrowsAsync(getStream(stream));
+});
+
+test('isReadonly is true when json option is provided', withServer, async (t, server, got) => {
+	server.post('/', postHandler);
+
+	const stream = got.stream.post({json: {foo: 'bar'}});
+	t.true(stream.isReadonly);
+
+	await t.notThrowsAsync(getStream(stream));
+});
+
+test('isReadonly is true when form option is provided', withServer, async (t, server, got) => {
+	server.post('/', postHandler);
+
+	const stream = got.stream.post({form: {foo: 'bar'}});
+	t.true(stream.isReadonly);
+
+	await t.notThrowsAsync(getStream(stream));
+});