test writing chunk wile flowing with buffer

Reproduces #27, cause of npm/cli#3884

test/buffer-chunk-when-flowing-stops.js

@@ -0,0 +1,35 @@
+// Reproduces an issue where write is called while the stream is flowing but a
+// destination cannot accept all of the buffered chunks.  minipass should add
+// the chunk to the end of the buffer instead of emitting it before the buffer
+// is cleared.
+//
+// This caused issues when piping make-fetch-happen stream to tar.extract
+// https://github.com/npm/cli/issues/3884
+const Minipass = require('../')
+const t = require('tap')
+
+class Pauser extends Minipass {
+  write (chunk, encoding, callback) {
+    super.write(chunk, encoding, callback)
+    return false
+  }
+}
+
+const src = new Minipass({encoding: 'utf8'})
+const pauser = new Pauser({encoding: 'utf8'})
+
+// queue up two chunks while the src is buffering
+src.write('1')
+src.write('2')
+
+// when the src starts flowing write a third chunk
+src.once('resume', () => src.write('3'))
+
+// pipe the src to the pauser which will request the src stops after the first
+// chunk.
+src.pipe(pauser)
+
+src.end()
+
+// we should expect the chunks in the original order.
+t.resolveMatch(pauser.collect(), ['1', '2', '3'], '123')