Deal with backpressure on RStream instances

Each RStream instance will now stop its libuv watcher when the buffer is full,
and automatically restart when some data is read with `rstream_read`.

src/os/rstream.c

@@ -147,6 +147,11 @@ uint32_t rstream_read(RStream *rstream, char *buf, uint32_t count)
         rstream->wpos - rstream->rpos);  // ...By the number of unread bytes
     rstream->wpos -= rstream->rpos;
     rstream->rpos = 0;
+
+    if (rstream->wpos < rstream->buffer_size) {
+      // Restart reading since we have freed some space
+      rstream_start(rstream);
+    }
   }
 
   return read_count;
@@ -167,8 +172,9 @@ static void alloc_cb(uv_handle_t *handle, size_t suggested, uv_buf_t *buf)
     return;
   }
 
-  buf->base = rstream->buffer + rstream->wpos;
   buf->len = rstream->buffer_size - rstream->wpos;
+  buf->base = rstream->buffer + rstream->wpos;
+
   // Avoid `alloc_cb`, `alloc_cb` sequences on windows
   rstream->reading = true;
 }
@@ -193,10 +199,17 @@ static void read_cb(uv_stream_t *stream, ssize_t cnt, const uv_buf_t *buf)
   // Data was already written, so all we need is to update 'wpos' to reflect
   // the space actually used in the buffer.
   rstream->wpos += cnt;
+
+  if (rstream->wpos == rstream->buffer_size) {
+    // The last read filled the buffer, stop reading for now
+    rstream_stop(rstream);
+  }
+
   // Invoke the callback passing in the number of bytes available and data
   // associated with the stream
   rstream->cb(rstream, rstream->data, false);
   rstream->reading = false;
+
 }
 
 // Called by the by the 'idle' handle to emulate a reading event
@@ -228,5 +241,11 @@ static void fread_idle_cb(uv_idle_t *handle)
 
   rstream->wpos += req.result;
   rstream->fpos += req.result;
+
+  if (rstream->wpos == rstream->buffer_size) {
+    // The last read filled the buffer, stop reading for now
+    rstream_stop(rstream);
+  }
+
   rstream->cb(rstream, rstream->data, false);
 }