lib: clarify maybeReadMore_ loop conditions

lib/_stream_readable.js

@@ -569,6 +569,30 @@ function maybeReadMore(stream, state) {
 
 function maybeReadMore_(stream, state) {
   var len = state.length;
+
+  // Attempt to read more data if we should.
+  //
+  // The conditions for reading more data are (one of):
+  // - Not enough data buffered (state.length < state.highWaterMark). The loop
+  //   is responsible for filling the buffer with enough data if such data
+  //   is available. If highWaterMark is 0 and we are not in the flowing mode
+  //   we should _not_ attempt to buffer any extra data. We'll get more data
+  //   when the stream consumer calls read() instead.
+  // - No data in the buffer, and the stream is in flowing mode. In this mode
+  //   the loop below is responsible for ensuring read() is called. Failing to
+  //   call read here would abort the flow and there's no other mechanism for
+  //   continuing the flow if the stream consumer has just subscribed to the
+  //   'data' event.
+  //
+  // In addition to the above conditions to keep reading data, the following
+  // conditions prevent the data from being read:
+  // - The stream has ended (state.ended).
+  // - There is already a pending 'read' operation (state.reading). This is a
+  //   case where the the stream has called the implementation defined _read()
+  //   method, but they are processing the call asynchronously and have _not_
+  //   called push() with new data. In this case we skip performing more
+  //   read()s. The execution ends in this method again after the _read() ends
+  //   up calling push() with more data.
   while (!state.reading && !state.ended &&
          (state.length < state.highWaterMark ||
           (state.flowing && state.length === 0))) {