diff: handle errors gracefully

If `git diff` fails in any project checkout (e.g. an incomplete
sync), make sure we print that error clearly rather than blowing
up, and exit non-zero in the process.

Bug: https://crbug.com/gerrit/11613
Change-Id: I12f278427cced20f23f8047e7e3dba8f442ee25e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239236
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

project.py

@@ -230,6 +230,7 @@ class DiffColoring(Coloring):
   def __init__(self, config):
     Coloring.__init__(self, config, 'diff')
     self.project = self.printer('header', attr='bold')
+    self.fail = self.printer('fail', fg='red')
 
 
 class _Annotation(object):
@@ -1136,10 +1137,18 @@ class Project(object):
       cmd.append('--src-prefix=a/%s/' % self.relpath)
       cmd.append('--dst-prefix=b/%s/' % self.relpath)
     cmd.append('--')
+    try:
       p = GitCommand(self,
                      cmd,
                      capture_stdout=True,
                      capture_stderr=True)
+    except GitError as e:
+      out.nl()
+      out.project('project %s/' % self.relpath)
+      out.nl()
+      out.fail('%s', str(e))
+      out.nl()
+      return False
     has_diff = False
     for line in p.process.stdout:
       if not hasattr(line, 'encode'):
@@ -1150,7 +1159,7 @@ class Project(object):
         out.nl()
         has_diff = True
       print(line[:-1])
-    p.Wait()
+    return p.Wait() == 0
 
 
 # Publish / Upload ##

subcmds/diff.py

@@ -37,5 +37,8 @@ to the Unix 'patch' command.
                  help='Paths are relative to the repository root')
 
   def Execute(self, opt, args):
+    ret = 0
     for project in self.GetProjects(args):
-      project.PrintWorkTreeDiff(opt.absolute)
+      if not project.PrintWorkTreeDiff(opt.absolute):
+        ret = 1
+    return ret