add isDisturbed helper

index.js

@@ -112,6 +112,8 @@ const WRITE_BACKPRESSURE_STATUS = WRITE_UNDRAINED | DESTROY_STATUS | WRITE_FINIS
 const WRITE_UPDATE_SYNC_STATUS = WRITE_UPDATING | OPEN_STATUS | WRITE_NEXT_TICK | WRITE_PRIMARY
 const WRITE_DROP_DATA = WRITE_FINISHING | WRITE_DONE | DESTROY_STATUS
 
+const DISTURBED_STATUS = OPEN_STATUS | READ_RESUMED_READ_AHEAD | WRITE_QUEUED
+
 const asyncIterator = Symbol.asyncIterator || Symbol('asyncIterator')
 
 class WritableState {
@@ -1134,6 +1136,10 @@ function isReadStreamx (stream) {
   return isStreamx(stream) && stream.readable
 }
 
+function isDisturbed (stream) {
+  return (stream._duplexState & DISTURBED_STATUS) !== OPENING
+}
+
 function isTypedArray (data) {
   return typeof data === 'object' && data !== null && typeof data.byteLength === 'number'
 }
@@ -1157,6 +1163,7 @@ module.exports = {
   pipelinePromise,
   isStream,
   isStreamx,
+  isDisturbed,
   isEnded,
   isFinished,
   getStreamError,

test/readable.js

@@ -1,6 +1,6 @@
 const test = require('brittle')
 const b4a = require('b4a')
-const { Readable } = require('../')
+const { Readable, isDisturbed } = require('../')
 
 test('ondata', function (t) {
   t.plan(4)
@@ -403,6 +403,16 @@ test('setEncoding empty string', async function (t) {
   }
 })
 
+test('is disturbed', function (t) {
+  const r = new Readable()
+
+  t.not(isDisturbed(r))
+
+  r.resume()
+
+  t.ok(isDisturbed(r))
+})
+
 function nextImmediate () {
   return new Promise(resolve => setImmediate(resolve))
 }