Merge ~cjwatson/launchpad:fix-export-is-stale into launchpad:master

[Colin Watson]

Fix test_bzrsync.TestMarkSnapsStale failures

Broken by https://code.launchpad.net/~cjwatson/launchpad/+git/launchpad/+merge/402642.

[Cristian Gonzalez (cristiangsp)]

review: Approve

diff --git a/lib/lp/code/model/branch.py b/lib/lp/code/model/branch.py
index 452bf4e..c9a98cd 100644
--- a/lib/lp/code/model/branch.py
+++ b/lib/lp/code/model/branch.py
@@ -689,7 +689,9 @@ class Branch(SQLBase, WebhookTargetMixin, BzrIdentityMixin):
             self,
             check_permissions=False)
         for snap in snaps:
-            snap.is_stale = True
+            # ISnapSet.findByBranch returns security-proxied Snap objects on
+            # which the is_stale attribute is read-only.  Bypass this.
+            removeSecurityProxy(snap).is_stale = True
 
     def addToLaunchBag(self, launchbag):
         """See `IBranch`."""