wsgi: fix Input.readline on Python 3

Previously, we would compare the last item of a byte string
with a newline in a native string. On Python 3, getting a
single item from a byte string give you an integer (which
will not be equal to any string), so readline would return
the entire request body.

While we're at it, fix the return type when the caller requests
that zero bytes be read.

eventlet/wsgi.py

@@ -157,7 +157,7 @@ def _chunked_read(self, rfile, length=None, use_readline=False):
             self.is_hundred_continue_response_sent = True
         try:
             if length == 0:
-                return ""
+                return b""
 
             if length and length < 0:
                 length = None
@@ -190,7 +190,7 @@ def _chunked_read(self, rfile, length=None, use_readline=False):
                         length -= datalen
                         if length == 0:
                             break
-                    if use_readline and data[-1] == "\n":
+                    if use_readline and data[-1:] == b"\n":
                         break
                 else:
                     try:

tests/wsgi_test.py

@@ -1773,6 +1773,9 @@ def application(self, env, start_response):
         elif pi == "/lines":
             for x in input:
                 response.append(x)
+        elif pi == "/readline":
+            response.extend(iter(input.readline, b''))
+            response.append(('\nread %d lines' % len(response)).encode())
         elif pi == "/ping":
             input.read()
             response.append(b"pong")
@@ -1876,6 +1879,16 @@ def test_chunked_readline(self):
         self.assertEqual(read_http(fd).body, b'this is chunked\nline 2\nline3')
         fd.close()
 
+    def test_chunked_readline_from_input(self):
+        body = self.body()
+        req = "POST /readline HTTP/1.1\r\nContent-Length: %s\r\n" \
+              "transfer-encoding: Chunked\r\n\r\n%s" % (len(body), body)
+
+        fd = self.connect()
+        fd.sendall(req.encode())
+        self.assertEqual(read_http(fd).body, b'this is chunked\nline 2\nline3\nread 3 lines')
+        fd.close()
+
     def test_chunked_readline_wsgi_override_minimum_chunk_size(self):
 
         fd = self.connect()