Swift store: do not send a 0 byte chunk

When a chunk of size 0 is added, it is immediately deleted. Swift might try to
delete it before its creation is complete, though, leading to a race condition.
Not sending this 0 byte chunk soves this issue.

Co-Authored-By: Stuart McLaren <stuart.mclaren@hp.com>
Change-Id: I2179a1e6e27a636bfca448fa18481eb11e64e6da
Closes-Bug: #1518431

3 files changed, 14 insertions, 11 deletions

diff --git a/glance_store/_drivers/swift/store.py b/glance_store/_drivers/swift/store.py
index bdf4bbd..591b16d 100644
--- a/glance_store/_drivers/swift/store.py
+++ b/glance_store/_drivers/swift/store.py
@@ -551,6 +551,9 @@ class BaseStore(driver.Store):
                             location.container, chunk_name, reader,
                             content_length=content_length)
                         written_chunks.append(chunk_name)
+                    except exceptions.ZeroSizeChunk:
+                        LOG.debug('Not writing zero-length chunk')
+                        break
                     except Exception:
                         # Delete orphaned segments from swift backend
                         with excutils.save_and_reraise_exception():
@@ -572,14 +575,6 @@ class BaseStore(driver.Store):
                             'chunk_etag': chunk_etag})
                     LOG.debug(msg)
 
-                    if bytes_read == 0:
-                        # Delete the last chunk, because it's of zero size.
-                        # This will happen if size == 0.
-                        LOG.debug("Deleting final zero-length chunk")
-                        connection.delete_object(location.container,
-                                                 chunk_name)
-                        break
-
                     chunk_id += 1
                     combined_chunks_size += bytes_read
 
@@ -960,6 +955,9 @@ class ChunkReader(object):
         if i > left:
             i = left
         result = self.fd.read(i)
+        if len(result) == 0 and self.bytes_read == 0:
+            # fd was empty
+            raise exceptions.ZeroSizeChunk()
         self.bytes_read += len(result)
         self.checksum.update(result)
         return result
diff --git a/glance_store/exceptions.py b/glance_store/exceptions.py
index ea7b61b..45c790d 100644
--- a/glance_store/exceptions.py
+++ b/glance_store/exceptions.py
@@ -81,6 +81,10 @@ class NotFound(GlanceStoreException):
     message = _("Image %(image)s not found")
 
 
+class ZeroSizeChunk(GlanceStoreException):
+    message = _("Zero size chunk")
+
+
 class UnknownScheme(GlanceStoreException):
     message = _("Unknown scheme '%(scheme)s' found in URI")
 
diff --git a/glance_store/tests/unit/test_swift_store.py b/glance_store/tests/unit/test_swift_store.py
index a71024c..813f623 100644
--- a/glance_store/tests/unit/test_swift_store.py
+++ b/glance_store/tests/unit/test_swift_store.py
@@ -1500,10 +1500,11 @@ class TestChunkReader(base.StoreBaseTest):
         bytes_read = 0
         while True:
             cr = swift.ChunkReader(infile, checksum, CHUNKSIZE)
-            chunk = cr.read(CHUNKSIZE)
-            bytes_read += len(chunk)
-            if not chunk:
+            try:
+                chunk = cr.read(CHUNKSIZE)
+            except exceptions.ZeroSizeChunk:
                 break
+            bytes_read += len(chunk)
         self.assertEqual(units.Ki, bytes_read)
         data_file.close()
         infile.close()