Report a config error for unsupported merge mode

This updates the branch cache (and associated connection mixin)
to include information about supported project merge modes.  With
this, if a project on github has the "squash" merge mode disabled
and a Zuul user attempts to configure Zuul to use the "squash"
mode, then Zuul will report a configuration syntax error.

This change adds implementation support only to the github driver.
Other drivers may add support in the future.

For all other drivers, the branch cache mixin simply returns a value
indicating that all merge modes are supported, so there will be no
behavior change.

This is also the upgrade strategy: the branch cache uses a
defaultdict that reports all merge modes supported for any project
when it first loads the cache from ZK after an upgrade.

Change-Id: I3ed9a98dfc1ed63ac11025eb792c61c9a6414384

1 files changed, 16 insertions, 0 deletions

diff --git a/zuul/source/__init__.py b/zuul/source/__init__.py
index faf97b48b..dd11aa9b6 100644
--- a/zuul/source/__init__.py
+++ b/zuul/source/__init__.py
@@ -15,6 +15,8 @@
 import abc
 import time
 
+from zuul import model
+
 
 class BaseSource(object, metaclass=abc.ABCMeta):
     """Base class for sources.
@@ -183,6 +185,20 @@ class BaseSource(object, metaclass=abc.ABCMeta):
 
         """
 
+    def getProjectMergeModes(self, project, tenant, min_ltime=-1):
+        """Get supported merge modes for a project
+
+        This method is called very frequently, and should generally
+        return quickly.  The connection is expected to cache merge
+        modes for all projects queried.
+
+        The default implementation indicates that all merge modes are
+        supported.
+
+        """
+
+        return model.ALL_MERGE_MODES
+
     @abc.abstractmethod
     def getProjectBranchCacheLtime(self):
         """Return the current ltime of the project branch cache."""