# HG changeset patch
# User Yuya Nishihara <yuya@tcha.org>
# Date 1476131409 -7200
# Node ID d61c42c1a35c1081d9eac5acdaf47221bc42ab79
# Parent  9626022feaa4733da7e1fff2465d7da0665539bf
revset: make follow() reject more than one start revisions

Taking only the last revision is inconsistent because ancestors(set) follows
all revisions given, and theoretically follow(startrev=set) == ancestors(set).
I'm planning to add a support for multiple start revisions, but that won't fit
to the 4.0 time frame. So reject multiple revisions now to avoid future BC.

len(revs) might be slow if revs were large, but we don't care since a valid
revs should have only one element.

diff -r 9626022feaa4 -r d61c42c1a35c mercurial/revset.py
--- a/mercurial/revset.py	Sat Oct 15 17:10:53 2016 -0700
+++ b/mercurial/revset.py	Mon Oct 10 22:30:09 2016 +0200
@@ -1027,10 +1027,11 @@
         x = getstring(l[0], _("%s expected a pattern") % name)
         rev = None
         if len(l) >= 2:
-            rev = getset(repo, fullreposet(repo), l[1]).last()
-            if rev is None:
+            revs = getset(repo, fullreposet(repo), l[1])
+            if len(revs) != 1:
                 raise error.RepoLookupError(
-                        _("%s: starting revision set cannot be empty") % name)
+                        _("%s expected one starting revision") % name)
+            rev = revs.last()
             c = repo[rev]
         matcher = matchmod.match(repo.root, repo.getcwd(), [x],
                                  ctx=repo[rev], default='path')