Add a 'smart sync' option to repo sync

This option allows the user to specify a manifest server to use when
syncing. This manifest server will provide a manifest pegging each
project to a known green build. This allows developers to work on a
known good tree that is known to build and pass tests, preventing
failed builds to hamper productivity.

The manifest used is not "sticky" so as to allow subsequent
'repo sync' calls to sync to the tip of the tree.

Change-Id: Id0a24ece20f5a88034ad364b416a1dd2e394226d

manifest_xml.py

@@ -65,8 +65,8 @@ class XmlManifest(object):
 
     self._Unload()
 
-  def Link(self, name):
-    """Update the repo metadata to use a different manifest.
+  def Override(self, name):
+    """Use a different manifest, just for the current instantiation.
     """
     path = os.path.join(self.manifestProject.worktree, name)
     if not os.path.isfile(path):
@@ -80,6 +80,11 @@ class XmlManifest(object):
     finally:
       self.manifestFile = old
 
+  def Link(self, name):
+    """Update the repo metadata to use a different manifest.
+    """
+    self.Override(name)
+
     try:
       if os.path.exists(self.manifestFile):
         os.remove(self.manifestFile)
@@ -123,6 +128,12 @@ class XmlManifest(object):
       root.appendChild(e)
       root.appendChild(doc.createTextNode(''))
 
+    if self._manifest_server:
+      e = doc.createElement('manifest-server')
+      e.setAttribute('url', self._manifest_server)
+      root.appendChild(e)
+      root.appendChild(doc.createTextNode(''))
+
     sort_projects = list(self.projects.keys())
     sort_projects.sort()
 
@@ -168,6 +179,11 @@ class XmlManifest(object):
     self._Load()
     return self._default
 
+  @property
+  def manifest_server(self):
+    self._Load()
+    return self._manifest_server
+
   @property
   def IsMirror(self):
     return self.manifestProject.config.GetBoolean('repo.mirror')
@@ -178,6 +194,7 @@ class XmlManifest(object):
     self._remotes = {}
     self._default = None
     self.branch = None
+    self._manifest_server = None
 
   def _Load(self):
     if not self._loaded:
@@ -246,6 +263,15 @@ class XmlManifest(object):
     if self._default is None:
       self._default = _Default()
 
+    for node in config.childNodes:
+      if node.nodeName == 'manifest-server':
+        url = self._reqatt(node, 'url')
+        if self._manifest_server is not None:
+            raise ManifestParseError, \
+                'duplicate manifest-server in %s' % \
+                (self.manifestFile)
+        self._manifest_server = url
+
     for node in config.childNodes:
       if node.nodeName == 'project':
         project = self._ParseProject(node)
@@ -315,7 +341,7 @@ class XmlManifest(object):
   def _ParseProject(self, node):
     """
     reads a <project> element from the manifest file
-    """ 
+    """
     name = self._reqatt(node, 'name')
 
     remote = self._get_remote(node)