From 6315eb5086a96e4ba614ba01c9bbfaec8bd8504b Mon Sep 17 00:00:00 2001
From: Rick Harris <rconradharris@gmail.com>
Date: Mon, 27 Feb 2012 21:01:49 +0000
Subject: [PATCH] Adds temporary chown to sparse_copy.

`sparse_copy` needs read and write access to the devices. Since we
cannot shell out to a run-as-root command here, we temporarily take
ownership of the device.

Change-Id: I891c38dbcba7177286dca729684c88ac065bd085
---
 nova/utils.py                | 20 ++++++++++++++++++
 nova/virt/xenapi/vm_utils.py | 39 ++++++++++++++++++++----------------
 2 files changed, 42 insertions(+), 17 deletions(-)

diff --git a/nova/utils.py b/nova/utils.py
index 0f3e61897707..6bb0dd0f269a 100644
--- a/nova/utils.py
+++ b/nova/utils.py
@@ -1523,3 +1523,23 @@ def read_file_as_root(file_path):
         return out
     except exception.ProcessExecutionError:
         raise exception.FileNotFound(file_path=file_path)
+
+
+@contextlib.contextmanager
+def temporary_chown(path, owner_uid=None):
+    """Temporarily chown a path.
+
+    :params owner_uid: UID of temporary owner (defaults to current user)
+    """
+    if owner_uid is None:
+        owner_uid = os.getuid()
+
+    orig_uid = os.stat(path).st_uid
+
+    if orig_uid != owner_uid:
+        execute('chown', owner_uid, path, run_as_root=True)
+    try:
+        yield
+    finally:
+        if orig_uid != owner_uid:
+            execute('chown', orig_uid, path, run_as_root=True)
diff --git a/nova/virt/xenapi/vm_utils.py b/nova/virt/xenapi/vm_utils.py
index e000e4ab109d..3c5264f9750a 100644
--- a/nova/virt/xenapi/vm_utils.py
+++ b/nova/virt/xenapi/vm_utils.py
@@ -1651,25 +1651,30 @@ def _sparse_copy(src_path, dst_path, virtual_size, block_size=4096):
                 "virtual_size=%(virtual_size)d block_size=%(block_size)d"),
                 locals())
 
-    with open(src_path, "r") as src:
-        with open(dst_path, "w") as dst:
-            data = src.read(min(block_size, left))
-            while data:
-                if data == EMPTY_BLOCK:
-                    dst.seek(block_size, os.SEEK_CUR)
-                    left -= block_size
-                    bytes_read += block_size
-                    skipped_bytes += block_size
-                else:
-                    dst.write(data)
-                    data_len = len(data)
-                    left -= data_len
-                    bytes_read += data_len
+    # NOTE(sirp): we need read/write access to the devices; since we don't have
+    # the luxury of shelling out to a sudo'd command, we temporarily take
+    # ownership of the devices.
+    with utils.temporary_chown(src_path):
+        with utils.temporary_chown(dst_path):
+            with open(src_path, "r") as src:
+                with open(dst_path, "w") as dst:
+                    data = src.read(min(block_size, left))
+                    while data:
+                        if data == EMPTY_BLOCK:
+                            dst.seek(block_size, os.SEEK_CUR)
+                            left -= block_size
+                            bytes_read += block_size
+                            skipped_bytes += block_size
+                        else:
+                            dst.write(data)
+                            data_len = len(data)
+                            left -= data_len
+                            bytes_read += data_len
 
-                if left <= 0:
-                    break
+                        if left <= 0:
+                            break
 
-                data = src.read(min(block_size, left))
+                        data = src.read(min(block_size, left))
 
     duration = time.time() - start_time
     compression_pct = float(skipped_bytes) / bytes_read * 100