publish: fix race conditions

* remove useless resource lock
  Resource locks need to be before the background task. creating same publish endpoint at the same time is unlikely...
* load data inside background tasks
  This fixes a flaw in async apis, which loaded the published repo from the DB and mutated it outside the task closure, before the task lock was acquired.
  Perform collection.LoadComplete inside maybeRunTaskInBackground and have tasks use a fresh copy of taskCollectionFactory, taskCollection
* lock source repos/snapshots for publish operations
  Concurrent tasks were not properly locking their resources, leading to inconsistent published indexes:
  SourceLocalRepo: iterate published.Sources (component -> source UUID), look up each local repo via localRepoCollection.ByUUID and append string(repo.Key()) to resources
  SourceSnapshot: iterate b.Snapshots,look up each snapshot via snapshotCollection.ByName and append string(snapshot.ResourceKey()) to resources.
* lock pool on non MultiDist publish
* revert mutex on LinkFromPool
* use uuids, since names can be renamed
* add test for MultiDist change

files/public.go

@@ -150,6 +150,7 @@ func (storage *PublishedStorage) LinkFromPool(publishedPrefix, publishedRelPath,
 
 	baseName := filepath.Base(fileName)
 	poolPath := filepath.Join(storage.rootPath, publishedPrefix, publishedRelPath, filepath.Dir(fileName))
+	destinationPath := filepath.Join(poolPath, baseName)
 
 	var localSourcePool aptly.LocalPackagePool
 	if storage.linkMethod != LinkMethodCopy {

files/public_test.go

@@ -632,16 +632,6 @@ func (s *DiskFullNoRootSuite) TestLinkFromPoolCopySyncErrorIsReturned(c *C) {
 	c.Check(strings.Contains(err.Error(), "error syncing file"), Equals, true)
 }
 
-func (s *DiskFullNoRootSuite) TestGetFileLockReusesMutex(c *C) {
-	a := getFileLock(filepath.Join(s.root, "a"))
-	b := getFileLock(filepath.Join(s.root, "a"))
-	c.Check(a == b, Equals, true)
-
-	c1 := getFileLock(filepath.Join(s.root, "c1"))
-	c2 := getFileLock(filepath.Join(s.root, "c2"))
-	c.Check(c1 == c2, Equals, false)
-}
-
 func (s *DiskFullNoRootSuite) TestPutFileFailsIfDestinationDirMissing(c *C) {
 	storage := NewPublishedStorage(s.root, "", "")