Make cooperative reader always support read()

glance.common.utils.CooperativeReader provides eventlet-friendly
cooperation on top of backends that support either read() or __iter__().
However, in the case of backends that only support __iter__(), such as
images returned by store.get(...), read() is not defined. This patch
adds read() in all cases.

Fixes bug 1057322.

Change-Id: I67d9b3e4d93fbefd7eeaf7cfc947ab635fe09534

glance/common/utils.py

@@ -116,9 +116,26 @@ def __init__(self, fd):
         :param fd: Underlying image file object
         """
         self.fd = fd
+        self.iterator = None
+        # NOTE(markwash): if the underlying supports read(), overwrite the
+        # default iterator-based implementation with cooperative_read which
+        # is more straightforward
         if hasattr(fd, 'read'):
             self.read = cooperative_read(fd)
 
+    def read(self, length=None):
+        """Return the next chunk of the underlying iterator.
+
+        This is replaced with cooperative_read in __init__ if the underlying
+        fd already supports read().
+        """
+        if self.iterator is None:
+            self.iterator = self.__iter__()
+        try:
+            return self.iterator.next()
+        except StopIteration:
+            return ''
+
     def __iter__(self):
         return cooperative_iter(self.fd.__iter__())
 

glance/tests/unit/test_utils.py

@@ -50,6 +50,17 @@ def test_cooperative_reader(self):
 
         self.assertEquals(bytes_read, BYTES)
 
+    def test_cooperative_reader_of_iterator(self):
+        """Ensure cooperative reader supports iterator backends too"""
+        reader = utils.CooperativeReader([l * 3 for l in 'abcdefgh'])
+        chunks = []
+        while True:
+            chunks.append(reader.read(3))
+            if chunks[-1] == '':
+                break
+        meat = ''.join(chunks)
+        self.assertEqual(meat, 'aaabbbcccdddeeefffggghhh')
+
     def test_limiting_reader(self):
         """Ensure limiting reader class accesses all bytes of file"""
         BYTES = 1024