download: handle shared projects a bit better

If a manifest checksout a project multiple times, repo download isn't
able to accurately pick the right project.  We were just picking the
first result which could be a bit random for the user.  If we hit that
situation, check if the cwd is one of the projects, and if it isn't,
we emit an error and tell the user it's an ambiguous request.

Bug: https://crbug.com/gerrit/13070
Change-Id: Id1059b81330229126b48c7312569b37504808383
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298702
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/download.py

@@ -16,7 +16,7 @@ import re
 import sys
 
 from command import Command
-from error import GitError
+from error import GitError, NoSuchProjectError
 
 CHANGE_RE = re.compile(r'^([1-9][0-9]*)(?:[/\.-]([1-9][0-9]*))?$')
 
@@ -60,6 +60,7 @@ If no project is specified try to use current directory as a project.
       if m:
         if not project:
           project = self.GetProjects(".")[0]
+          print('Defaulting to cwd project', project.name)
         chg_id = int(m.group(1))
         if m.group(2):
           ps_id = int(m.group(2))
@@ -76,7 +77,23 @@ If no project is specified try to use current directory as a project.
                 ps_id = max(int(match.group(1)), ps_id)
         to_get.append((project, chg_id, ps_id))
       else:
-        project = self.GetProjects([a])[0]
+        projects = self.GetProjects([a])
+        if len(projects) > 1:
+          # If the cwd is one of the projects, assume they want that.
+          try:
+            project = self.GetProjects('.')[0]
+          except NoSuchProjectError:
+            project = None
+          if project not in projects:
+            print('error: %s matches too many projects; please re-run inside '
+                  'the project checkout.' % (a,), file=sys.stderr)
+            for project in projects:
+              print('  %s/ @ %s' % (project.relpath, project.revisionExpr),
+                    file=sys.stderr)
+            sys.exit(1)
+        else:
+          project = projects[0]
+          print('Defaulting to cwd project', project.name)
     return to_get
 
   def ValidateOptions(self, opt, args):