Let VCSes sometimes bypass clobber fallback.

(Some fetch errors are detectably unrecoverable. In
these cases, it's better to keep DVCS clones around
for future use, instead of wiping them out for no reason.)

slave/buildslave/commands/base.py

@@ -366,6 +366,9 @@ def maybeDoVCFallback(self, rc):
             return rc
         if self.interrupted:
             raise AbandonChain(1)
+        # Let VCS subclasses have an opportunity to handle
+        # unrecoverable errors without having to clobber the repo
+        self.maybeNotDoVCFallback(self, rc)
         msg = "update failed, clobbering and trying again"
         self.sendStatus({'header': msg + "\n"})
         log.msg(msg)
@@ -382,6 +385,14 @@ def doVCFallback2(self, res):
         d.addCallback(self._abandonOnFailure)
         return d
 
+    def maybeNotDoVCFallback(self, rc):
+        """Override this in a subclass if you want to detect unrecoverable
+        checkout errors where clobbering the repo wouldn't help, and stop
+        the current VC chain before it clobbers the repo for future builds.
+
+        Use 'raise AbandonChain' to pass up a halt if you do detect such."""
+        pass
+
     def maybeDoVCRetry(self, res):
         """We get here somewhere after a VC chain has finished. res could
         be::

slave/buildslave/commands/git.py

@@ -5,6 +5,7 @@
 from buildslave.commands.base import SourceBaseCommand
 from buildslave import runprocess
 from buildslave.commands import utils
+from buildslave.commands.base import AbandonChain
 
 
 class Git(SourceBaseCommand):
@@ -102,6 +103,12 @@ def _didFetch(self, res):
         command = ['reset', '--hard', head]
         return self._dovccmd(command, self._didHeadCheckout)
 
+    def maybeNotDoVCFallback(self, res):
+        # If we were unable to find the branch/SHA on the remote,
+        # clobbering the repo won't help any, so just abort the chain
+        if "Couldn't find remote ref" in self.stderr:
+            raise AbandonChain(-1)
+
     # Update first runs "git clean", removing local changes,
     # if the branch to be checked out has changed.  This, combined
     # with the later "git reset" equates clobbering the repo,
@@ -131,7 +138,7 @@ def _doFetch(self, dummy):
             command.append('--progress')
         self.sendStatus({"header": "fetching branch %s from %s\n"
                                         % (self.branch, self.repourl)})
-        return self._dovccmd(command, self._didFetch)
+        return self._dovccmd(command, self._didFetch, keepStderr=True)
 
     def _didClean(self, dummy):
         # After a clean, try to use the given revision if we have one.