Directly copy a file URL from glance.

That patch allows nova to use shutil to copy a file directly from the
glance store.  For this to work glance needs to enable direct_url
descriptions in the image metadata.  This is done by setting
show_image_direct_url to True in glance-api.conf.  The path used to back
glance also must match the path used to back nova instance storage.  This
should be a large performance increase for the cases where both glance
and nova instance storage are backed by a shared file system like gluster
or ceph.

A new option has been added to nova: allowed_direct_url_schemes
This option is set to a list of schemes that can be directly accessed.
For this first simple patch only file:// is valid, and the
implementation is simple and inline.  In the future it would be better
to make this into propagation modules which are dynamically loaded
according to the scheme.

DocImpact
Implements: blueprint direct-file-copy
Change-Id: I041b5524f8a0ea59e89ece0202f0503abf2f9d8e

2 files changed, 56 insertions, 1 deletions

diff --git a/nova/image/glance.py b/nova/image/glance.py
index 6a5406d9e..8bda7285f 100644
--- a/nova/image/glance.py
+++ b/nova/image/glance.py
@@ -22,6 +22,7 @@ from __future__ import absolute_import
 import copy
 import itertools
 import random
+import shutil
 import sys
 import time
 import urlparse
@@ -58,7 +59,12 @@ glance_opts = [
     cfg.IntOpt('glance_num_retries',
                default=0,
                help='Number retries when downloading an image from glance'),
-]
+    cfg.ListOpt('allowed_direct_url_schemes',
+                default=[],
+                help='A list of url scheme that can be downloaded directly '
+                     'via the direct_url.  Currently supported schemes: '
+                     '[file].'),
+    ]
 
 LOG = logging.getLogger(__name__)
 CONF = cfg.CONF
@@ -254,6 +260,18 @@ class GlanceImageService(object):
 
     def download(self, context, image_id, data):
         """Calls out to Glance for metadata and data and writes data."""
+        if 'file' in CONF.allowed_direct_url_schemes:
+            location = self.get_location(context, image_id)
+            o = urlparse.urlparse(location)
+            if o.scheme == "file":
+                with open(o.path, "r") as f:
+                    # FIXME(jbresnah) a system call to cp could have
+                    # significant performance advantages, however we
+                    # do not have the path to files at this point in
+                    # the abstraction.
+                    shutil.copyfileobj(f, data)
+                return
+
         try:
             image_chunks = self._client.call(context, 1, 'data', image_id)
         except Exception:
diff --git a/nova/tests/image/test_glance.py b/nova/tests/image/test_glance.py
index 7c13796a6..9dd9e5121 100644
--- a/nova/tests/image/test_glance.py
+++ b/nova/tests/image/test_glance.py
@@ -17,7 +17,10 @@
 
 
 import datetime
+import filecmp
+import os
 import random
+import tempfile
 import time
 
 import glanceclient.exc
@@ -468,6 +471,40 @@ class TestGlanceImageService(test.TestCase):
         self.flags(glance_num_retries=1)
         service.download(self.context, image_id, writer)
 
+    def test_download_file_url(self):
+        class MyGlanceStubClient(glance_stubs.StubGlanceClient):
+            """A client that returns a file url."""
+
+            (outfd, s_tmpfname) = tempfile.mkstemp(prefix='directURLsrc')
+            outf = os.fdopen(outfd, 'w')
+            inf = open('/dev/urandom', 'r')
+            for i in range(10):
+                _data = inf.read(1024)
+                outf.write(_data)
+            outf.close()
+
+            def get(self, image_id):
+                return type('GlanceTestDirectUrlMeta', (object,),
+                            {'direct_url': 'file://%s' + self.s_tmpfname})
+
+        client = MyGlanceStubClient()
+        (outfd, tmpfname) = tempfile.mkstemp(prefix='directURLdst')
+        writer = os.fdopen(outfd, 'w')
+
+        service = self._create_image_service(client)
+        image_id = 1  # doesn't matter
+
+        self.flags(allowed_direct_url_schemes=['file'])
+        service.download(self.context, image_id, writer)
+        writer.close()
+
+        # compare the two files
+        rc = filecmp.cmp(tmpfname, client.s_tmpfname)
+        self.assertTrue(rc, "The file %s and %s should be the same" %
+                        (tmpfname, client.s_tmpfname))
+        os.remove(client.s_tmpfname)
+        os.remove(tmpfname)
+
     def test_client_forbidden_converts_to_imagenotauthed(self):
         class MyGlanceStubClient(glance_stubs.StubGlanceClient):
             """A client that raises a Forbidden exception."""